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june 2008 rev 6 1/37 37 l6732 adjustable step-down controller with synchronous rectification features input voltage range from 1.8 v to 14 v supply voltage range from 4.5 v to 14 v adjustable output voltage down to 0.6 v with 0.8 % accuracy over line voltage and temperature (0 c~125 c) fixed frequency voltage mode control t on lower than 100 ns 0 % to 100 % duty cycle external input voltage reference soft-start and inhibit high current embedded drivers predictive anti-cross conduction control programmable high-side and low-side r ds(on) sense over-current-protection selectable switching frequency 250 khz / 500 khz pre-bias start up capability power good output master/slave synchronization with 180 phase shift over voltage protection thermal shutdown package: htssop16 applications lcd and pdp tv high performance / high density dc-dc modules low voltage distributed dc-dc nipol converters ddr memory supply graphic cards htssop16 (exposed pad) table 1. device summary order codes package packing l6732 htssop16 tube l6732tr htssop16 tape and reel www.st.com
contents l6732 2/37 contents 1 summary description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 maximum rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 pin connection and function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4 electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5 device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.1 oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.2 internal ldo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.3 bypassing the ldo to avoid the voltage drop with low vcc . . . . . . . . . . . 11 5.4 internal and external references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.5 error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.6 soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.7 driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 5.8 monitoring and protections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 5.9 thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.10 synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.11 minimum on-time (ton, min) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5.12 bootstrap anti-discharging system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.12.1 fan's power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6 application details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 6.1 inductor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 6.2 output capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 6.3 input capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 6.4 compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 7 l6732 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
l6732 contents 3/37 7.1 20 a board description and pcb layout . . . . . . . . . . . . . . . . . . . . . . . . . . 26 7.2 5 a board description and pcb layout . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 8 package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 9 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
summary description l6732 4/37 1 summary description the controller is an integrated circuit re alized in bcd5 (bicmos-dmos, version 5) fabrication that provides complete control logic and protection for high performance step- down dc-dc and nipol converters. it is designed to drive n-channel mosfets in a synchronous rectified buck topology. the output voltage of the converter can be precisely regulated down to 600 mv with a maximum tolerance of 0.8 % and it is also possible to use an external reference from 0 v to 2.5 v. the input voltage can range from 1.8 v to 14 v, while the supply voltage can range from 4.5 v to 14 v. high peak current gate drivers provide for fast switching to the external power section, and the output current can be in excess of 20 a. the pwm duty cycle can range from 0 % to 100 % with a minimum on-time (t on , min ) lower than 100 ns making possible conversions with very low duty cycle at high switching frequency. the device provides voltage-mode control that includes a selectab le frequency oscillator ( 250 khz or 500 khz). the error amplifier features a 10 mhz gain-bandwidth-product and 5 v/s slew-rate that permits to realize high converter bandwidth for fast transient response. the device monitors the current by using the r ds(on) of both the high-side and low-side mosfet(s), eliminating the need for a current sensing resistor and guaranteeing an effective over-current-protection in all the application conditions. when necessary, two different current limit protections can be externally set through two external resistors. during the soft-start phase a constant current protection is provided while after the soft-start the device enters in hiccup mode in case of over-current. during the soft-start, the sink mode capability is disabled in order to allow a proper start-up also in pre-biased output voltage conditions. after the soft-start the device can sink current. other features are power good, master/slave synchronization (with 180 phase shift), over-voltage-protection, feed-back disconnection and thermal shutdown. the htssop16 package allows the realization of really co mpact dc/dc converters.
