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| general description the MAX8688 is a fully integrated digital power-supply controller and monitor ic that works with any existing pol (point-of-load) power supply to provide complete digital programmability. by interfacing to the reference input, feedback node, and output enable, the MAX8688 takes control of the pol to provide functions such as perfect tracking, sequencing, margining, and dynamic adjustment of the output voltage. the MAX8688 offers an accurate 12-bit analog-to-digi- tal converter (adc) accompanied with two differential amplifiers for accurately monitoring both voltage and current. an integrated 12-bit digital-to-analog converter (dac) is also available to margin power supplies as well as dynamically adjust the output voltage with 0.2% accuracy over temperature using this closed-loop sys- tem. an internal temperature sensor provides an addi- tional level of system monitoring. the user-programmable registers provide flexible and accurate control of time events such as a delay time and transition period, monitoring for overvoltage and undervoltage, overcurrent, reverse-current, overtemper- ature fault and warning handling. the closed-loop oper- ation is also programmable to make sure the MAX8688 works with any existing pol to provide superior regula- tion accuracy and accurate margining. the MAX8688 operates using a pmbus -compliant communication protocol. the device is programmed using this protocol or simply with the use of a free graphic-user interface (gui) available from the maxim website that significantly reduces development time. once the configuration is complete, the results can be saved into an eeprom or loaded onto the part through the pmbus at power-up. this allows remote configura- tion of any pol using the MAX8688, replacing expen- sive recalls or field service. module current sharing is also supported, since accurate current measurement and fine resolution voltage control are available. the MAX8688 can be programmed with up to 127 distinct addresses to support large systems. the MAX8688 is offered in a space-saving, 24-pin, lead-free 4mm x 4mm tqfn package. applications telecom networking dc-dc modules and pols servers high-reliability infrastructure equipment features pmbus interface for programming, monitoring, sequencing up and down, and accurate output- voltage control controls output voltage with 0.2% accuracy for line, load, and temperature variations output voltage, output current, and temperature monitoring with adjustable monitor rate current measurement with 2.6% accuracy and temperature compensation option programmable soft-start and soft-stop ramp rates controls up to 26 power supplies with hardwired address pins and up to 127 pols with software addressing compatible with refin and fb terminals of pol power supplies protection for pol against overvoltage, undervoltage, overcurrent, negative current and overtemperature faults with no action, latch and retry (hiccup) options open-drain flt signal for fault detection master-slave clocking option eliminates external clock requirement and provides accurate timing reference external eeprom interface for auto-programming on power-up MAX8688 digital power-supply controller/monitor with pmbus interface ________________________________________________________________ maxim integrated products 1 part temp range pin- package pkg code MAX8688aletg+ -40? to +85? 24 tqfn-ep* t2444-4 MAX8688ahetg+ -40? to +85? 24 tqfn-ep* t2444-4 MAX8688bletg+ -40? to +85? 24 tqfn-ep* t2444-4 MAX8688bhetg+ -40? to +85? 24 tqfn-ep* t2444-4 ordering information part accuracy (%) enout power-up default state MAX8688aletg+ 0.2 low MAX8688ahetg+ 0.2 high MAX8688bletg+ 0.4 low MAX8688bhetg+ 0.4 high selector guide 19-3201; rev 0; 1/08 for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. pin configuration appears at end of data sheet. evaluation kit available pmbus is a trademark of smif, inc. + denotes a lead-free package. * ep = exposed pad.
MAX8688 digital power-supply controller/monitor with pmbus interface 2 _______________________________________________________________________________________ absolute maximum ratings electrical characteristics (avdd = dvdd = 3.3v, t a = -40? to +85?, v rs+ - v rs- = 2v, v rs- = agnd, unless otherwise stated.) (note 1) stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. avdd, dvdd to agnd .........................................-0.3v to +4.5v dgnd to agnd ..................................................................?.3v rs+, rs-, isn+, isn- to agnd ................................-0.3v to +6v rs_c, isn_c, a1/scle, a2/sdae, a3/onoff to agnd ............................-0.3v to (avdd + 0.3v) dacout to agnd ..................................-0.3v to (avdd + 0.3v) refo to agnd......................................................-0.3v to +4.5v continuous power dissipation (t a = +70?) 24-pin tqfn (derate 27.8mw/? above +70?)...........2222mw* ja .................................................................................36?/w jc ................................................................................2.7?/w scl, sda, clkio, rst to dgnd .........................-0.3v to +4.5v enout, flt to dgnd..............................................-0.3v to +6v thermal resistance from junction to exposed pad ......2.7?/w operating temperature .......................................-40? to +85? junction temperature ........................................................150? storage temperature.........................................-65? to +150? lead temperature (soldering, 10s) .................................+300? parameter symbol conditions min typ max units general avdd/dvdd operating range 3.0 3.6 v avdd and dvdd operating supply current v rs+ = v rs- = v isn+ = v isn- = agnd 6.7 8.5 ma rising 2.70 2.8 2.95 v avdd uvlo hysteresis 100 mv output-voltage sensing MAX8688a, t a = 0? to +85?, v rs+ = 1v, v rs- = 0v -0.2 +0.2 MAX8688a, t a = -40? to +85?, v rs+ = 1v, v rs- = 0v -0.3 +0.3 MAX8688b, t a = 0? to +85?, v rs+ = 1v, v rs- = 0v -0.4 +0.4 voltage regulation accuracy (2v range, table 8) (note 2) MAX8688b, t a = -40? to +85?, v rs+ = 1v, v rs- = 0v -0.5 +0.5 % MAX8688a, t a = 0? to +85?, v rs+ = 2.5v, v rs- = 0v -0.3 +0.3 MAX8688a, t a = -40? to +85?, v rs+ = 2.5v, v rs- = 0v -0.4 +0.4 MAX8688b, t a = 0? to +85?, v rs+ = 2.5v, v rs- = 0v -0.4 +0.4 voltage regulation accuracy (5.5v range, table 8) (note 2) MAX8688b, t a = -40? to +85?, v rs+ = 2.5v, v rs- = 0v -0.5 +0.5 % rs+, rs- differential mode range 0 5.5 v rs- to agnd differential voltage -250 +250 mv 2v range, v rs+ = -0.25v to 2v -10 +15 rs+ input bias current 5.5v range, v rs+ = -0.25v to 5.5v -10 +60 ? rs- input bias current 2v or 5.5v range, v rs- = -0.25v to +0.25v -10 0 a * as per jedec51 standard (multilayered board). MAX8688 digital power-supply controller/monitor with pmbus interface _______________________________________________________________________________________ 3 parameter symbol conditions min typ max units output current sense v isn+ = 1v, v isn+ - v isn- = 20mv, t a = +25? to +85? -2.6 +2.6 current-sense accuracy (note 2) v isn+ = 1v, v isn+ - v isn- = 20mv, t a = -40? to +85? -7 +7 % isn+, isn- common-mode range 0 5.5 v isn+, isn- common-mode current-sense error v cm = 0 to 5.5v, v dm = 20mv 0.9 % isn+, isn- differential mode range -10 +40 mv isn+, isn- input bias current isn+, isn- to agnd = 0 or 5.5v -20 +40 ? rs_c, isn_c rs_c, isn_c output impedance 0.2 k ? temperature sensing temperature sensing accuracy exposed pad = -40? to +100? ? ? clkio clkio input-logic low voltage 0.8 v clkio input-logic high voltage 2.1 v clkio input bias current clkio = 0 or 3.6v -1 +1 ? clkio input clock duty cycle f clkio = 100khz to 2500khz 20 80 % clkio output low voltage clkio in output mode, i sink = 4ma 0.4 v clkio output high leakage clkio = dvdd = avdd = 3.6v -1 +1 ? clkio input/output clock rise time pullup = 560 ? , c load = 20pf 20 ns clkio input/output clock fall time pullup = 560 ? , c load = 20pf 2 ns clkio pullup voltage dvdd v clkio input frequency f ext_clk 100 2500 khz clkio output frequency accuracy 0.95 1.00 1.05 mhz enout, flt open-drain logic outputs enout, flt output low voltage i sink = 4ma 0.4 v enout, flt output high leakage enout = flt = 5.5v, dvdd = avdd = 3.6v -1 +1 ? enout, flt pullup voltage 5.5 v dac dac resolution 12 bits dac output-voltage range no load refo - 1 lsb v electrical characteristics (continued) (avdd = dvdd = 3.3v, t a = -40? to +85?, v rs+ - v rs- = 2v, v rs- = agnd, unless otherwise stated.) (note 1) MAX8688 digital power-supply controller/monitor with pmbus interface 4 _______________________________________________________________________________________ electrical characteristics (continued) (avdd = dvdd = 3.3v, t a = -40? to +85?, v rs+ - v rs- = 2v, v rs- = agnd, unless otherwise stated.) (note 1) parameter symbol conditions min typ max units dac output-voltage slew rate 0.6 v/? dac output resistance 5 ? dac driving capability 2ma adc MAX8688a 12 adc resolution MAX8688b 11 bits three-state address pins (a3/onoff, a2/sdae, a1/scle) three-state address pins input low voltage 0.3 v three-state address pins input low threshold hysteresis 50 mv three-state address pins input high voltage avdd - 0.4 v three-state address pins input high threshold hysteresis 50 mv three-state address pins input bias current avdd = 3.6v, a3/onoff = a2/sdae = a1/scle = avdd or agnd -12 +12 ? three-state address pins (a2/sdae (data) and a1/scle (clock) with eeprom) (note 3) a2/sdae, a1/scle output low voltage output sink current = 100? (note 4) 0.3 x avdd v a2/sdae, a1/scle output high voltage output source current = 100? (note 5) 0.7 x avdd v three-state address pin (a3/onoff as pol on/off control) minimum a3/onoff control pulse low time t a3_low 250 ? minimum a3/onoff control pulse high time t a3_high 750 ? scl, sda smbus signals maximum smbus speed 100 khz scl, sda input low voltage v smb_il dvdd = 3.0v to 3.6v 0.8 v scl, sda input high voltage v smb_ih dvdd = 3.0v to 3.6v 2.1 dvdd v scl, sda output low voltage v smb_ol dvdd = 3.0v to 3.6v at i sink = 4ma 0.4 v scl, sda input leakage per device pin i smb_ileak dvdd = 3.6v, scl = sda = 0 or 3.6v -1 +1 ? smbus is a trademark of intel corp. MAX8688 digital power-supply controller/monitor with pmbus interface _______________________________________________________________________________________ 5 electrical characteristics (continued) (avdd = dvdd = 3.3v, t a = -40? to +85?, v rs+ - v rs- = 2v, v rs- = agnd, unless otherwise stated.) (note 1) parameter symbol conditions min typ max units rst input rst input low voltage v rst_il dvdd = 3.0v to 3.6v 0.8 v rst input high voltage v rst_ih dvdd = 3.0v to 3.6v 2.1 v rst input bias current rst = dvdd or dgnd 10 ? minimum smbus interface reset pulse width t smb_rst 1 455 ? smbus interface recovery time after interface reset t smb_wait 15 ? minimum reset pulse width t rst 565 ? recovery time after device reset t rst_wait 15 ? other timing parameters pmbus command response time t pmb_rsp 300 ? overvoltage fault 5 fault response time t fault_rsp overcurrent fault 5 ms note 1: production tested at t a = +25? and t a = +85?. specifications from t a = -40? to +25? are guaranteed by design, unless otherwise noted. note 2: production tested at t a = +85? only. all other temperatures are guaranteed by design. note 3: when an eeprom is connected to a2/sdae and a1/scle, these pins cannot be hardwired to ground or supply. they must be connected through 33k ? ?