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general description the max1973/max1974 are constant-frequency1.4mhz pulse-width-modulated (pwm) current-mode step-down regulators. the output voltage can be set as low as 0.75v using an external voltage-divider, or it can be set to preset outputs of 1v, 1.5v (max1974), 1.8v, or 2.5v (max1973) without requiring external resistors. the max1973 also includes a voltage-margining fea- ture that offsets the output voltage up or down 4% to facilitate board-level production testing. a fixed 1.4mhz operating frequency ensures operation outside the dsl frequency band, provides fast transient response, and allows the use of small external compo- nents. only 4.7? input and output ceramic capacitors are needed for 1a applications. forced pwm operation ensures a constant switching frequency over all load conditions. output voltage accuracy is ?% over load, line, and temperature operating ranges. the max1973 features voltage margining; the max1974 provides a pok out- put to indicate when the output has reached 90% of its nominal regulation voltage. both devices are available in small 10-pin ?ax packages. applications network equipmentcellular base stations dsl and wireless modems/routers optical modules central-office dsl and telecom dsp/asic core and io supplies features ? tiny circuit footprint of 0.19in 2 ? ultra-low circuit height of 1.8mm ? 4.7? ceramic input and output capacitors ? 2.6v to 5.5v input voltage ? 1a output current ? 1% accurate ? built-in ?% logic-controlled voltage margining(max1973) ? preset 1v, 1.5v, 1.8v, 2.5v, or 0.75v to v in adjustable output ? fixed-frequency pwm current-mode operation ? 1.4mhz switching frequency, operate outsidedsl band ? 100% duty-cycle dropout capability ? small external components max1973/max1974 smallest 1a, 1.4mhz step-down regulators ________________________________________________________________ maxim integrated products 1 1 23 4 5 10 98 7 6 ctl1in lx pgnd ss fb comp fbsel max1973 max top view ctl2 gnd 1 23 4 5 10 98 7 6 onin lx pgnd ss fb comp fbsel max1974 max pok gnd pin configurations ordering information max1973 in comp fbsel ss gnd lx fb ctl1 ctl2 pgnd input2.6v to 5.5v max1974 output down to 0.75v voltagemargining on/off output1.25v to v in 1a typical operating circuit 19-2547; rev 0; 7/02 for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. part temp range pin-package max1973 eub -40 c to +85 c 10 ?ax max1974 eub -40 c to +85 c 10 ?ax selector guide appears at end of data sheet. downloaded from: http:///
max1973/max1974 smallest 1a, 1.4mhz step-down regulators 2 _______________________________________________________________________________________ absolute maximum ratings stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. in, pok, ctl1, ctl2, fbsel, on to gnd ..............-0.3v to +6v comp, fb, ss to gnd ................................-0.3v to (v in + 0.3v) pgnd to gnd .......................................................-0.3v to +0.3v lx current (note 1) ...............................................-2.4a to +2.4a continuous power dissipation (t a = +70?) 10-pin ?ax (derate 5.6mw/? above +70?) .......... 444mw operating temperature range ...........................-40? to +85? storage temperature range .............................-65? to +150? lead temperature (soldering, 10s) .................................+300? electrical characteristics(v in = v ctl_ = 3.3v, fb = out, t a = 0? to +85? , unless otherwise noted. typical values are at t a = +25?.) parameter conditions min typ max units in in voltage range 2.6 5.5 v in undervoltage lockoutthreshold rising and falling, hysteresis is 25mv (typ) 2.2 2.35 2.5 v supply current switching with no load 4.0 7.5 ma supply current in dropout v out set for 3.6v 3 5 ma shutdown supply current v in = 5.5v 0.1 10 ? fb max1973 1.25 v in output voltage range max1974 0.75 v in v fbsel not connected 1.2375 1.25 1.2625 fbsel = gnd 1.7820 1.8 1.8180 max1973 fbsel = in 2.4750 2.5 2.5250 fbsel not connected 0.7425 0.75 0.7575 fbsel = gnd 0.99 1.00 1.01 fb regulation voltage max1974 fbsel = in 1.485 1.500 1.515 v fb regulation voltagepositive voltage margining max1973, ctl1 = gnd, ctl2 = in +3 +4 +5 % fb regulation voltagenegative voltage margining max1973, ctl1 = in, ctl2 = gnd -3 -4 -5 % fb input resistance to gnd inpreset output modes 10 30 70 k fb input bias current fbsel not connected -0.1 0.01 +0.1 ? ss (reference output) max1974 0.75 ss voltage max1973 1.25 v note 1: lx has internal clamp diodes to in and pgnd. applications that forward bias these diodes should take care not to exceedthe ic package power dissipation limit. downloaded from: http:///