l6732 summary description 5/37 1.1 functional description figure 1. block diagram vin=1.8v-14v vcc = 4.5 v - 1 4 v l6732 pgnd phase gnd lgate boot hgate och fb ss monitor protection and ref osc + - + - e/a pwm vccdr - + - 0.6v ocl comp earef ldo pgnd phase gnd lgate boot hgate och v out fb ss monitor protection and ref osc + - + - + - e/a pwm vccdr - + - ocl comp earef ldo pgood synch
electrical data l6732 6/37 2 electrical data 2.1 maximum rating 2.2 thermal data table 2. absolute maximum ratings symbol parameter value unit v cc v cc to gnd and pgnd, och, pgood -0.3 to 18 v v boot - v phase boot voltage 0 to 6 v v hgate - v phase 0 to v boot - v phase v v boot boot -0.3 to 24 v v phase phase -1 to 18 v phase spike, transient < 50 ns (f sw = 500 khz) -3 +24 ss, fb, earef, sync, ocl, lgate, comp, v ccdr -0.3 to 6 v och pin maximum withstanding voltage range test condition: cdf-aec -q100-002 ?human body model? acceptance criteria: ?normal performance? 1500 v pgood pin 1000 other pins 2000 table 3. thermal data symbol description value unit r thja thermal resistance junction to ambient 50 c/w t stg storage temperature range -40 to +150 c t j junction operating temperature range -40 to +125 c t a ambient operating temperature range -40 to +85 c
l6732 pin connection and function 7/37 3 pin connection and function figure 2. pin connection (top view) table 4. pin functions pin n. name function 1 pgood this pin is an open collector output an d it is pulled low if the output voltage is not within the specified thresholds (9 0 %-110 %). if not used it may be left floating. pull-up this pin to v ccdr with a 10 k resistor to obtain a logical signal. 2 synch it is a master-slave pin. two or more devices can be synchronized by simply connecting the synch pins together. the device operating with the highest fsw will be the master. the slave devices will operate with 180 phase shift from the master. the best way to synchronize devices together is to set their fsw at the same value. if it is not used the synch pin can be left floating. 3 sgnd all the internal references are referred to this pin. 4 fb this pin is connected to the error ampl ifier inverting input. connect it to v out through the compensation network. this pin is also used to sense the output voltage in order to manage the over voltage conditions and the pgood signal. 5 comp this pin is connected to the erro r amplifier output and is used to compensate the voltage control feedback loop. 6 ss/inh the soft-start time is programmed con necting an external capacitor from this pin and gnd. the internal current generator forces a current of 10 a through the capacitor. when the voltage at this pin is lower than 0.5 v the device is disabled. 1 2 3 4 5 6 11 12 htssop16 13 14 15 16 sgnd earef fb pgood synch comp vcc lgate phase ss/inh pgnd 7 10 vccdr boot hgate 8 9 ocl och
pin connection and function l6732 8/37 pin n. name function 7 earef by setting the voltage at this pin is po ssible to select the internal/external reference and the switching frequency: v earef 0-80 % of v ccdr -> external reference/f sw = 250 khz v earef = 80 %-95 % of v ccdr -> v ref = 0.6 v/f sw = 500 khz v earef = 95 %-100 % of v ccdr -> v ref = 0.6 v/f sw = 250 khz an internal clamp limits the maximum v earef at 2.5 v (typ.). the device captures the analog value present at this pin at the start-up when v cc meets the uvlo threshold. 8 ocl a resistor connected from this pin to ground sets the valley- current-limit. the valley current is sensed through the low-side mosfet(s). the internal current generator sources a current of 100 a (i ocl ) from this pin to ground through the external resistor (r ocl ). the over-current threshold is given by the following equation: connecting a capacitor from this pin to gnd helps in reducing the noise injected from v cc to the device, but can be a low impedance path for the high-frequency noise related to the gnd. connect a capacitor only to a ?clean? gnd. 9 och a resistor connected from this pin and the high-side mosfet(s) drain sets the peak-current-limit. the peak current is sensed through the high-side mosfet(s). the internal 100 a current generator (i och ) sinks a current from the drain through the external resistor (r och ). the over-current threshold is given by the following equation: 10 phase this pin is connected to the source of the high-side mosfet(s) and provides the return path for the high-side driver. this pin monitors the drop across both the upper and lower mosfet (s) for the current limit together with och and ocl. 11 hgate this pin is connected to the high-side mosfet(s) gate. 12 boot through this pin is supplied the high-s ide driver. connect a capacitor from this pin to the phase pin and a diode from v ccdr to this pin (cathode versus boot). 13 pgnd this pin has to be connected closely to the low-side mosfet(s) source in order to reduce the noise injection into the device. 14 lgate this pin is connected to the low-side mosfet(s) gate. 15 v ccdr 5 v internally regulated voltage. it is used to supply the internal drivers. filter it to ground with at least 1 f ceramic cap. 16 v cc supply voltage pin. the operative supply voltage range is from 4.5 v to 14 v. table 4. pin functions (continued) i valley i ocl r ocl ? 2r dsonls ? ---------------------------------- = i peak i och r och ? r dsonhs --------------------------------- - =