% resistors. note 4: equivalent of having 33k ? pulldown resistor to dgnd. note 5: equivalent of having 33k ? pullup resistor to dvdd. typical operating characteristics (avdd = dvdd = 3.3v, t a = +25?, unless otherwise noted.) refin mode soft-start with tracking MAX8688 toc01 2ms/div v out_4 = 1.8v v out_3 = 1.5v v out_2 = 1.2v v out_1 = 1.0v 0.5v/div refin mode soft-stop with tracking MAX8688 toc02 2ms/div v out_4 = 1.8v v out_3 = 1.5v v out_2 = 1.2v v out_1 = 1.0v 0.5v/div MAX8688 digital power-supply controller/monitor with pmbus interface 6 _______________________________________________________________________________________ typical operating characteristics (continued) (avdd = dvdd = 3.3v, t a = +25?, unless otherwise noted.) refin mode soft-start with sequencing MAX8688 toc03 2ms/div v out_4 = 1.8v v out_3 = 1.5v v out_2 = 1.2v v out_1 = 1.0v 0.5v/div refin mode soft-stop with sequencing MAX8688 toc04 2ms/div v out_4 = 1.8v v out_3 = 1.5v v out_2 = 1.2v v out_1 = 1.0v 0.5v/div refin mode immediate off with sequencing MAX8688 toc05 2ms/div v out_4 = 1.8v v out_3 = 1.5v v out_2 = 1.2v v out_1 = 1.0v 0.5v/div refin mode soft-start from a3 with sequencing MAX8688 toc06 2ms/div a3/onoff 2v/div v out_1 , v out_2 = 1.2v 500mv/div refin mode soft-stop from a3 with sequencing MAX8688 toc07 2ms/div a3/onoff 2v/div v out_1 , v out_2 = 1.2v 500mv/div MAX8688 digital power-supply controller/monitor with pmbus interface _______________________________________________________________________________________ 7 typical operating characteristics (continued) (avdd = dvdd = 3.3v, t a = +25?, unless otherwise noted.) refin mode margining 200mv high at 1mv/ s MAX8688 toc08 200 s/div 1.2v 0.6v 0.85v v out 200mv/div v dacout 200mv/div v rsc 500mv/div refin mode margining 200mv low at 1mv/ s MAX8688 toc09 200 s/div 0.8v 0.4v 1.25v v out 200mv/div v dacout 200mv/div v rsc 500mv/div fb mode margining high from 1.0v to 1.2v MAX8688 toc10 40ms/div 1.2v 0.6v 0.85v v out 200mv/div v dacout 500mv/div v rsc 500mv/div fb mode margining low from 1.0v to 0.8v MAX8688 toc11 40ms/div 0.8v 1v 1.25v v out 200mv/div v dacout 500mv/div v rsc 500mv/div MAX8688 digital power-supply controller/monitor with pmbus interface 8 _______________________________________________________________________________________ pin description pin name function 1, 3 dvdd digital power-supply input (3.3v nominal ). connect avdd to dvdd externally. connect a 0.1? capacitor to dgnd. 2 dgnd digital ground. connect agnd to dgnd externally close to the device. 4 clkio clock input/output. user-configurable clock input/output signal. the system controller can provide a clock input to synchronize the time bases of multiple MAX8688 devices. alternatively, a MAX8688 can provide a 1mhz output clock to other MAX8688s for synchronization. see the mfr_mode (d1h) section. 5 rst active-low smbus interface and device reset line. active-low logic input. see the rst operation section. 6 sda smbus data line 7 scl smbus clock line 8, 9 n.c. no connection. not internally connected. 10 enout on/off signal. typically used to turn on/off a pol power supply under the pmbus command or a3/onoff control. open-drain output. see the enout operation and mfr_mode (d1h) sections . 11 flt fault output. active-low open-drain output. typically connected to system controller/master interrupt input. 12 a3/onoff dual-functioned three-state MAX8688 slave address identifier (msb) and pol on/off control using the mfr_mode command. see the MAX8688 address assignment and a3/onoff operation sections. 13 a2/sdae dual-functioned three-state MAX8688 slave address identifier and eeprom i 2 c data line. see the MAX8688 address assignment and external eeprom interface sections . 14 a1/scle dual-functioned three-state MAX8688 slave address identifier (lsb) and eeprom i 2 c clock line. see the MAX8688 address assignment and external eeprom interface sections . 15 dacout analog voltage output of internal 12-bit dac. connect to refin or fb of the dc-dc module with high impedance in shutdown. 16 agnd analog ground. connect agnd to dgnd externally close to the device. 17 avdd analog power-supply input (3.3v nominal ). connect avdd to dvdd externally. connect 0.1? capacitor to agnd. 18 refo buffered reference output. connect a 1? capacitor to ground for bypass. 19 rs_c filter capacitor for v sense amplifier 20 rs- differential remote-sense input return of the dc-dc output voltage. connect to the return terminal at the load. 21 rs+ differential remote-sense input of dc-dc output voltage. connect to the load terminal where the output must be regulated. 22 isn- differential-sense input return of dc-dc output current. connect to the negative end of the current-sense resistor (figure 3). in case of dcr sensing, connect to the return terminal of filter capacitor c s (figure 7). 23 isn+ differential-sense input of dc-dc output current. connect to the positive end of the current-sense resistor. in case of dcr sensing, connect to the junction of filter resistor and capacitor (r s and c s ) (figure 7). 24 isn_c filter capacitor for i sense amplifier ep exposed pad. connect ep to the agnd plane for the pol for best temperature measurement performance. MAX8688 digital power-supply controller/monitor with pmbus interface _______________________________________________________________________________________ 9 dvdd dgnd avdd agnd 0.1 f 1 f 200 ? rs- rs+ rs_c 200 ? isn- isn+ isn_c refo dacout s1 power monitor smbus controller internal reference temp sensor adc MAX8688 a1/scle a2/sdae a3/onoff scl sda rst 0.1 f 3.3v pmbus command and status registers dac maxq c enout flt clkio figure 1. MAX8688 functional diagram maxq is a registered trademark of maxim integrated products, inc. detailed description for many applications, it is desirable to control the out- put voltage of a pol power supply to a much higher accuracy than the offered standard 1% overtempera- ture specification. many designs are required to access information such as output voltage, output cur- rent, and temperature of individual power supplies in a board, for monitoring system health as well as logging fault information to help in failure analysis. moreover, it is desirable to sequence startup and shutdown of multi- ple power supplies in an application with programma- ble start, stop delays, and soft-start ramp rates to avoid latchup and stressing of esd structures. the MAX8688 solves these problems by providing the required func- tions in a small compact ic that is capable of interfac- ing with a master controller through an on-board pmbus interface. up to 127 MAX8688s can reside on the same pmbus bus, each controlling its own pol, under command from the system controller, as shown in figure 2. long traces from pols located at various system board locations for voltage sensing and current sensing are avoided resulting in a cleaner layout for the system designer. pols can therefore be placed close to the load where they provide the best transient response with short power plane runs. MAX8688 digital power-supply controller/monitor with pmbus interface 10 ______________________________________________________________________________________ MAX8688 operating modes reference input (refin) mode figure 3 shows the typical manner in which the MAX8688 is used in an application where the pol has a reference input, refin, to which it regulates its output voltage between the vo+ and vo- terminals. in the refin application, the dacout of the MAX8688 is con- nected to the refin input of the pol. the output voltage of the pol is sensed using rs+ and rs-. the sensed voltage is suitably filtered by an internal 200 ? resistor and external capacitor connected to rs_c and is multi- plexed to a 12-bit adc that uses an accurate internal ref- erence voltage. on receipt of either an operation on command or a turn-on signal from a3/onoff, the MAX8688 commences the startup operation that has been programmed for the pol being controlled. after the programmed t on_delay time, the MAX8688 open-drain enout output goes active and the pol out- put voltage is ramped up to its target vout_command value precisely in the programmed t on_rise time. this facilitates easy implementation of tracking of multiple out- put rails. on reaching the target output voltage, the MAX8688 continuously monitors the pol output voltage obtained at the rs+ and rs- inputs, and regulates it to within ?.2% for line, load, and temperature variations by incrementing or decrementing the dacout output 1 lsb (0.5mv) at a time. the MAX8688 output-voltage correc- tion rate is programmable up to 10khz by the mfr_vout_correction_rate parameter. once the requested target pol voltage is reached, it is possible to easily margin up or down the pol voltage at a prepro- grammed slew rate set by the parameter vout_transi- tion_rate. to achieve this, the MAX8688 increments or decrements the dacout output in a suitable number of steps that depend on the programmed transition rate. in addition, the user needs to program the vout_scale_loop parameter equal to any voltage- divider ratio implemented on the pol from its output volt- age node to the inverting input of its error amplifier. this allows the MAX8688 to correctly calculate the number of dacout steps and voltage increments/decrements per step and thus achieves the programmed rise time and transition time. scl sda system controller irq rst scl sda flt rst a3/onoff a2/sdae a1/scle a3/onoff a2/sdae a1/scle a3/onoff a2/sdae a1/scle a3/onoff a2/sdae a1/scle MAX8688 vo+ vo- io+ pol 1 io- fb en rs+ rs- isn+ isn- dacout enout scl sda flt rst MAX8688 vo+ vo- io+ pol 2 io- fb en rs+ rs- isn+ isn- dacout enout scl sda flt rst MAX8688 vo+ vo- io+ pol 3 io- fb en rs+ rs- isn+ isn- dacout enout scl sda flt rst MAX8688 vo+ vo- io+ pol n io- fb en rs+ rs- isn+ isn- dacout enout figure 2. system application showing multiple MAX8688s controlling pol power supplies MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 11 since the reference voltage input is provided by the MAX8688, the refin mode provides complete control of the pol in terms of soft-start, soft-stop, and margin- ing transitions. it may be noted that the slew rates dur- ing soft-start (t on_rise ) and during margining (vout_transition_rate) should be programmed with pol current-limit consideration. an excessively fast slew rate causes the pol to trip due to overcurrent. a general guideline for setting the output-voltage slew rate is as follows: where c out is the output capacitance on the pol out- put, i load is the load current being delivered by the pol, and i limit is the current-limit setting of the pol. on receipt of either an operation off command or a turn-off signal from a3/onoff, the MAX8688 com- mences the shutdown operation that has been pro- grammed for the pol being controlled. after the programmed t off_delay time, the MAX8688 ramps down the output voltage to zero precisely in the pro- grammed t off_fall time, and deasserts its open-drain enout output. slew rate ii c limit load out () ? dvdd dgnd avdd agnd 0.1 f 1 f 200 ? rs- rs+ rs_c 200 ? isn- isn+ isn_c refo dacout refin vo+ vo- s1 power monitor smbus controller internal reference temp sensor adc v in+ r sense load v in- MAX8688 a1/scle a2/sdae a3/onoff scl sda rst 0.1 f 3.3v pol en pmbus command and status registers dac maxq c enout flt clkio clkout irq system controller figure 3. typical system application?efin mode MAX8688 digital power-supply controller/monitor with pmbus interface 12 ______________________________________________________________________________________ feedback (fb) mode in applications where a refin input is not available, the MAX8688 interfaces to the inverting input of the pol error amplifier, the feedback node (fb), through a resistor