max1973/max1974 smallest 1a, 1.4mhz step-down regulators _______________________________________________________________________________________ 3 electrical characteristics (continued)(v in = v ctl_ = 3.3v, fb = out, t a = 0 c to +85 ? , unless otherwise noted. typical values are at t a = +25 c.) parameter conditions min typ max units ss source current -25 -20 -15 ? ss sink current 10 20 35 ? ss to gnd resistance inshutdown 54 0 100 fbsel low input threshold 0.3 v high input threshold v in - 0.3 v input bias current fbsel = gnd or in, v in = 5.5v -20 10 +20 ? comp transconductance from fb tocomp 40 60 80 ? comp to gnd resistance inshutdown 54 0 100 clamp voltage low 0.6 0.9 1.2 v clamp voltage high 1.35 1.75 2.15 v lx on-resistance high v in = 3.3v 0.23 0.46 on-resistance low v in = 3.3v 0.16 0.32 current-sense transresistance 0.275 0.335 0.425 v/a positive current-limit threshold 1.1 1.6 1.75 a negative current-limit threshold -1.2 -0.8 -0.4 a v lx = v in = 5.5v 20 lx shutdown leakage current lx = gnd, v in = 5.5v -20 ? switching frequency 1.2 1.4 1.6 mhz ctl1, ctl2 (max1973), on (max1974) logic-low input threshold 0.6 v logic-high input threshold 1.6 v logic input current -1 +1 ? pok (max1974 only) output low voltage pok sinking 1ma 10 100 mv rising 90 92.5 95 output valid threshold for pok percentage of nominalregulation voltage falling 88 90 92 % downloaded from: http:///
max1973/max1974 smallest 1a, 1.4mhz step-down regulators 4 _______________________________________________________________________________________ electrical characteristics (continued)(v in = v ctl_ = 3.3v, fb = out, t a = 0 c to +85 ? , unless otherwise noted. typical values are at t a = +25 c.) parameter conditions min typ max units thermal shutdown thermal-shutdown threshold +170 ? thermal-shutdown hysteresis 20 ? electrical characteristics(v in = v fb = v ctl_ = 3.3v, t a = -40 c to +85 c , unless otherwise noted.) (note 2) parameter conditions min typ max units in in voltage range 2.6 5.5 v in undervoltage lockoutthreshold rising and falling, hysteresis is 25mv (typ) 2.2 2.5 v supply current switching with no load 7.5 ma supply current in dropout v out set for 3.6v 5 ma shutdown supply current v in = 5.5v 10 ? fb max1973 1.25 v in output voltage range max1974 0.75 v in v fbsel not connected 1.2375 1.2625 fbsel = gnd 1.7820 1.8180 max1973 fbsel = in 2.4750 2.5250 fbsel not connected 0.7425 0.7575 fbsel = gnd 0.99 1.01 fb regulation voltage max1974 fbsel = in 1.485 1.515 v fb regulation voltagepositive voltage margining max1973, ctl1 = gnd, ctl2 = in 3 5 % fb regulation voltagenegative voltage margining max1973, ctl1 = in, ctl2 = gnd -3 -5 % fb input resistance to gnd inpreset output modes 10 70 k fb input bias current fbsel not connected -0.15 +0.15 ? ss (reference output) ss source current -25 -15 ? ss sink current 10 35 ? downloaded from: http:///
max1973/max1974 smallest 1a, 1.4mhz step-down regulators _______________________________________________________________________________________ 5 electrical characteristics(v in = v ctl_ = 3.3v, fb = out, t a = -40 c to +85 ? , unless otherwise noted.) (note 2) parameter conditions min typ max units ss to gnd resistancein shutdown 5 40 100 fbsel low input threshold 0.3 v high input threshold v in - 0.4 v input bias current fbsel = gnd or in, v in = 5.5v -20 +20 ? comp transconductancefrom fb to comp 40 80 ? comp to gnd resistancein shutdown 5 100 clamp voltage low 0.6 1.2 v clamp voltage high 1.3 2.2 v lx on-resistance high v in = 3.3v 0.46 on-resistance low v in = 3.3v 0.32 current-sense transresistance 0.275 0.425 v/a positive current-limit threshold 1.10 1.85 a negative current-limit threshold -1.20 -0.35 a v lx = v in = 5.5v 20 lx shutdown leakage current lx = gnd, v in = 5.5v -20 ? switching frequency 1.2 1.6 mhz ctl1, ctl2 (max1973), on (max1974) logic-low input threshold 0.6 v logic-high input threshold 1.6 v logic input current -1 1 ? pok (max1974 only) output low voltage pok sinking 1ma 100 mv rising 90 95 output valid thresholdfor pok percentage of nominalregulation voltage falling 88 92 % note 2: specifications to -40 c are guaranteed by design and not production tested. downloaded from: http:///