l6732 electrical characteristics 9/37 4 electrical characteristics table 5. electrical characteristics (v cc = 12 v, t a = 25 c, unless otherwise specified) symbol parameter test condition min typ max unit v cc supply current i cc v cc stand by current osc = open; ss to gnd 7 9 ma v cc quiescent current osc = open; hg = open, lg = open, ph = open 8.5 10 power-on v cc tu r n - o n v cc threshold v och = 1.7 v 4.0 4.2 4.4 v tu r n - o f f v cc threshold v och = 1.7 v 3.6 3.8 4.0 v v in ok tu r n - o n v och threshold 1.1 1.25 1.47 v v in ok tu r n - o f f v och threshold 0.9 1.05 1.27 v v ccdr regulation v ccdr voltage v cc = 5.5 v to 14 v i dr = 1 ma to 100 ma 4.555.5v soft-start and inhibit i ss soft-start current ss = 2 v 7 10 13 a ss = 0 to 0.5 v 20 30 45 oscillator f osc accuracy 237 250 263 khz 450 500 550 khz ? v osc ramp amplitude 2.1 v output voltage v fb output voltage v dis = 0 to v th 0.597 0.6 0.603 v error amplifier r earef earef input resistance vs gnd 70 100 150 k ? i fb i.i. bias current v f = 0 v 0.290 0.5 a ext ref clamp 2.3 v v offset error amplifier offset vref = 0.6 v -5 +5 mv g v open loop voltage gain guaranteed by design 100 db gbwp gain-bandwidth product guaranteed by design 10 mhz sr slew-rate comp = 10 pf guaranteed by design 5v/ s
electrical characteristics l6732 10/37 symbol parameter test condition min typ max unit gate drivers r hgate_on high side source resistance v boot - v phase = 5 v 1.7 ? r hgate_off high side sink resistance v boot - v phase = 5 v 1.12 ? r lgate_on low side source resistance v ccdr = 5 v 1.15 ? r lgate_off low side sink resistance v ccdr = 5 v 0.6 ? protections i och och current source v och = 1.7 v 90 100 110 ? i ocl ocl current source 90 100 110 ? ovp over voltage trip (vfb / vearef) v fb rising v earef = 0.6 v 120 % v fb falling v earef = 0.6 v 117 % under voltage threshold (v b fb b / v b earef b ) v b fb b falling 80 % power good upper threshold (v b fb b / v b earef b ) v b fb b rising 108 110 112 % lower threshold (v b fb b / v b earef b ) v b fb b falling 88 90 92 % v b pgood b pgood voltage low i b pgood b = -5 ma 0.5 v table 5. electrical characteristics (v cc = 12 v, t a = 25 c, unless otherwise specified) (continued) table 6. thermal characteristics (v cc = 12 v) symbol parameter test condition min typ max unit output voltage v fb output voltage t j = 0 c ~ 125 c 0.596 0.6 0.605 v t j = -40 c ~ 125 c 0.593 0.6 0.605
l6732 device description 11/37 5 device description 5.1 oscillator the switching frequency can be fixed to two values: 250 khz or 500 khz by setting the proper voltage at the earef pin (see table 4. pins function and section 4.3 internal and external reference). 5.2 internal ldo an internal ldo supplies the internal circuitry of the device. the input of this stage is the v cc pin and the output (5 v) is the v ccdr pin ( figure 3. ). the ldo can be by-passed, providing directly a 5 v voltage to v ccdr . in this case v cc and v ccdr pins must be shorted together as shown in figure 4. v ccdr pin must be filtered with at least 1 f capacitor to sustain the internal ldo during the recharge of the bootstrap capacitor. v ccdr also represents a voltage reference for pgood pin (see table 4. pins function). figure 3. ldo block diagram ldo 4.5 14v
device description l6732 12/37 5.3 bypassing the ldo to avoid the voltage drop with low vcc if v cc 5 v the internal ldo works in dropout with an output resistance of about 1 ? . the maximum ldo output current is about 100 ma and so the output voltage drop is 100 mv, to avoid this the ldo can be by passed. 5.4 internal and external references it is possible to set the internal/external reference and the switching frequency by setting the proper voltage at the earef pin. the maximum value of the external reference depends on the v cc : with v cc = 4 v the clamp operates at about 2 v (typ.), while with v cc greater than 5v the maximum external reference is 2.5 v (typ.). v earef from 0 % to 80 % of v ccdr -> external reference/fsw = 250 khz v earef from 80 % to 95 % of v ccdr -> v ref = 0.6v/fsw = 500 khz v earef from 95 % to 100 % of v ccdr -> v ref = 0.6v/fsw = 250 khz providing an external reference from 0 v to 450 mv the output voltage will be regulated but some restrictions must be considered: ov threshold saturates to a minimum value of 300 mv (ov is tracking the reference; tracking small references will result in a narrow threshold reducing noise immunity) the under-voltage-protection doesn't work; the pgood signal remains low; to set the resistor divider it must be considered that a 100 k pull-down resistor is integrated into the device (see figure 5. ). finally it must be taken into account that the voltage at the earef pin is captured by the device at the start-up when v cc is about 4 v. figure 4. bypassing the ldo