r fb as shown in figure 5. in steady-state oper- ation, the MAX8688 controls the pol voltage as mea- sured between rs+ and rs- to 0.2% accuracy over line, load, and temperature variations, by adjusting dacout 1 lsb (0.5mv) at a time, up and down as required. this mode of operation is termed fb mode. since the MAX8688 does not have control over the pol error-amplifier reference voltage, this mode relies on the pol soft-start setting to implement the required soft-start time. on receipt of either an operation on command or a turn-on signal from a3/onoff, the MAX8688 commences startup operations that have been programmed for the pol being controlled. after the programmed t on_delay time, the MAX8688 open- drain enout output goes active, causing the pol to ramp up its output voltage to its target value. the soft- start time taken by the pol to ramp from zero to its commanded output voltage should be entered into the MAX8688 with the t on_rise parameter. during t on_rise , the MAX8688 maintains dacout in a high-impedance state by keeping the s1 switch open. this allows the voltage at dacout to equal that of the fb node of the pol. at the end of the t on_rise delay time, the internal dac output is initialized to the external voltage measured on dacout and switch s1 is closed. if the pol has completed its soft-start and settled down at its output voltage, the dac output is initialized to the steady-state value of the pol fb voltage. therefore, when switch s1 is closed, the voltages on either side of the resistor r fb are equal. under these conditions, zero current flows into the fb node from dacout and no perturbations are introduced to the output voltage. from this point on, the MAX8688 adjusts the voltage at dacout to provide accurate output voltage control. in fb mode, the user is required to supply t on_delay and t on_rise parameters. if those parameters are not set (the default values are zero), s1 closes prematurely and cause an initial error in the voltage monitor. on receipt of either an operation off command or a turn-off signal from a3/onoff, the MAX8688 com- mences the shutdown operation that has been pro- grammed for the pol being controlled. after the programmed t off_delay time, the MAX8688 deasserts its open-drain enout output, and turns off the pol. for the fb mode, the value of r fb is selected based on following formula: rr v v fb dac o = 1 ? ? t pmb_rsp t pmb_rsp t on_delay close s1 t off_delay t off_fall t on_rise turn-on turn-off dac output enout pol v out pol operation figure 4. refin mode timing MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 13 where r 1 is the upper feedback divider resistor, ? v o is the required change in output voltage, and ? v dac is the dacout output voltage change that the user allows. the recommended operating range for the dacout voltage for pol output voltage adjustment is between 30mv and 2v. it should be noted that ? v dac is the difference between the steady-state pol fb node voltage, v fb , and the voltage limits on dacout. this is best illustrated with an example as follows: consider an application involving a pol with v fb = 0.6v. let the desired margining be ?0% for a pol output voltage of 1v. for a pol with an upper voltage- divider resistor r 1 = 10k ? , r fb is calculated as follows: this value of r fb allows the MAX8688 to margin the pol output voltage up by 10%. it is useful to check the margin low condition by using the formula: the effective margining range for the 57k ? resistor therefore turns out to be between +10% and -24.5%. it should be noted that the vout_transition_rate parameter has no effect on fb mode. the transition time for margining in the fb mode of operation is a function of the mfr_vout_correction_rate parameter, r fb and r 1 , and is given by the following formula: t r r v mfr vout correction rate fb fb out = 1 ? __ _ ? ? ? ? vr v r k vv k v o dac fb = = = ? 1 10 20 06 57 0 245 (. . ) . rk vv v k fb = ? = 10 06 003 01 57 ?? (. . ) . dvdd dgnd avdd agnd 0.1 f 1 f 200 ? rs- rs+ rs_c 200 ? isn- isn+ isn_c refo dacout fb vo+ vo- s1 power monitor smbus controller internal reference temp sensor adc v in+ r sense load v in- MAX8688 a1/scle a2/sdae a3/onoff scl sda rst 0.1 f 3.3v pol en pmbus command and status registers dac maxq c enout flt clkio clkout irq system controller r 1 r 2 r fb figure 5. typical system application?eedback mode MAX8688 digital power-supply controller/monitor with pmbus interface 14 ______________________________________________________________________________________ current sensing isn+ and isn- are the inputs of the MAX8688 current- sense amplifier. these pins may be connected to a cur- rent-sense element such as a current-sense resistor, as shown in figures 3 and 5. the voltage proportional to the sensed current is suitably filtered by an internal 200 ? resistor and external capacitor connected to isn_c and is multiplexed to a 12-bit adc that uses an accurate internal reference voltage. a scale factor can be programmed with an iout_scale pmbus com- mand to translate the sensed voltage information to the current. the MAX8688 accommodates a current-sense range of +40mv/-10mv across the isn+ and isn- inputs. the common-mode voltage range for the cur- rent-sense signal can be between 0 and 5.5v. when a negative current is sensed by the MAX8688, flt is asserted indicating a negative fault current flow into the pol output. the dc resistance of the output inductor (dcr) in a switch-mode power supply can also function as a cur- rent-sense element, as shown in figure 7. the rc filter formed by r s and c s is designed with a time constant of about 10 times larger than the lo/dcr time constant. under these conditions, the dc voltage across c s is equal to the product of the average current flowing through the output inductor, essentially the output load current and the dcr. the resistor r bias equal to r s is placed in the current-sensing path as shown, to cancel the effect of the input bias current voltage drop across r s . the MAX8688 offers a temperature compensation coefficient for the current-sense element resistance. see the mfr_iout_temp_coeff (dch) section. t pmb_rsp t pmb_rsp t on_delay close s1 open s1 t off_delay t off_fall t pol_fall t on_rise t pol_rise turn-on high- impedance high- impedance turn-off dac output enout pol v out in fb mode, t pol_rise and t pol_fall are not controlled by the MAX8688 and are dependent on pol implementation. pol operation figure 6. feedback mode timing MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 15 temperature sensing it is intended that the MAX8688 be placed in close prox- imity to the pol. an on-chip temperature sensor on the MAX8688 senses the temperature of the die, which is related to the exposed pad temperature of the MAX8688 by the junction-to-case thermal resistance. the exposed pad of the MAX8688 can be connected to the heat dis- sipating ground plane of the pol, and the pol board may be characterized to obtain the relationship between the pol temperature and temperature as measured by the MAX8688. this information may be used to set overtemperature fault settings in the MAX8688. external eeprom interface the MAX8688 is capable of communicating with an eeprom attached to the a1/scle and a2/sdae. the MAX8688 communicates to the eeprom with an address byte of ?010 0000?for writing and ?010 0001 for reading. for the data values of 2 bytes, the most sig- nificant byte is stored in the lower offset, whereas the least significant byte is stored in the higher offset. upon reset, the MAX8688 tests for the presence of a configuration eeprom. it searches for the signature bytes in the attached eeprom. if the signature bytes are present, it concludes that it has a valid con- figuration eeprom and starts reading configuration information from the attached eeprom. if slave address information is present, this overrides the slave address information previously set by the address a3:a1 pins. table 1 shows the contents and offsets of the configu- ration information expected by the MAX8688. this infor- mation is for reference only. it is recommended to use a properly configured, working MAX8688 and to save its state to the eeprom and limit modifications to as few fields as possible (such as the slave address). some ?eserved fields may contain data other than 0 when the state is saved to the eeprom. these locations are ignored on restoration from the eeprom or are fre- quently recomputed. some reserved fields need to be set to greater than 0 to guarantee proper operation timing. temperature, voltage, and current values are stored in internal representation, which is not identical to the for- mat used by the corresponding pmbus command(s). for details on eeprom internal representation, see the notes following table 1. for example, to store to the eeprom, vout_command = 3.0v, m = 19995, b = 0, r = -1. first calculate the pmbus command value, which is 5998. if the voltage range is 2v, no conversion is required. hence write 17h to offset 14 and 6eh to offset 15. if the voltage range is 5.5v, the stored eeprom value = 5998/2.75 = 2181. so at offset 14, write 08h and offset 15, write 85h. note that the conversion is automatically handled by the MAX8688 as it restores and stores configuration information into the eeprom. v in l o isn+ isn- v0- rs- rs+ v0+ r s r bias c s q1 c1 q2 figure 7. dcr sensing MAX8688 digital power-supply controller/monitor with pmbus interface 16 ______________________________________________________________________________________ offset (bytes) values (note 6) pmbus command notes 0 mfr_temperature_peak d6h note 7 2 mfr_vout_peak d4h note 8 4 mfr_iout_peak d5h note 9 6 mfr_status_word (set to 0) d8h 8?3 reserved (set to 0) 14 vout_command 21h note 8 16 vout_scale_loop 29h 18 ton_rise 61h 10 ton_delay 60h 22 mfr_tick_reload this value equals to 0fe84h when using the internal clock, else computed as 65535 ?mfr_mode[15:8] d1h 24 vout_margin_high 25h note 8 26 vout_margin_low 26h note 8 28 mfr_vout_correction_rate d2h 30 mfr_sample_rate d3h 32 vout_ov_fault_limit 40h note 8 34 vout_ov_warn_limit 42h note 8 36 vout_uv_fault_limit 44h note 8 38 vout_uv_warn_limit 43h note 8 40 iout_oc_fault_limit 46h note 9 42 iout_oc_warn_limit 4ah note 9 44 ot_fault_limit 4fh note 7 46?3 reserved (set to 0) 54 mfr_mode must match mfr_tick_reload setting d1h 56 mfr_fault_retry dah 58 mfr_fault_response d9h 60?3 reserved (set to 1) 64?1 reserved (set to 0) 72 ot_warn_limit 51h note 7 74 iout_scale 38h 76 toff_delay 64h 78 vout_transition_rate 27h 80 reserved (set to 0) 82 mfr_filter_mode d7h 84 mfr_set_address low byte: smbus slave address, high byte: reserved dbh 86 toff_fall 65h 88 mfr_iout_temp_coeff dch 90 reserved (set to 0) 92?25 reserved (set to 0) 126 signature (set to 4453h) n/a table 1. eeprom contents note 6: for a 2-byte value, the most significant byte is written first (lower offset) and then the least significant byte is written la st (higher offset). note 7: to store temperature values to the eeprom, add 3010 (decimal) to the pmbus value. note 8: to store voltage values to the eeprom, no conversion is needed in 2v mode. for 5.5v mode value, divide the pmbus value by 2.75. note 9: to store current values to the eeprom, multiply the pmbus value by iout_scale and divide by 37.24. MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 17 figure 8 shows how the MAX8688 interfaces to an exter- nal serial sot23 eeprom (such as atmel at24c01a) using the a1/scle and a2/sdae in applications where a master controller does not exist or is not required. using the gui, the user can select each MAX8688 device and configure all the required output-voltage settings and sequencing/tracking information. once the configuration is complete, the results can be saved to the external eeprom by using the store_default_all command and configuration restored on the MAX8688 power-on reset. the eeprom can also be preprogrammed prior to board assembly in the manufacturing environment. a3/onoff can be used as a control signal to turn on/off the pol in a similar fashion as the operation command. MAX8688 operation upon reset (power-on reset or applying the device reset pulse to rst ), the MAX8688 goes through an ini- tialization process as shown in figure 9. after initialization, the MAX8688 constantly monitors the pmbus and executes the pmbus command according- ly. in addition, if the pol has been commanded to turn on, the MAX8688 also monitors the pol output voltage, current, and temperature at the mfr_sample_rate. the system controller monitors the pol health by issu- ing various inquiries and status commands. dgnd dvdd agnd avdd 0.1 f 1 f rs- rs+ rs_c isn- isn+ isn_c refo dacout refin vo+ vo- v in+ r sense load v in- MAX8688 a1/scle a2/sdae 0.1 f 3.3v pol en enout clkout irq a3/onoff scl sda rst flt clkio system controller sda scl gnd a3 a2 a1 v cc eeprom 33k ? 