max1973/max1974 smallest 1a, 1.4mhz step-down regulators 6 _______________________________________________________________________________________ typical operating characteristics (circuits of figure 2, 3, and 4; t a = +25 c, unless otherwise noted.) 100 0 0.01 0.1 1 efficiency vs. load current 20 10 max1973/74 toc01 load current (a) efficiency (%) 40 30 60 70 50 80 90 v out = 2.5v v out = 1v v out = 1.8v v in = 3.3v 100 0 0.01 0.1 1 efficiency vs. load current 20 10 max1973/74 toc02 load current (a) efficiency (%) 40 30 60 70 50 80 90 v out = 3.3v v out = 2.5v v in = 5v 0 0.100.05 0.200.15 0.300.25 0.35 dropout voltage vs. load current max1973/74 toc03 load current (a) dropout voltage (v) 0 0.2 0.4 0.6 0.8 v out = 3.3v v out = 2.5v 0 42 86 10 12 02 13 4 5 no-load current vs. input voltage max1973/74 toc04 input voltage (v) no-load current (ma) v out = 2.5v 1.244 1.245 1.246 1.247 1.248 1.249 1.250 1.251 1.252 0 0.2 0.4 0.6 0.8 1.0 fb voltage vs. load current max1973/74 toc05 load current (a) fb voltage (v) max1973v in = 5v r1 = 22k r2 = 13k -4.0 -2.5-3.0 -3.5 -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 0.01 1 0.1 10 100 1000 change in output voltage vs. load current max1973/74 toc06 load current (a) change in output voltage (mv) v in = 5v v out = 1.8v 2.5v 1v 1.10 1.201.15 1.301.25 1.35 1.40 24 35 6 switching frequency vs. supply voltage max1973/74 toc07 supply voltage (v) switching frequency (mhz) t a = +85 c t a = +25 c t a = -40 c downloaded from: http:///
v in = 5v, 100k pullup resistor pok to v out max1974 startup waveforms pok on v out 1v/div 10v/div0 2v/div 0 0 max1973/74 toc08 i in 0 200ma/div max1973/max1974 smallest 1a, 1.4mhz step-down regulators _______________________________________________________________________________________ 7 v in = 5v, v out = 2.5v, i out = 800ma max1973 high-current switching waveforms v lx v out i l 5v/div 20mv/div 0 max1973/74 toc09 0 500ma/div 500ns/div v in = 5v, v out = 2.5v, i out = 10ma max1973 low-current switching waveforms v lx v out i l 5v/div 20mv/div 0 max1973/74 toc10 0 500ma/div 500ns/div v in = 5v, v out = 2.5v, i out = 400ma to 800ma max1973 load transient v out i l 20mv/div 0 max1973/74 toc11 500ma/div 500ns/div v in = 5v, v out = 2.5v, i out = 600ma to 800ma max1973 load transient v out i l 200mv/div 0 max1973/74 toc12 500ma/div 20 s/div v in = 3.3v to 5v to 3.3v, i out = 800ma max1973 line transient v out v in 50mv/div 0 max1973/74 toc13 2v/div 200 s/div typical operating characteristics (continued) (circuits of figure 2, 3, and 4; t a = +25 c, unless otherwise noted.) downloaded from: http:///
max1973/max1974 smallest 1a, 1.4mhz step-down regulators 8 _______________________________________________________________________________________ pin description name pin max1973 max1974 function 1 fbsel fbsel feedback-mode selector. connect fbsel to gnd to set the output voltage to 1.8v(max1973) or 1v (max1974). connect fbsel to in to set the output voltage to 2.5v (max1973) or 1.5v (max1974). leave fbsel unconnected to set the output voltage using a resistor-divider at fb. 2 comp comp compensation. connect a series rc network to gnd. comp is internally pulled tognd when the device is in shutdown or in undervoltage lockout (see the compen- sation components section). 3f bf b feedback input. connect to the output if a preset voltage is used, or to a resistor-divider from the output to gnd for an adjustable output voltage. 