l6732 device description 13/37 5.5 error amplifier 5.6 soft-start when both v cc and v in are above their turn-on thresholds (v in is monitored by the och pin) the start-up phase takes place. otherwise the ss pin is internally shorted to gnd. at start-up, a ramp is generated charging the external capacitor c ss with an internal current generator. the initial value for this current is 35 a and charges the capacitor up to 0.5 v. after that it becomes 10 a until the final charge value of approximately 4 v (see figure 6 ). the output of the error amplifier is clamped with this voltage (v ss ) until it reaches the programmed value. no switching activity is observable if v ss is lower than 0.5 v and both mosfets are off. when v ss is between 0.5 v and 1.1 v the low-side mosfet is turned on because the comp signal is lower than the valley of the triangular wave and so the duty- cycle is 0 %. as v ss reaches 1.1 v (i.e. the oscillator tria ngular wave inferior limit) even the high-side mosfet begins to switch and the output voltage starts to increase. the l6732 can only source current during the soft-start phase in order to manage the pre-bias start-up applications. this means that when the start-up occurs with output voltage greater than 0v (pre-bias startup), even when vss is between 0.5 v and 1.1 v the low-side mosfet is kept off (see figure 7 and figure 8 ). figure 5. error amplifier reference
device description l6732 14/37 figure 6. device start-up: voltage at the ss pin figure 7. start-up without pre-bias v cc v in t t 0.5v 4v v cc v in v ss 4.2v 1.25v lgate v out i l v ss
l6732 device description 15/37 the l6732 can sink or source current after the soft-start phase (see figure 9. ). if an over current is detected during the soft-start phase, the device provides a constant-current- protection. in this way, in case of short soft-start time and/or small inductor value and/or high output capacitors value and so, in case of high ripple current during the soft-start, the converter can start in any case, limiting the current (see section 4.6 monitoring and protections) but not entering in hiccup mode. during normal operation, if any under-voltage is detected on one of the two supplies, the ss pin is internally shorted to gnd and so the ss capacitor is rapidly discharged. figure 8. start-up with pre-bias figure 9. inductor current during and after soft-start v ss i l v out lgate v out i l v ss v cc
device description l6732 16/37 5.7 driver section the high-side and low-side drivers allow using different types of power mosfets (also multiple mosfets to reduce the r ds(on) maintaining fast switching transitions. the low-side driver is supplied by v ccdr while the high-side driver is supplied by the boot pin. a predictive dead time control avoids mosfets cross-conduction maintaining very short dead time duration in the range of 20 ns. the control monitors the phase node in order to sense the low-side body diode recirculation. if the phase node voltage is less than a certain threshold (-350 mv typ.) during the dead time, it will be reduced in the next pwm cycle. the predictive dead time control doesn't work when the high-side body diode is conducting because the phase node doesn't go negative. this situation happens when the converter is sinking current for example and, in this case, an adaptive dead time control operates. 5.8 monitoring and protections the output voltage is monitored by means of pin fb. if it is not within 10 % (typ.) of the programmed value, the power good (pgood) output is forced low. the device provides over-voltage-protection: when the voltage sensed on fb pin reaches a value 20% (typ.) greater than the reference, the low-side driver is turned on as long as the over voltage is detected (see figure 10. ). it must be taken into account that there is an electrical network between the output terminal and the fb pin and therefore the voltage at the pin is not a perfect replica of the output voltage. however due to the fact that the converter can sink current, in the most of cases the low-side will turn-on before the output voltage ex ceeds the over-voltage threshold, because the error amplifier will throw off balance in advance. even if the device doesn't report an over-voltage, the behavior is the same, because the low-side is turned-on immediately. the following figure shows the device behavior during an over-voltage event. the output voltage figure 10. ovp lgate fb