33k ? figure 8. typical system application with external eeprom MAX8688 digital power-supply controller/monitor with pmbus interface 18 ______________________________________________________________________________________ rst operation the rst signal can be used to reset either the smbus interface or the MAX8688 depending on the rst pulse width. to reset the smbus interface, the rst signal is held low for t smb_rst . this resets the smbus interface, thus flushing any pending pmbus commands or portion of commands received thus far. none of the other MAX8688 internal registers are affected by an smbus reset. if the host controller applies an active-low pulse to rst for t rst , the MAX8688 undergoes a device reset and repeats the initialization process. enout operation to ensure a known and controlled pol power-up state, the MAX8688 is factory-programmed to a specific enout initial power-up state. there are two types of pols?ctive-high enable or active-low enable. by default, the MAX8688 assumes that the initial power-up state is the off-state. to operate properly, the pol on- state has to be configured. to configure the enout active state, use the mfr_mode command enout polarity select. MAX8688 address assignment the MAX8688 address can be assigned in one of the three ways described below: 1) hardwire by a3:a2:a1. 2) restore from eeprom. 3) by system controller using the mfr_set_address command. address assignment order is shown in figure 10. reset n n n y y y initialize internal registers valid eeprom? internal reference stable? set address according to a3:a1 restore configuration from eeprom enable pmbus communication assert flt execute pmbus command pmbus command? figure 9. MAX8688 initialization enout default startup state enout polarity select enout active state 0 (low) 0 active high 0 (low) 1 active low 1 (high) 0 active low 1 (high) 1 active high table 2. enout active state MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 19 the MAX8688 reads a3:a2:a1 address pins upon device reset and determines its address according to table 3. for example, to configure the MAX8688 to have a slave address of 010 0101 (25h), set a3:a2:a1 = h:l:z. the MAX8688 also responds to the broadcast address (00h). while it is possible to configure the address pins such that the resulting address is 00h, the MAX8688 with such address is then only responsive to the broad- cast address. this may cause undesired results if other pmbus devices are also present on the same bus. therefore, the hardwire address pins option give 3 3 - 1 = 26 address options. if an eeprom with valid signature bytes is attached to the MAX8688, the MAX8688 also tries to restore its slave address from the eeprom. this overrides the address set by the address pins. this gives a total of 127 useful slave addresses (address 00h is normally for broadcast address). if the address bit 7 from the eeprom is set to 1, this is an invalid address and the MAX8688 continues using the address set by the address pins. when an eeprom is attached to a2/sdae and a1/scle, these pins assume either a logic-high or logic-low level, therefore, the resulting number of possible addresses set by the a3:a2:a1 pins in this scenario is 2 3 - 1 = 7. in addition, for the MAX8688 with an eeprom attached, the system controller can change the MAX8688 slave address by sending the new address with the mfr_set_address command. however, the new address is not immediately effective. the new address must be stored to the eeprom first. then, a device reset has to be applied to the MAX8688 which undergoes the address assignment procedure and recalls the new address from the eeprom. reset y y y n n n valid eeprom? mfr_set_addr? set slave address according to a3:a1 restore slave address from eeprom save to eeprom? slave address unchanged. store new address temporarily. save new address to eeprom figure 10. MAX8688 address assignment * the address 00h is reserved for broadcast. ** the shaded addresses are not available if an external eeprom is attached. a3/onoff a2/sdae a1/scle address l l l 00h* llz 01h** lzl 02h lzz 03h zll 04h zlz 05h zzl 06h zzz 07h l l h 09h lzh 0bh zlh 0dh zzh 0fh l h l 12h lhz 13h zhl 16h zhz 17h l h h 1bh zhh 1fh h l l 24h hlz 25h hzl 26h hzz 27h h l h 2dh hzh 2fh h h l 36h hhz 37h h h h 3fh table 3. MAX8688 a3:a1 address assignment MAX8688 digital power-supply controller/monitor with pmbus interface 20 ______________________________________________________________________________________ a3/onoff operation in addition to providing address information to the MAX8688, a3/onoff can also be used as a control sig- nal for turning the pol on or off, similar to the operation command. to use a3/onoff to control the pol, config- ure the a3 control enable bit in mfr_mode command. when a3 control is enabled, a transition of a3/onoff from low to high turns the pol on, as if the MAX8688 has received an operation on command. a transition of a3/onoff from high to low initiates a soft-off to the pol ?as if the MAX8688 has received an operation off command (soft-off, with sequencing). the MAX8688 still responds to the pmbus operation command while a3 control is enabled. to detect the a3/onoff input, the a3/onoff signal pulse width has to satisfy the t a3_low and t a3_high requirement to be detected. if disabled (cleared to 0), the MAX8688 ignores the a3/onoff state and function as directed by the oper- ation command only. the dual functionality of a3/onoff of the MAX8688 requires the system enable signal to be isolated from a3/onoff until the address setting has been read and latched by the MAX8688. figure 11 shows one imple- mentation for the three possible states of the a3/onoff setting. in each case, the system enable signal (MAX8688_en) is applied to the input of a three- state buffer whose output is kept in the high-impedance state by a control input signal (hiz_en) for a time peri- od during which the MAX8688 reads and latches the a3/onoff address setting. after this period, the con- trol signal hiz_en goes low and allows the system enable signal to be applied to the MAX8688 a3/onoff pin. after a t a3_low , the MAX8688_en signal transi- tions from low to high and causes the MAX8688s to commence pol startup operations. a3/onoff a2/sdae a1/scle MAX8688_en hiz_en refo hiz_en places u1, u2, and u3 outputs in high-impedance state when asserted. u1, u2, and u3 are not necessary if an eeprom is attached to a1/scle and a2/sdae. a1/scle, a2/sdae are either pulled up or pulled down with 33k ? when connected to an eeprom. 33k ? u1 1 f a3/onoff a2/sdae a1/scle u2 a3/onoff a2/sdae a1/scle u3 3.3v hiz_en MAX8688_en refo t rst_wait a3/onoff t a3_low avdd MAX8688_uvlo refo 1 f refo 1 f 33k ? MAX8688 MAX8688 MAX8688 figure 11. application diagram with a3/onoff as both address and on/off control signal MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 21 pmbus digital interface from a software perspective, the MAX8688 appears as a pmbus device capable of executing a subset of pmbus commands. a pmbus 1.0-compliant device uses the smbus version 1.1 for transport protocol and responds to the smbus slave address. in this data sheet, the term smbus is used to refer to the electrical characteristics of the pmbus communication using the smbus physical layer. the term pmbus is used to refer to the pmbus command protocol. the MAX8688 employs five standard smbus protocols (write word, read word, write byte, read byte and send byte (see figures 12?5)) to program output voltage and warning/faults thresholds, read monitored data, and pro- vide access to all manufacturer-specific commands. the MAX8688 also supports the group command. the group command is used to send commands to more than one pmbus device. it is not required that all the devices receive the same command. however, no more than one command can be sent to any one device in one group command packet. the group command must not be used with commands that require receiving devices to respond with data, such as the status_byte command. when the MAX8688 receives a command through this protocol, it immediately begins execution of the received command after detecting the stop condition. when the data word is transmitted, the lower order byte is sent first and the higher order byte is sent last. within any byte, the most significant bit (msb) is sent first and the least significant bit (lsb) is sent last. addr slave address command byte: selects which command parameter to write to data byte: data for the command set by the command byte write byte format sw p a a a 7 bits command 8 bits data 8 bits addr slave address command byte: selects which command parameter to read from data byte: data for the command set by the command byte read byte format ssr w p a r a a na 7 bits addr slave address: repeated due to change in data-flow direction 7 bits command 8 bits data 8 bits addr slave address command byte: selects which command parameter to write to data byte: data for the command set by the command byte write word format sw p a a a 7 bits command 8 bits data0 8 bits a data1 8 bits addr slave address command byte: send command with no data send byte format sw s = start condition. sr = repeated start condition. p = stop condition. = slave transmission. p a a 7 bits command 8 bits addr slave address command byte: selects which command parameter to read from data byte: data for the command set by the command byte read word format ssr w p a r a a a 7 bits addr slave address: repeated due to change in data-flow direction 7 bits command 8 bits data0 8 bits na data1 8 bits figure 12. smbus protocols MAX8688 digital power-supply controller/monitor with pmbus interface 22 ______________________________________________________________________________________ addrn slave address of device n command byte for device n: no data byte sr w s = start condition. sr = repeated start condition. p = stop condition. = slave transmission. p a a 7 bits cmdn 8 bits addr1 slave address of device 1 command byte for device 1 data bytes for device 1 sw a a a 7 bits cmd1 8 bits data0 8 bits a data1 8 bits addr2 slave address of device 2 command byte for device 2 data bytes for device 2 sr w a a 7 bits cmd2 8 bits a data 8 bits addr3 slave address of device 3 command byte for device 3 data bytes for device 3 sr w a a a 7 bits cmd3 8 bits data0 8 bits a data1 8 bits figure 13. smbus group command protocol scl sda a a = start condition. b = msb of address clocked into slave. c = lsb of address clocked into slave. d = r/w bit clocked into slave. e = slave pulls sda line low. f = acknowledge bit clocked into master. g = msb of data clocked into slave. h = lsb of data clocked into slave. i = slave pulls sda line low. j = acknowledge clocked into master. k = acknowledge clock pulse. l = stop condition, data executed by slave. m = new start condition. bcd ef ghijklm t su;sta t hd;sta t su;dat t hd;dat t su;sto t buf t low t high figure 14. smbus write timing diagram scl sda a a = start condition. b = msb of address clocked into slave. c = lsb of address clocked into slave. d = r/w bit clocked into slave. e = slave pulls sda line low. f = acknowledge bit clocked into master. g = msb of data clocked into master. h = lsb of data clocked into master. i = master pulls sda line low. j = acknowledge clocked into slave. k = acknowledge clear pulse. l = stop condition. m = new start condition. bcd ef ghijklm t su;sta t hd;sta t su;dat t hd;dat t su;sto t buf t low t high figure 15. smbus read timing diagram MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 23 the MAX8688 smbus interface does not support pack- et error checking (pec). it does not support the 35ms timeout either. therefore, to reset the MAX8688 smbus interface, the host controller has to hold rst low for t smb_rst . this resets the smbus interface. see the rst operation section. pmbus protocol support the MAX8688 supports a subset of the commands defined in the power system management protocol specification part ii - command language revision 1.0. for detailed specifications and the complete list of pmbus commands, refer to part ii of the pmbus specifi- cation available at www.pmbus.org . the supported pmbus commands and the corresponding MAX8688 behavior are described in this document. all data values are represented in direct format, unless otherwise stated. whenever the resolution of the data is less than the number of bits required, data are right justified (only the lower bits are significant) and the higher order bits are zero-padded, unless otherwise stated. for example, for a 2-byte value where the MAX8688 only has 12-bit data to return, the MAX8688 returns data in the lower 12 bits and zero-padded the upper 4 bits. whenever the pmbus specification refers to the ?mbus device,?it is referring to MAX8688 oper- ating in conjuction with a pol device. while the com- mand may call for turning on or off the pmbus device, the MAX8688 always remains on to continue communi- cating with the pmbus master and the MAX8688 trans- fers the command to the pol device accordingly. data format voltage data for commanding or reading the output voltage or related parameters (such as the overvoltage threshold) are presented in direct format. direct format data is a 2-byte, two? complement binary value. direct format data may be used with any command that sends or reads a parametric value. the direct format uses an equation and defined coefficients to cal- culate the desired values. the coefficients used by the MAX8688 can be found in table 4. command code command name transaction type no. of bytes mbr 1 01h operation r/w byte 1 2 03h clear_faults send byte 0 3 11h store_default_all send byte 0 4 12h restore_default_all send byte 0 5 21h vout_command r/w word 2 19995 0 -1 6 25h vout_margin_high r/w word 2 19995 0 -1 7 26h vout_margin_low r/w word 2 19995 0 -1 8 27h vout_transition_rate r/w word 2 256 0 0 9 29h vout_scale_loop r/w word 2 128 0 0 10 38h iout_scale r/w word 2 1 0 1 11 40h vout_ov_fault_limit r/w word 2 19995 0 -1 12 42h vout_ov_warn_limit r/w word 2 19995 0 -1 13 43h vout_uv_warn_limit r/w word 2 19995 0 -1 14 44h vout_uv_fault_limit r/w word 2 19995 0 -1 15 46h iout_oc_fault_limit r/w word 2 23109 0 -2 16 4ah iout_oc_warn_limit r/w word 2 23109 0 -2 17 4fh ot_fault_limit r/w word 2 -7612 335 -3 table 4. MAX8688 pmbus command summary MAX8688 digital power-supply controller/monitor with pmbus interface 24 ______________________________________________________________________________________ the MAX8688 uses direct format for all parameters with the exception of manufacturer-specific commands. refer to the manufacturer command details on data format. interpreting received direct format values the host system uses the following equation to convert the value received from the pmbus device, in this case the MAX8688, into a reading of volts, amperes, degrees celsius or other units as appropriate: where x is the calculated, real world value in the appro- priate units (a, v, ?, etc.); m , the slope coefficient, is a 2-byte, two? complement integer; y is a 2-byte two? complement integer received from the pmbus device; b, the offset, is a 2-byte, two? complement integer; and r, the exponent, is a 1-byte, two? complement integer. x m yb r =? () ? 1 10 command code command name transaction type no. of bytes mbr 18 51h ot_warn_limit r/w word 2 -7612 335 -3 19 60h ton_delay r/w word 2 1 0 1 20 61h ton_rise r/w word 2 1 0 3 21 64h toff_delay r/w word 2 1 0 1 22 65h toff_fall r/w word 2 1 0 3 23 78h status_byte read byte 1 24 8bh read_vout read word 2 19995 0 -1 25 8ch read_iout read word 2 23109 0 -2 26 8dh read_temperature_1 read word 2 -7612 335 -3 27 98h pmbus_revision read byte 1 28 99h mfr_id read word 2 29 9ah mfr_model read word 2 30 9bh mfr_revision read word 2 31 d0h mfr_smb_loopback r/w word 2 32 d1h mfr_mode r/w word 2 33 d2h mfr_vout_correction_rate r/w word 2 34 d3h mfr_sample_rate r/w word 2 35 d4h mfr_vout_peak r/w word 2 19995 0 -1 36 d5h mfr_iout_peak r/w word 2 23109 0 -2 37 d6h mfr_temperature_peak r/w word 2 7612 335 -3 38 d7h mfr_filter_mode r/w word 2 39 d8h mfr_fault_status read word 2 40 d9h mfr_fault_response r/w word 2 41 dah mfr_fault_retry r/w word 2 42 dbh mfr_set_address r/w word 2 43 dch mfr_iout_temp_coeff r/w word 2 6888 0 -5 table 4. MAX8688 pmbus command summary (continued) MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 25 sending a direct format value to send a value, the host must use the equation below to solve for y: y = (mx + b) x 10 r where: y is the 2-byte two? complement integer to be sent to the unit; m , the slope coefficient, is the 2-byte, two? comple- ment integer; x is a real world value, in units such as amperes or volts, to be converted for transmission; b, the offset, is the 2-byte, two? complement integer; and r, the exponent, is the decimal value equivalent to the 1 byte, two? complement integer. the following example demonstrates how the host can send and retrieve values from the MAX8688. from table 4, the coefficients used in the following parameters are: vout_command: m = 19995, b = 0, r = -1 read_vout: m = 19995, b = 0, r = -1 if a host wants to request the pol to output a voltage of 3.0v, the corresponding vout_command value is: y = (mx + b) x 10 r y = (19995 x 3.0 + 0) x 10 -1 = 5998.5 (decimal) = 176eh (hex) conversely, if the host received a value of 176eh on a read_vout command, this is equivalent to: x = 1 (176eh x 10 -(-1) - 0) 19995 = 59980/19995 = 2.999750 which is within 0.0083% of 3.0v. power supplies and power converters generally have no way of knowing how their outputs are connected to ground. within the power supply, all output voltages are most commonly treated as positive. accordingly, all out- put voltages and output voltage-related parameters of pmbus devices are commanded and reported as posi- tive values. it is up to the system to know that a particu- lar output is negative, if that is of interest to the system. all output voltage-related commands use 2 data bytes. fault management and reporting for reporting faults/warnings to the host on a real-time basis, the MAX8688 asserts the open-drain flt pin and sets the appropriate bit in the status_byte and mfr_fault_status registers, respectively. on recognition of the flt assertion, the host or system manager is expected to poll multiple MAX8688s and retrieves fault/warning information. the manufacturer fault status register, mfr_fault_status, provides more detailed information on fault/warning. faults/warn- ings are cleared when any one of the following condi- tions occurs: a clear_faults command is received. enout is commanded through the operation command or a3/onoff to turn off and then turn on. ? rst signal is asserted for longer than t rst where the MAX8688 is internally reset. bias power to the MAX8688 is removed and then reapplied. the MAX8688 responds to fault conditions according to the manufacturer fault response command (mfr_fault_response). this command byte deter- mines how the MAX8688 should respond to each par- ticular fault. in addition, the MAX8688 responds to the following error conditions. 1) if the internal reference fails to operate, flt is asserted. to clear this fault, the MAX8688 has to go through a device reset. 2) the MAX8688 responds to unsupported commands with a nack. 3) when the host sends insufficient data (too few bytes), the MAX8688 sets the cml bit and asserts flt . 4) when the host sends too much data (too many bytes), the MAX8688 sets the cml bit and asserts flt . when a read request is issued to a write-only com- mand, the read operation is aborted and no warning is issued. x m yb r =? () ? 1 10 MAX8688 digital power-supply controller/monitor with pmbus interface 26 ______________________________________________________________________________________ pmbus commands a summary of the pmbus commands supported by the MAX8688 is described in table 4. operation (01h) the operation command is used to turn the pol on and off in conjunction with enout according to the enout polarity select setup. the operation com- mand is also used to cause the pol to set the output voltage to the upper or lower margin voltages. the pol stays in the commanded operating mode until a subse- quent operation command or change in the state of a3/onoff (if enabled) instructs the pol to change to another state. the valid operation command byte values are shown in table 5. the operation command controls how the MAX8688 responds when commanded to change the output. when the command byte is 00h, the MAX8688 turns the pol off immediately and ignores any programmed turn-off delay and fall time. when the command byte is set to 40h, the MAX8688 powers down according to the programmed turn-off delay and fall time. in table 5, act on fault means that if an output overvolt- age warning or output overvoltage fault is detected when the output is margined high, the MAX8688 treats this as a warning or fault and responds as programmed by the warning limit or fault response command. similarly, if an output undervoltage warning or output undervoltage fault is detected when the output is margined low, the MAX8688 treats this as a qualified warning/fault event and responds as programmed by the warning/fault limit or fault response command. any command value not shown in table 5 is an invalid command. if a MAX8688 receives a data byte that is not listed in table 5, then it may treat this as invalid data, declares a communications fault (set cml bit and assert flt ), and responds as described in the fault management and reporting section. the default operation value is 00h. clear_faults (03h) the clear_faults command is used to clear any fault bits that have been set. this command clears all bits in the status_byte and mfr_fault_status registers simultaneously. it also deasserts flt . the clear_faults command does not cause a pol that has latched off for a fault condition to restart. the status of enout under fault conditions is not affected by this command and should change only if command- ed through the operation command or a3/onoff. if the fault is still present after the clear_faults command is