4s ss s soft-start pin and reference output. bypass to gnd with at least 0.01?. connect0.1? to gnd for a soft-start ramp time of 6.25ms for the max1973, or 3.75ms for the max1974. ss is internally pulled to gnd when the device is shut down or in undervoltage lockout. 5 gnd gnd ground ctl2 control input 2. controls enable/disable and voltage margining (see table 1). 6 pok power-ok output. open-drain output goes low when output is below 90% of nominaloutput. pok is also low when the device is shut down or in undervoltage lockout. 7 pgnd pgnd power ground 8 lx lx inductor connection. connect an inductor from lx to the output. 9i ni n input supply voltage. input voltage range is 2.6v to 5.5v. connect a 4.7? capacitorfrom in to pgnd. ctl1 control input 1. controls enable/disable and voltage margining (see table 1). 10 on enable input. connect to in or drive high for normal operation. drive low to put devicein shutdown. max1974 with 100k pullup resistor from pok to in, i load = 100ma max1974 pok and input voltage v pok 2v/div 0 max1973/74 toc15 20ms/div v in 1v/div v out 2v/div 0 0 v in = 5v, vout = 2.5v, i out = 800ma, -4% to +4% to -4% max1973 voltage margin step change response v out i in 1v/div 0 max1973/74 toc14 500ma/div 1ms/div v ctl1 v ctl2 010v/div 0 10v/div 0 typical operating characteristics (continued) (circuits of figure 2, 3, and 4; t a = +25 c, unless otherwise noted.) downloaded from: http:///
detailed description the max1973/max1974 are 1.4mhz fixed-frequencypwm current-mode step-down dc/dc converters. a high 1.4mhz switching frequency allows use of small inductors and small capacitors for filtering and decou- pling. an internal synchronous rectifier improves effi- ciency and eliminates the need for an external schottky freewheeling diode. on-chip current sensing uses the on-resistance of the internal mosfets, eliminating cur- rent-sensing resistors and improving efficiency. the input voltage range is 2.6v to 5.5v. the output volt- age is selectable to one of two presets, or adjustable by using a resistor-divider. the output voltage of the max1973 is preset to 1.8v or 2.5v by connecting fbsel to gnd or in, respectively. the max1974 is preset to 1.0v or 1.5v by connecting fbsel to gnd or in, respec- tively. in adjustable mode (see the output voltage selection section), the output voltage is programmable down to 0.75v on the max1974, and down to 1.25v onthe max1973. pwm control scheme the max1973/max1974 use a fixed-frequency pwmcurrent-mode control scheme. the heart of the pwm current-mode controller is an open-loop comparator that compares the integrated voltage feedback signal against the sum of the amplified current-sense signal and the slope compensation ramp (see figure 1). at each rising edge of the internal clock, the internal high- side mosfet turns on until the pwm comparator trips. during this on-time, current ramps up through the inductor, sourcing current to the output and storing energy in a magnetic field. the current-mode feedback system regulates the peak inductor current as a function of the output voltage error signal. because the average inductor current is nearly the same as the peak inductor current (assuming that the inductor value is relatively high to minimize ripple current), the circuit acts as a switch-mode transconduc- tance amplifier. it pushes the output lc filter pole, nor- mally found in a voltage-mode pwm, to a higher frequency. to preserve inner loop stability and eliminate inductor staircasing, an internal slope-compensation ramp is summed into the main pwm comparator. during the second half of the switching cycle (off-time), the internal high-side mosfet turns off and the internal low-side n-channel mosfet turns on. the inductor releases the stored energy as its current ramps down while still providing current to the output. the output capacitor stores charge when the inductor current exceeds the load current and discharges when the inductor current is lower, smoothing the voltage across the load. under overload conditions, when the inductorcurrent exceeds the current limit, the high-side mosfet is not turned on at the rising edge of the clock, and the low-side mosfet remains on to let the inductor current ramp down. 100% duty-cycle operation the max1973/max1974 can operate at 100% dutycycle. in this state, the high-side p-channel mosfet is turned on (not switching). the dropout voltage in 100% duty-cycle operation is the output current multiplied by the sum of the on-resistance of the p-channel mosfet (r ds(on)p ) and the inductor resistance (r l ). v dropout = i out ? ( r ds(on)p + r l ) current sense and current limit the current-sense circuit amplifies the current-sensevoltage generated by the high-side mosfet s on-resis- tance and the inductor current (r ds(on) ? inductor). this amplified current-sense signal and the internalslope compensation signal are summed together at the pwm comparator s inverting input. the pwm compara- tor turns off the internal high-side mosfet when thissum exceeds the integrated feedback voltage. the internal high-side mosfet has a current limit of 1.6a (typ). if the current flowing out of lx exceeds this maximum, the high-side mosfet turns off and the syn- chronous rectifier mosfet turns on. this lowers the duty cycle and causes the output voltage to droop until the current limit is no longer exceeded. there is also a synchronous rectifier current limit of -0.85a, to protect the device from current flowing into lx. if this negative current limit is exceeded, the synchronous rectifier turns off, and the inductor current continues to flow through the high-side mosfet body diode back to the input until the beginning of the next cycle, or until the inductor current drops to zero. soft-start to reduce the supply inrush current, soft-start circuitryramps up the output voltage during startup by charging the ss capacitor with a 20? current source. when ss reaches its nominal value, the output is in full regula- tion. the soft-start time (t ss ) is determined from: where v ss is the soft-start (reference) voltage (1.25v for the max1973; 0.75v for the max1974), i ss is 20?, and c ss is the value of the capacitor connected to ss. soft-start occurs when power is first applied and whenthe device exits shutdown. the part also goes through t v i c ss ss ss ss = max1973/max1974 smallest 1a, 1.4mhz step-down regulators _______________________________________________________________________________________ 9 downloaded from: http:///
max1973/max1974 soft-start when coming out of undervoltage lockout(uvlo) or thermal-overload protection. undervoltage lockout (uvlo) if v in drops below 2.35v (typ), the max1973/max1974 assume that the supply voltage is too low to provide avalid output voltage, and the uvlo circuit inhibits switching. once v in rises above 2.4v, uvlo is dis- abled and the soft-start sequence begins. thermal-overload protection thermal-overload protection limits total power dissipa- tion and protects the ic from damage in case of an overload or short-circuit condition. when the ic junction temperature (t j ) exceeds +170 c, the device shuts down. the part turns on again after the junction temper-ature cools by 20 c. this results in a pulsed output dur- ing continuous thermal-overload conditions. smallest 1a, 1.4mhz step-down regulators 10 ______________________________________________________________________________________ pwm logic block pmos current sense voltage clamp nmos current limit slope comp chip enable reference ready undervoltage lock out feedback select voltage margining soft-start reference bias max1974 only pok in lx pgnd pwm comparator error amp 1.4mhz oscillator in gnd ss comp out fb ctl1 ctl2 on max1973 only max1974 only max1973max1974 figure 1. functional diagram downloaded from: http:///