executed, the fault bit shall be set again and the host notified by the usual means. this command is write-only. there is no data byte for this command. store_default_all (11h) the store_default_all command instructs the MAX8688 to store configuration information to an exter- nal i 2 c eeprom device attached to a1/scle and a2/sdae. if an error occurs during storing, flt asserts and the cml bit is set to 1. it is permitted to use the store_default_all com- mand while the pol is operating. however, the MAX8688 is unresponsive to pmbus commands while storing the configuration. enout maintains its state. this command is write-only. there is no data byte for this command. for information on eeprom contents, see the external eeprom interface section . restore_default_all (12h) the restore_default_all command instructs the MAX8688 to restore configuration information from an external i 2 c eeprom device attached to a1/scle and a2/sdae. the restore_default_all command can only be executed when the pol is off. otherwise, a communication fault occurs (cml = 1) and flt asserts. if an error occurs during restoration, flt asserts and the cml bit is set to 1. the status_byte and mfr_fault_status values are not restored by the restore_default_all command. this command is write-only. there is no data byte for this command. for information on eeprom contents, see the external eeprom interface section. command byte pol on or off margin state 00h immediate off (no sequencing) 40h soft-off (with sequencing) 80h on margin off (nominal) 98h on margin low (act on fault) a8h on margin high (act on fault) table 5. operation command byte MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 27 vout_command (21h) the vout_command command loads the MAX8688 with the voltage to which the pol output is to be changed when it is commanded to turn on using the operation command or a3/onoff transition when enabled. once the pol is turned on, changing the vout_command has no effect on the pol output voltage. the MAX8688 only adjusts the pol to the new vout_command voltage after receiving a new turn- on command. the 2 data bytes are in direct format. valid values are from 0 to 2.048v when the input range is 2.048v, and 0 to 5.5v when the input range is 5.5v. the default vout_command value is 00h. vout_margin_high (25h) the vout_margin_high command loads the MAX8688 with the voltage to which the pol output is to be changed when the operation command is set to margin high. if the pol is already operating at margin high, changing vout_margin_high has no effect on the output voltage. the MAX8688 only adjusts the pol to the new vout_margin_high voltage after receiv- ing a new margin high operation command. the 2 data bytes are in direct format. valid values are the same as vout_command. the default vout_margin_high value is 00h. vout_margin_low (26h) the vout_margin_low command loads the MAX8688 with the voltage to which the pol output is to be changed when the operation command is set to margin low. if the pol is already operating at margin low, changing vout_margin_low has no effect on the output voltage. the MAX8688 only adjusts the pol to the new vout_margin_low voltage after receiv- ing a new margin low operation command. the 2 data bytes are in direct format. valid values are the same as vout_command. the default vout_margin_low value is 00h. vout_transition_rate (27h) the vout_transition_rate command sets the rate in mv/? at which the pol output voltage should change when the pol is commanded to change between the margin high, margin low, and margin off (on) opera- tion mode. this commanded rate of change does not apply when the pol is commanded to turn on or turn off. in that case, t on_rise and t off_fall applies. the 2 data bytes are in direct format. valid values are from 0 to 128mv/?. the default vout_transition_rate value is 0mv/?. when vout_transition_rate is cleared to 0, vout_transition_rate is ignored and the voltage out- put is not changed even when commanded by the oper- ation commands. when vout_transition_rate is set to 07fffh, the voltage output is adjusted as quickly as possible. if a vout_transition_rate parameter results in the dac outputting an out of range value (valid range 0 to 4090) during transition, the cml flag is set and flt is asserted to issue a warning. the vout_transition_rate command applies to refin mode only and is ignored in feedback mode. vout_scale_loop (29h) in a typical application, the output voltage of a power converter is sensed through a resistive voltage-divider, as illustrated in figure 16. the resistive voltage-divider reduces or scales the output voltage, v out . the pmbus commands specify the actual pol output voltages and not the input voltage to the control circuit. to allow the MAX8688 to map between the command- ed voltage (such as 3.3v), and the voltage at the con- trol circuit input (perhaps 3.3v divided down to match a reference voltage of 2.0v), the vout_scale_loop command is used. the 2 data bytes are in direct format. valid values are from 0 to 1.0. note that due to m, b, r representation restriction, the supplied value is rounded off to multi- ples of 1/128. therefore, to ensure optimum operation, circuit design should choose a value as close to multi- ples of 1/128 as possible to avoid rounding errors and thus ensure the final accuracy of v out . this value is dimensionless. the default vout_scale_loop value is 00h. the vout_scale_loop command is ignored in feed- back mode. vout scale loop r rr __ = + 2 12 MAX8688 rs- rs+ pol vo- vo+ load r2 r1 figure 16. vout_scale_loop MAX8688 digital power-supply controller/monitor with pmbus interface 28 ______________________________________________________________________________________ iout_scale (38h) the iout_scale command is used to set the ratio of the voltage at the current-sense pins to the sensed cur- rent. for devices using a fixed current-sense resistor, it is the same value as the resistance. the unit of the iout_scale factor is m ? . the 2 data bytes are in direct format. valid values are from 0.5m ? to 5m ? . the default iout_scale value is 0m ? . vout_ov_fault_limit (40h) the vout_ov_fault_limit command sets the value of the output voltage measured across rs+ and rs- that causes an output overvoltage fault. the 2 data bytes are in direct format. valid values are the same as vout_command. the default vout_ov_fault_limit value is 00h. in response to the vout_ov_fault_limit being exceeded, the MAX8688: 1) sets the vout_ov bit in the status_byte. 2) sets the vout_ov_fault bit in the mfr_fault_status register. 3) responds as specified by vout_ov_fault_ response bits in the mfr_fault_response register. 4) notifies the host through flt assertion. vout_ov_warn_limit (42h) the vout_ov_warn_limit command sets the value of the output voltage measured across rs+ and rs-, which causes an output-voltage high warning. this value is typically less than the output overvoltage threshold in vout_ov_fault_limit. the 2 data bytes are in direct format. valid values are the same as the vout_command. the default vout_ov_warn_limit value is 00h. in response to the vout_ov_warn_limit being exceeded, the MAX8688: 1) sets the other bit in the status_byte. 2) sets the vout_ov_warn bit in the mfr_fault_status register. 3) notifies the host through flt assertion. vout_uv_warn_limit (43h) the vout_uv_warn_limit command sets the value of the output voltage measured across rs+ and rs-, which causes an output-voltage low warning. this value is typically greater than the output undervoltage fault threshold in vout_uv_fault_limit. this warning is masked until the output voltage reaches the programmed voltage at startup, and also during turn-off when the pol is disabled. the 2 data bytes are in direct format. valid values are the same as vout_command. the default vout_uv_warn_limit value is 00h. in response to violation of the vout_uv_warn_limit, the MAX8688: 1) sets the other bit in the status_byte. 2) sets the vout_uv_warn bit in the mfr_fault_status register. 3) notifies the host using flt assertion. vout_uv_fault_limit (44h) the vout_uv_fault_limit command sets the value of the output voltage measured across rs+ and rs-, which causes an output undervoltage fault. this fault is masked until the output voltage reaches the programmed voltage at startup, and also during turn-off when the pol is disabled. the 2 data bytes are in direct format. valid values are the same as vout_command. the default vout_uv_fault_limit value is 00h. in response to violation of the vout_uv_fault_limit, the MAX8688: 1) sets the other bit in the status_byte. 2) sets the vout_uv_fault bit in the mfr_fault_status register. 3) responds as specified by vout_uv_fault_ response bits in the mfr_fault_response register. 4) notifies the host using flt assertion. iout_oc_fault_limit (46h) the iout_oc_fault_limit command sets the value of the output current, in amperes, measured across isn+ and isn- that causes the overcurrent detector to indicate an overcurrent fault condition. the 2 data bytes are in direct format. valid values are from 0 to 50/iout_scale amperes. the default iout_oc_fault_limit value is 00h. in response to the iout_oc_fault_limit being exceeded, the MAX8688: 1) sets the iout_oc bit in the status_byte. 2) sets the iout_oc_fault bit in the mfr_fault_status register. MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 29 3) responds as specified by iout_oc_fault_ response bits in the mfr_fault_response register. 4) notifies the host using flt assertion. iout_oc_warn_limit (4ah) the iout_oc_warn_limit command sets the value of the output current, in amperes, measured across isn+ and isn- that causes an output overcurrent warning. the 2 data bytes are in direct format. valid values are the same as the iout_oc_fault_limit. the default iout_oc_warn_limit value is 00h. in response to the iout_oc_warn_limit being exceeded, the MAX8688: 1) sets the other bit in the status_byte. 2) sets the iout_oc_warn bit in the mfr_fault_status register. 