voltage margining and shutdown a voltage-margining feature is provided on themax1973 to shift the output voltage up or down by 4%. voltage margining is useful for the automatic testing of systems at high and low supply conditions to find potential failures. see table 1 for the max1973 voltage margining and shutdown truth table. a shutdown feature is included on both the max1973 and the max1974. shutdown turns off the ic and reduces the supply current about 0.1?. for the max1974, drive on high for normal operation, or low for shutdown. for the max1973, drive both ctl1 and ctl2 high for normal operation, or drive both low for shutdown. for a simple enable/shutdown function with no voltage margining on the max1973, connect ctl1 to ctl2 and drive as one input. power-ok output (pok) a power-ok output (pok) is provided on the max1974.this is an open-drain output indicating when the output voltage is in regulation. if the output voltage falls below 90% of its nominal value, pok goes low. pok remains low until the output voltage rises to 92.5% of its nominal value. at that point, pok goes high impedance. to use pok as a logic output, connect a 10k to 100k pullup resistor from pok to the power supply of the logicreceiving the pok signal. pok continues to function in shutdown or uvlo. note that a minimum voltage of 1v at in is required to ensure that pok provides a valid output. when v in drops to zero, pok is high imped- ance. see the typical operating characteristics. applications information output voltage selection the output voltage can be set to one of two preset val-ues, or can be set by an external resistor-divider. for preset output voltages, connect fb to the output as shown in figures 2 and 3. connect fbsel to gnd or in to select the desired preset output voltage (see table 2). to set the output voltage to a value other than the preset values, fbsel is not connected, and fb is connected to a voltage-divider as shown in figures 4 and 5. select a value for r2 in the 1k to 22k range, and then calcu- late the value of r1 from the following equation: for the max1973, v fb = 1.25v, allowing its output to be set down to 1.25v. for the max1974, v fb = 0.75v, allowing its output to be set down to 0.75v the max1973/max1974 pwm circuitry is capable of a stable minimum duty cycle of 17%. this limits the mini- mum output voltage that can be generated to 0.17 ? v in . instability may result for v in /v out ratios below 0.17. inductor selection a 2.2? to 4.7? inductor with a saturation current of atleast 1.25a is recommended for full-load (1ma) applica- tions. for lower load currents, the inductor current rating can be reduced. for most applications, use an inductor with a current rating 1.25 times the maximum required output current. for best efficiency, the inductor s dc resistance should be as small as possible. see table 3for recommended inductors and manufacturers. for most designs, the inductor value (l init ) can be derived from the following equation: l vvv v lir i f init out in out in out max sw = ? () () rr v v out fb 12 1 = ? ?? ? ?? ? max1973/max1974 smallest 1a, 1.4mhz step-down regulators ______________________________________________________________________________________ 11 ctl1 ctl2 function gnd gnd shutdown gnd in positive voltage margining,regulation voltage increased 4% from normal operation in gnd negative voltage margining,regulation voltage lowered 4% from normal operation in in normal operation table 1. ctl_ input functions (max1973) output voltage fbsel max1973 max1974 gnd 1.8v 1v in 2.5v 1.5v not connected adjustable down to 1.25v adjustable down to 0.75v table 2. preset output voltages downloaded from: http:///
max1973/max1974 where f sw is the switching frequency (1.4 ? 10 6 hz), and lir is the inductor ripple current as a percentage of themaximum load current. keep lir between 20% and 40% for best compromise of cost, size, and perfor- mance. the peak inductor current is approximately: input capacitor a 4.7? ceramic input capacitor is recommended formost applications because of its low equivalent series resistance (esr), equivalent series inductance (esl), and cost. to ensure stability over a wide temperature range, an x5r or x7r dielectric is recommended. the input capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuit s switching. the input capacitor must meet the ripple current requirement(i rms ) imposed by the switching currents defined by the following equation:choose a capacitor that exhibits less than 10 c tem- perature rise at the maximum operating rms current foroptimum long-term reliability. output capacitor a 4.7? ceramic output capacitor is recommended formost applications because of its low esr, esl, and lower cost. to ensure stability over a wide temperature range, an x5r or x7r dielectric is recommended. key selection parameters for a ceramic output capacitor are capacitance, esr, and voltage rating. these affect the overall stability, output ripple voltage, and transient i i v vvv rms out in out in out =? () i lir i l peak out max () () =+ ?? ?? ?? ?? 