3) notifies the host using flt assertion. ot_fault_limit (4fh) the ot_fault_limit command sets the temperature, in degrees celsius, of the on-chip temperature sensor at which an overtemperature fault is detected. the 2 data bytes are in direct format. valid values are from -142.5? to +395.4?. the default ot_fault_limit value is 00h. in response to the ot_fault_limit being exceeded, the MAX8688: sets the temperature bit in the status_byte. sets the ot_fault bit in the mfr_fault_status register. responds as specified by ot_fault_response bits in the mfr_fault_response register. notifies the host using flt assertion. ot_warn_limit (51h) the ot_warn_limit command sets the temperature, in degrees celsius, of the on-chip temperature sensor at which an overtemperature warning is detected. the 2 data bytes are in direct format. valid values are the same as the ot_fault_limit. the default ot_warn_limit value is 00h. in response to the ot_warn_limit being exceeded, the MAX8688: sets the temperature bit in the status_byte. sets the ot_warn bit in the mfr_fault_status register. notifies the host through flt using assertion. ton_delay (60h) ton_delay sets the time, in milliseconds, from when a start condition is received (a valid operation com- mand or through a3/onoff when enabled) until the pol output voltage starts to rise. during ton_delay, the pol is disabled (enout deasserted) until ton_delay expires. also, the undervoltage fault and warning are masked off during ton_delay. the 2 data bytes are in direct format. valid values are from 0 to 3276.7ms. the default ton_delay value is 0ms. ton_rise (61h) the ton_rise sets the time, in milliseconds, from when the pol output voltage starts to rise until the volt- age has entered the regulation band. during ton_rise, the voltage, current, and temperature-relat- ed faults and warnings are masked off. meanwhile, the MAX8688 still responds to the pmbus command. if a ton_rise parameter results in the dac outputting an out-of-range value (valid range 0 to 4090) immediately during ton_rise, the cml flag is set and flt asserts to issue a warning. in feedback mode, the MAX8688 leaves the s1 switch on the dac open during soft-start. it waits for the ton_rise time to expire before adjusting the dac out- put to equal the dacout feedback and then closes s1. the 2 data bytes are in direct format. in refin mode, the ton_rise parameter is a 16-bit value and the valid values are from 0.01ms to 32.767ms. in fb mode, the ton_rise is a 14-bit value and the valid values are from 0.01ms to 16.383ms (the upper 2 bits are ignored). the default ton_rise value is 0.01ms. setting any value less than this minimum value defaults back to 0.01ms. toff_delay (64h) the toff_delay sets the time, in milliseconds, from when a stop condition is received (a soft-off operation command or through a3/onoff when enabled) until the pol stops transferring energy to the output. when commanded to turn off immediately through the operation command, the toff_delay value is ignored. when commanded to turn off through a3/onoff (when enabled), the toff_delay parame- ter is used. the 2 data bytes are in direct format. valid values are from 0 to 3276.7ms. the default toff_delay value is 0ms. MAX8688 digital power-supply controller/monitor with pmbus interface 30 ______________________________________________________________________________________ toff_fall (65h) the toff_fall command sets the time, in millisec- onds, from the end of the turn-off delay time until the output voltage is commanded to zero. note that this command can only be used with a device whose out- put can sink enough current to cause the output volt- age to decrease at a controlled rate. when commanded to turn off immediately through the operation command, the toff_fall value is ignored. when commanded to turn off through the operation soft-off command or a3/onoff (when enabled), the toff_fall parameter is used. if a toff_fall parameter results in the dac outputting an out-of-range value (valid range 0 to 4090) immediately during toff_fall, the cml flag is set and flt asserts to issue a warning. in feedback mode, this value is not used. the MAX8688 disables enout and opens the dac switch immediately after the toff_delay. the 2 data bytes are in direct format. valid values are from 0.01ms to 32.767ms. the default toff_fall value is 0.01ms. setting any value less than this minimum value defaults back to 0.01ms. status_byte (78h) the status_byte command returns 1 byte of informa- tion with a summary of the most critical faults. in the MAX8688, status information is binary. a value of 1 indi- cates that a fault or warning has occurred and a 0 indi- cates otherwise. bits for unsupported features shall be reported as 0. the status_byte cannot be restored by restore_default_all command. the status_byte message content is described in table 6. this command is read only. the default status_byte value is 40h (pol is off). read_vout (8bh) the read_vout command returns the actual mea- sured (not commanded) output voltage across rs+ and rs-. if filter mode is enabled, the filtered value is returned. the 2 data bytes are in direct format. valid values are the same as vout_command. the default read_vout value is 00h. read_iout (8ch) the read_iout command returns the measured out- put current in amperes across isn+ and isn-. the 2 data bytes are in direct format. valid values are the same as iout_oc_fault_limit. the default read_iout value is 00h. read_temperature_1 (8dh) the MAX8688 supports only one temperature reading, read_temperature_1. the MAX8688 returns the actual on-chip measured temperature in degrees celsius. the 2 data bytes are in direct format. valid values are the same as ot_fault_limit. the default read_temperature_1 value is 00h. bit number status bit name meaning 7 busy a fault was declared because the device was busy and unable to respond. (the MAX8688 does not support the busy bit. this bit always returns 0). 6 off this bit is asserted if enout is presently disabling the pol, regardless of the reason, including simply not being enabled. 5 vout_ov an output overvoltage fault has occurred. 4 iout_oc an output overcurrent fault has occurred. 3 vin_uv an input undervoltage fault has occurred. (the MAX8688 does not support the vin_uv bit. this bit always returns 0). 2 temperature a temperature fault or warning has occurred. 1 cml a communication, memory, or logic fault has occurred. 0 other a fault or warning not listed in bits [7:1] has occurred. see the mfr_fault_status (d8h) section for more information. table 6. status_byte message contents MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 31 pmbus_revision (98h) the pmbus_revision command returns the revision of the pmbus specification to which the MAX8688 is compliant. the command has 1 data byte. bits [7:5] indicate the revision of pmbus specification part i to which the MAX8688 is compliant. bits [4:0] indicate the revision of pmbus specification part ii to which the MAX8688 is compliant. the permissible values are shown in table 7. this command is read only. the default pmbus_revision value is 00h which indi- cates that the MAX8688 is compliant with part i rev 1.0 and part ii rev 1.0. mfr_id (99h) the mfr_id command returns the MAX8688 manufac- turer? identification. the default mfr_id value is 4d01h. this command is read only. mfr_model (9ah) the mfr_model command returns the MAX8688 model number. the default mfr_model value is 4101h. this command is read only. mfr_revision (9bh) the mfr_revision command reads the ascii charac- ters that contain the MAX8688 revision number with a block read command. the default mfr_revision value is 3201h. this command is read only. mfr_smb_loopback (d0h) the mfr_smb_loopback command returns the data word previously received by the MAX8688. the smbus master writes a data word to the MAX8688 using this command and then retrieves the data word. a valid communication channel is established if the master reads back the same word. note that if another command is sent in between the write mfr_smb_loopback command and the read mfr_smb_loopback command, the MAX8688 returns whatever last command data word it receives. mfr_mode (d1h) the mfr_mode command is used to configure the MAX8688 to support manufacturer specific commands. the mfr_mode command is described in table 8. the default mfr_mode value is 00h. bits [7:5] part i revision bits [4:0] part ii revision 000 1.0 00000 1.0 table 7. pmbus revision data byte contents bit bit name description 15:8 input clock time factor this is equivalent to the number of external clock cycles provided to clkio in 100? - 2. mfr_mode[15:8] = f ext_clk /10khz - 2 where f ext_clk is the frequency of the external clock. for example, when f ext_clk = 1mhz, f ext_clk /10khz = 100, mfr_mode[15:8] = 100 ?2 = 98. valid external input clock range is from 100khz (mfr_mode[15:8] = 8) to 2.5mhz (mfr_mode[15:8] = 248). these bits are ignored if the internal clock source is selected as the time base (clock source select bit = 0) 7 clock out enable the clock out enable bit allows the output of a 1mhz reference clock to clkio for synchronizing multiple MAX8688s. setting this bit to 1 enables the 1mhz output on clkio. when this bit is cleared to 0, no reference clock is outputted. 6 a3 control enable setting this bit to 1 enables a3/onoff to function as a pol on/off input control. clearing this bit to 0 ignores the a3/onoff state and the MAX8688 is controlled by the operation command alone. see the a3/onoff operation section. table 8. mfr_mode bit definition MAX8688 digital power-supply controller/monitor with pmbus interface 32 ______________________________________________________________________________________ mfr_vout_correction_rate (d2h) the mfr_vout_correction_rate command sets the frequency (hz) at which the MAX8688 adjusts 1 lsb of dacout (0.5mv) after the voltage has entered the regulation band. correction rate = 10khz/mfr_vout_correction_rate the 2 data bytes are formatted as positive integers. valid values are from 1 to 65535. setting this value to 0 dis- ables the dacout adjustment in the regulation band. the default mfr_vout_correction_rate value is 10 which is equivalent to a correction rate of 1khz. mfr_sample_rate (d3h) the mfr_sample_rate command sets the frequency (hz) at which the pol output voltage, output current, and temperature fault/warning conditions are monitored. monitor sample rate =10khz/mfr_sample_rate the 2 data bytes are formatted as positive integers. valid values are from 1 to 65535. setting this value to 0 disables all fault/warning monitoring. the default mfr_sample_rate value is 50 which is equivalent to a sample rate of 200hz. bit bit name description 5 eeprom lock enable the eeprom lock enable bit is used to protect external eeprom data from being overwritten. when this bit is set to 1, the store_default_all command is ignored. the restore_default_all command is still valid. when this bit is cleared to 0, the store_default_all command initiates a store configuration operation to the eeprom attached to a1/scle and a2/sdae. 4 correction bypass enable correction bypass enable. setting this bit to 1 disables a correction algorithm made to voltage, current, and temperature readings. clearing this bit to 0 applies a correction algorithm to voltage, current, and temperature measurement, thus resulting in high-accuracy readings. for optimal operation, this bit should be cleared to 0. 3 input range select the input range select bit determines the input range of rs+ and rs-. setting this bit to 1 extends the input range to 5.5v. clearing this bit to 0 sets the input range to 2.048v. prior to setting any voltage-related values, the user application must first configure the desired input range. all voltage-related commands use the selected input range to convert the commanded value into internal register values. it is not recommended to change the input range selection while the pol is operating, since all voltage-related commands continue to refer to the input range that was in use when the commanded voltage was received. this results in unpredictable and catastrophic operation. 2 enout polarity select the enout polarity select bit selects the enout active-on polarity. setting this bit to 1 configures the enout asserted on-state the same as the default startup state. clearing this bit to 0 configures the enout deasserted off-state the same as the default startup state. in effect, writing a 1 to this bit means that the asserted state of enout is the default startup state. see the enout operation section. 