1 2 smallest 1a, 1.4mhz step-down regulators 12 ______________________________________________________________________________________ max1973 in comp fbsel ss gnd lx fb ctl1 ctl2 pgnd 3.3 h 1.8v 4.7 f 0.1 f 470pf 36k 4.7 f v in = 2.6v to 5.5v figure 2. max1973 with 1.8v preset output max1974 in comp fbsel ss gnd lx fb pok on pgnd 3.3 h 1.5v 4.7 f 0.1 f 330pf 43k 4.7 f v in = 2.6v to 5.5v pok 100k in in figure 3. max1974 with preset 1.5v output max1973 in comp fbsel ss gnd lx fb ctl1 ctl2 pgnd 3.3 h 1.25v to v in 4.7 f 0.1 f c c r c 4.7 f v in = 2.6v to 5.5v r1 r2 figure 4. max1973 with adjustable output voltage set by r1and r2 max1974 in comp fbsel ss gnd lx fb on pok pgnd 3.3 h 0.75v to v in 4.7 f 0.1 f c c r c 4.7 f v in = 2.6v to 5.5v r1 r2 in in 100k pok figure 5. max1974 with adjustable output voltage set by r1and r2 downloaded from: http:///
response of the dc-dc converter. with ceramic capaci-tors, the voltage ripple from esl is negligible. output ripple is generated by variations in the charge stored in the output capacitance, and the voltage drop across the capacitor esr. the output voltage ripple due to the output capacitance is: the output voltage ripple due to capacitor esr is: i p-p is the peak-to-peak inductor current: these equations are suitable for initial capacitor selec- tion, but final values should be set by testing a proto- type or evaluation circuit. as a rule, a smaller ripple current results in less output voltage ripple. because the inductor ripple current is inversely proportional to inductor value, output voltage ripple decreases with larger inductance. load transient response depends on the selected output capacitor. during a load transient, the output voltage instantly changes by esr ? ? i load . before the controller can respond, the output deviates further, depending onthe inductor and output capacitor values. after a short time (see the typical operating characteristics ), the controller responds by regulating the output voltageback to its nominal state. the controller response time depends on the closed-loop bandwidth. with a higher bandwidth the response time is faster. however, to main- tain stable operation, the bandwidth should not be set above f sw /10. compensation components an internal transconductance error amplifier compen- sates the control loop. connect a series resistor and capacitor between comp and gnd to form a pole-zeropair. the external inductor, output capacitor, compen- sation resistor, and compensation capacitor determine the loop bandwidth and stability. the inductor and out- put capacitor are chosen based on performance, size, and cost. additionally, the compensation resistor and capacitor are selected to optimize the control loop. table 4 and table 5 list typical component values. the rest of this section is a more detailed discussion on cal- culating compensation components. the controller uses a current-mode control scheme that regulates the output voltage by forcing the required current through the external inductor. the voltage across the internal high-side mosfet s on-resistance is used to sense inductor current. current-mode controleliminates the double pole caused by the inductor and output capacitor found in other control schemes. simple type 1 compensation with a single resistor (r c ) and capacitor (c c ) is all that is needed to provide a stable and high-bandwidth loop.use the formula below to calculate the value of c c , then use the nearest standard value:where v fb is 1.25v for the max1973 and 0.75v for the max1974, the current-sense transresistance (r cs ) is 0.26 (typ), and the transconductance from fb to comp (g m ) is 50? (typ). for best stability and response performance, the closed-loop unity-gain fre- quency (f c ) should be approximately 140khz (one- tenth the switching frequency).use the following equation to calculate r c : below is a numerical example of calculating compen- sation values for a circuit using the max1973 with 2.5v output and maximum output current of 1a: max1973 r c c v i c out c out out max = 05 . () c v ir g f c fb out max cs m c = 05 11 2 . () i vv fl v v pp in out sw out in ? = ? v i esr ripple esr p p () = ? v i cf ripple c pp out sw () = ? 8 vv v ripple ripple c ripple esr =+ () ( ) max1973/max1974 smallest 1a, 1.4mhz step-down regulators ______________________________________________________________________________________ 13 manufacturer part inductance (?) esr (m ) saturationcurrent (a) dimensions l ? w ? h (mm) coilcraft lpo1704-32m 3.3 160 1.3 5.5 ? 6.6 ? 1 sumida cdrd3d16-r3 3.3 85 1.1 4 ? 4 ? 1.8 toko a682ay-3r3m 3.3 134 0.97 4.4 ? 4.4 ? 3.1 table 3. recommended inductors downloaded from: http:///
max1973/max1974 v out = 2.5v i out(max) = 1a c out = 4.7? v fb = 1.25v r cs = 0.26 g m = 50? f c = 140khz select the nearest standard value: c c = 560pf select the nearest standard value: r c = 43k pc board layout a properly designed pc board layout is important inany switching regulator. the switching power stage requires particular attention. follow these guidelines for good pc board layout: 1) place decoupling capacitors as close to ic pins as possible. keep the power ground plane (connectedto pgnd) and signal ground plane (connected to gnd) separate. connect the two ground planes with a single connection from pgnd to gnd. 2) input and output capacitors are connected to the power ground plane; all other capacitors are con- nected to the signal ground plane. 3) keep the high-current paths as short and wide as possible. 4) if possible, connect in, lx, and pgnd separately to a large land area to help cool the ic to furtherimprove efficiency and long-term reliability. 5) ensure all feedback connections are short and direct. place feedback resistors (if used) as close tothe ic as possible. 6) route high-speed switching nodes (lx) away from sensitive analog areas (fb, comp, ss). r c c v i k c out c out out max = = = ? ? 05 47 10 560 10 25 05 1 41 9 6 12 . .. . . () c v ir g f pf c fb out max cs m c = = = ? 05 11 2 125 05 1 1 026 50 10 1 2 140000 547 6 . . .. () smallest 1a, 1.4mhz step-down regulators 14 ______________________________________________________________________________________ chip information transistor count: 1998process: bicmos v out (v) c in (?) c out (?) c c (pf) r c (k ) 2.5 4.7 4.7 560 43 1.8 4.7 4.7 560 30 table 4. recommended components for the max1973 v out (v) c in (?) c out (?) c c (pf) r c (k ) 1.5 4.7 4.7 330 43 1.0 4.7 4.7 330 27 table 5. recommended components for the max1974 selector guide part features output preset max1973eub voltage margining 1.8v or 2.5v max1974eub power-ok output 1v or 1.5v downloaded from: http:///
max1973/max1974 smallest 1a, 1.4mhz step-down regulators maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circuit patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 ____________________ 15 2002 maxim integrated products printed usa is a registered trademark of maxim integrated products. package information (the package drawing(s) in this data sheet may not reflect the most current specifications. for the latest package outline information, go to www.maxim-ic.com/packages .) 10lumax.eps package outline, 10l umax/usop 1 1 21-0061 i rev. document control no. approval proprietary information title: top view front view 1 0.498 ref 0.0196 ref s 6 side view bottom view 0 0 6 0.037 ref 0.0078 max0.006 0.0430.118 0.1200.199 0.0275 0.118 0.0106 0.120 0.0197 bsc inches 1 10 l1 0.0035 0.007 e c b 0.187 0.0157 0.114 hl e2 dim 0.116 0.114 0.116 0.002 d2e1 a1d1 min - a 0.940 ref 0.500 bsc 0.090 0.177 4.75 2.89 0.40 0.200 0.270 5.05 0.70 3.00 millimeters 0.052.89 2.952.95 - min 3.003.05 0.153.05 max 1.10 10 0.60.1 0.60.1 ? 0.500.1 h 4x s e d2 d1 b a2 a e2 e1 l l1 c gage plane a2 0.030 0.037 0.75 0.95 a1 downloaded from: http:///

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