1 feedback mode select the feedback mode select bit determines the MAX8688 operation mode. when this bit is set to 1, the MAX8688 operates in the feedback mode while when cleared to 0, the MAX8688 operates in the refin mode. 0 clock source select the clock source select bit determines the MAX8688 reference clock time source. when the clock source select bit is set to 1, an external clock must be supplied to clkio and is used as the MAX8688 reference clock. when this bit is cleared to 0, an internal clock is used. table 8. mfr_mode bit definition (continued) MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 33 mfr_vout_peak (d4h) the mfr_vout_peak command returns the maximum actual measured (not commanded) output voltage in volts across rs+ and rs-. if the filter mode is enabled, instead of returning the instantaneous value, the filtered output voltage is returned. to reset this value to 0, write to this command with a data value of 0. any other val- ues written by this command are used as a comparison for future peak updates. the 2 data bytes are in direct format. valid values are the same as vout_command. the default mfr_vout_peak value is 0. mfr_iout_peak (d5h) the mfr_iout_peak command returns the maximum measured output current in amperes across isn+ and isn-. to reset this value to 0, write to this command with a data value of 0. any other values written by this com- mand are used as a comparison for future peak updates. the 2 data bytes are in direct format. valid values are the same as iout_oc_fault_limit. the default mfr_iout_peak value is 00h. mfr_temperature_peak (d6h) the mfr_temperature_peak command returns the maximum actual on-chip measured temperature in degrees celsius. to reset this value to its lowest value, write to this command with a data value of 0ffffh. any other values written by this command are used as a comparison for future peak updates. the 2 data bytes are in direct format. valid values are the same as ot_fault_limit. the default mfr_temperature_peak value is 00h. mfr_filter_mode (d7h) the mfr_filter_mode command is used to set v out sample count and sample delays. whenever filtering is enabled (mfr_filter_mode[7:0] not equal to 0), voltage-related readings are computed as an average over the sample count. fault and warning limits are compared against the averaged value to determine if a fault/warning has occurred. the sample delay bit value (mfr_filter_mode[15:8]) sets the time between successive voltage readings. note that when filtering is disabled (mfr_filter_mode[7:0] = 0), the sample delay bit value is ignored. the default mfr_filter_mode value is 00h (1 sample count bit and no delay). when filtering is in progress, the pmbus command is ignored. mfr_fault_status (d8h) when a warning or fault condition is detected, the MAX8688 sets the corresponding bit in the mfr_fault_status register to 1 and notifies the host using flt assertion. the status_byte is computed from mfr_fault_status[7:0]. this register is cleared to 0 together with the status_byte register by any of the fault/warning clear- ing methods mentioned earlier in the clear_faults command. bits bit name description 15:8 sample delay the upper byte defines the time interval between each sample. the sampling delay is 1.75? + mfr_filter_mode[15:8] x 250ns. 7:0 sample count the lower byte defines the number of samples to acquire in each monitoring sequence. the number of samples is determined by 2^ mfr_filter_mode[7:0]. table 9. mfr_filter_mode bit definition bit fault/warning bit name 15 reserved. read returns 0. 14 ot_warn 13 ot_fault 12 iout_oc_warn 11 iout_nc_fault 10 vout_uv_warn 9 vout_uv_fault 8 vout_ov_warn 7 reserved. read returns 0. 6 off 5 vout_ov_fault 4 iout_oc_fault 3 reserved. read returns 0. 2 temperature. set when either ot_warn or ot_fault is set. 1 cml 0 other. set when any bit (other than those temperature related bits) in the high byte is set. table 10. mfr_fault_status bit definition MAX8688 digital power-supply controller/monitor with pmbus interface 34 ______________________________________________________________________________________ iout_nc_fault is set to 1 when a negative current flow is detected. the mfr_fault_status command value cannot be restored by the restore_default_all command. this command is read only. mfr_fault_response (d9h) the mfr_fault_response command specifies the response to each fault condition supported by the MAX8688. each fault has 2 response bits that describe how the MAX8688 should respond to that particular fault. it is also used to record the condition under which a fault occurs. the store_peak function is used to store peak values (mfr_vout_peak, mfr_iout_peak, and mfr_tem- perature_peak) to the eeprom (if present) on a fault detection regardless of the status of the eeprom lock enable bit. setting the store_peak bit to 1 enables the store function. table 12 describes how the MAX8688 responds to fault conditions. if the fault response requires enout to be deasserted, enout deasserts immediately. meanwhile the dac output ramps down slowly. the fault bit is cleared according to the fault management and reporting section . if enout is deasserted due to a fault condition, the pol shall remain off until instructed to change. the default mfr_fault_response value is 00h. mfr_fault_retry (dah) the mfr_fault_retry command sets the time between restarting the pol if the fault response is to restart the pol at specified intervals. this command sets the retry time delay in multiples of 100?. this command value is used for all fault responses that require delay retry. delay retry time = mfr_fault_retry[15:0] x 100? the 2 data bytes are in direct format. valid values are from 0 to 3.2768s. when mfr_fault_retry = 00h, the MAX8688 restarts the pol at the next available time period. the default mfr_fault_retry value is 00h. mfr_set_address (dbh) the mfr_set_address command is used to change the MAX8688 slave address. by default the MAX8688 address is set by a3:a1 upon reset according to table 3. after reset, the slave address can be changed by the mfr_set_address command. see the MAX8688 address assignment section. this command has 2 data bytes. the slave address is contained in bits [6:0] of the first data byte. bit 7 of the first data byte must be 0. bits fault response bit name 15 store_peak 14:10 reserved 9:8 nc_fault_response[1:0] 7:6 ot_fault_response[1:0] 5:4 iout_oc_fault_response[1:0] 3:2 vout_uv_fault_response[1:0] 1:0 vout_ov_fault_response[1:0] table 11. mfr_fault_response bit definition response [1:0] fault response option 11 reserved. same response as 00 except this option also stores peak data to eeprom if enabled. 10 set the corresponding fault bit in the fault status register, assert flt , shutdown the pol (deasserted enout) and restart the pol every t (?), where t is set in the mfr_fault_retry register. store peak data to eeprom if enabled. 01 set the corresponding fault bit in the fault status register, assert flt and shutdown the pol. store peak data to eeprom if enabled. 00 set the corresponding fault bit in the fault status register, assert flt and continue operation without any action. table 12. fault response options MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 35 mfr_iout_temp_coeff (dch) the mfr_iout_temp_coeff command sets the tem- perature coefficient of current-sense resistor, r sense , in ppm/?. the voltage readings across the current-sense resistor are temperature compensated as followed: temperature compensation = mfr_iout_temp_coeff[15:0] x 14.518ppm/? in practice, it should be noted that the temperature measured by the MAX8688 may be different from that of the current-sense element being used due to differ- ent physical locations on the board. this must be taken into account while programming the MAX8688 for the current-sense element temperature coefficient. characterization of the board may be required to establish the required ppm number to be programmed. the 2 data bytes are in direct format. valid values are from 0 to 4000ppm/?. the default mfr_iout_temp_coeff value is 00h (0ppm/?). performance consideration the MAX8688 can be viewed as a task scheduler where it periodically goes through its task list and per- forms the required tasks. while it may be tempting to monitor the pol at the highest supported frequency, mfr_sample_rate, it must be noted that doing so takes away resources from other tasks, such as com- munication with the system controller or adjusting the dacout voltage. the same is true with employing an increasing number of mfr_sample_rate and mfr_filter_mode parameters. since each applica- tion is unique in its own merit, it is impossible to pre- scribe a solution that suffices in all conditions. system designers are thus urged to derive optimum configura- tion based on the particular system needs. graphic user interface (gui) the MAX8688 evaluation kit comes with a free gui that eliminates the need for any software development and provides a simple and user-friendly method for configur- ing large systems in a short time. once the configuration is complete, the results can be saved to the external eeprom for MAX8688 configuration on power-up, or loaded at power-up onto the MAX8688 through the pmbus by a master controller. the powerful MAX8688 feature set can be inferred from the gui screenshot shown in figure 17, where the programmable parame- ters are displayed. these parameter values set serve as data bytes for pmbus and manufacturer-specific com- mands supported by the MAX8688. for details on using the gui, refer to the MAX8688 evaluation kit. pcb considerations pcb layout for the MAX8688 is simple. it is easy to achieve accurate voltage and current measurement and voltage regulation by following these guidelines for good pcb layout: 1) place ic decoupling and filter capacitors for avdd, dvdd, refo, rs_c, and isn_c as close to the ic pins as possible. if using an external eeprom, place it close to the MAX8688 and use short direct traces for interconnections. 2) use kelvin connections for the traces from the isn+ and isn- to the current-sense resistor for accurate current sensing. in the case of dcr sensing (see the current sensing section), connect the rc filter across the pol output inductor terminals using kelvin connections. route differential pair traces from the capacitor of the rc filter to isn+ and isn-. 3) use kelvin connections for the differential pair traces from the desired remote-sense points on the pol output-voltage power plane to rs+ and rs- of the MAX8688 for accurate pol output-voltage sensing. 4) connect the analog ground (agnd) and digital ground (dgnd) of the MAX8688 to a ground plane right at the ic. terminate all other ground connec- tions to this ground plane. connect this ground plane to the quiet analog ground plane of the pol so that the reference voltage to the pol is unaffect- ed by switching noise. use a single-point (star) grounding technique to connect the analog ground plane of the pol to the heat dissipating power ground plane of the pol. place the MAX8688 as close as possible to the pol for best temperature measurement performance. 5) refer to the MAX8688 evaluation kit for sample layout. MAX8688 digital power-supply controller/monitor with pmbus interface 36 ______________________________________________________________________________________ figure 17. MAX8688 graphic user interface 23 24 22 21 8 + 7 9 dgnd clkio rst sda 10 dvdd avdd dacout a1/scle refo a2/sdae 12 isn- 456 17 18 16 14 13 isn+ isn_c enout n.c. n.c. scl MAX8688 dvdd agnd 3 15 rs+ 20 11 flt rs- 19 12 a3/onoff rs_c thin qfn top view pin configuration MAX8688 digital power-supply controller/monitor with pmbus interface ______________________________________________________________________________________ 37 24l qfn thin.eps package information (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation go to www.maxim-ic.com/packages .) MAX8688 digital power-supply controller/monitor with pmbus interface maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. 38 ____________________maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 2008 maxim integrated products is a registered trademark of maxim integrated products, inc. package information (continued) (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline info rmation go to www.maxim-ic.com/packages .) |
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