l6732 device description 17/37 rises with a slope of 100 mv/s, emulating in this way the breaking of the high-side mosfet as an over-voltage cause. the device realizes the over-current-protection (ocp) sensing the current both on the high-side mosfet(s) and the low-side mosfet(s) and so 2 current limit thresholds can be set (see och pin and ocl pin in table 4. pins function): peak current limit valley current limit the peak current protection is active when the high-side mosfet(s) is turned on, after a masking time of about 100 ns. the valley-current-protection is enabled when the low-side mosfet(s) is turned on after a masking time of about 400 ns. if, when the soft-start phase is completed, an over current event occurs during the on time (peak-current-protection) or during the off time (valley-cu rrent-protection ) the device enters in hiccup mode: the high- side and low-side mosfet(s) are turned off, the soft-start capacitor is discharged with a constant current of 10 a and when the voltage at the ss pin reaches 0.5 v the soft-start phase restarts. during the soft-start phase the ocp provides a constant-current-protection. if during the t on the och comparator triggers an over current the high-side mosfet(s) is immediately turned off (after the masking time and the internal delay) and returned on at the next pwm cycle. the limit of this protection is that the t on can't be less than masking time plus propagation delay because during the masking time the peak-current-protection is disabled. in case of very hard short circuit, even with this short t on , the current could escalate. the valley-current-protection is very helpful in this case to limit the current. if during the off-time the ocl comparator triggers an over current, the high-side mosfet(s) is not turned on until the current is over the va lley-current-limit. this implies th at, if it is necessary, some pulses of the high-side mosfet(s) will be skipped, guaranteeing a maximum current due to the following formula: figure 11. ovp: the low-side mosfet is turned-on in advance v out v fb lgate 109%
device description l6732 18/37 equation 4 during soft-start the oc acts in constant current mode: a current control loop limits the value of the error amplifier output (comp), in order to avoid its saturation and thus recover faster when the output returns in regulation. figure 12. shows the behavior of the device during an over current condition that persists also in the soft-start phase. l6732 provides under voltage (uv) protection: when the voltage on fb pin falls below 80 % of the reference, the ic will enter hiccup mode. feedback disconnection is also provided by sourcing a 100 na current from fb pin. if fb results being floating, the ic will detect and ov so latching its condition with low side mosfet firmly on 5.9 thermal shutdown when the junction temperature reaches 150 c 10 c the device enters in thermal shutdown. both mosfets are turned off and the soft-start capacitor is rapidly discharged with an internal switch. the device doesn't restart until the junction temperature goes down to 120 c and, in any case, until the voltage at the soft-start pin reaches 500 mv. 5.10 synchronization the presence of many converters on the same board can generate beating frequency noise. to avoid this it is important to make them operate at the same switching frequency. moreover, a phase shift between different modules helps to minimize the rms current on the common input capacitors. figure 13 and figure 14 shows the results of two modules in synchronization. two or more devices can be synchronized simply connecting together the synch pins. the device with th e higher switching frequency w ill be the master while the figure 12. constant current and hiccup mode during an ocp min on valley max t l vout vin i i , ? ? + = vss vcomp i l
l6732 device description 19/37 other one will be the slave. the slave contro ller will increase its switching frequency reducing the ramp amplitude pr oportionally and then the modulator gain will be increased. to avoid a huge variation of the modulator gain, the best way to synchronize two or more devices is to make them work at the same switching frequency and, in any case, the switching frequencies can differ for a maximum of 50 % of the lowest one. if, during synchronization between two (or more) l6732, it's important to know in advance which the master is, it's timely to set its switching frequency at least 15 % higher than the slave. using an external clock signal (f ext ) to synchronize one or more devices that are working at a different switching frequency (f sw ) it is recommended to follow the below formula: equation 5 the phase shift between master and slaves is approximately 180 . figure 13. synchronization: pwm signal figure 14. synchronization: inductor currents sw ext sw f f f ? 3 , 1
device description l6732 20/37 5.11 minimum on-time (t on , min ) the device can manage minimum on-times lower than 100 ns. this feature comes down from the control topology and from the particular over-current-protection system of the l6732. in fact, in a voltage mode controller the current has not to be sensed to perform the regulation and, in the case of l6732, neither for the over-current protection, given that during the off-time the valley-current-protection can operate in every case. the first advantage related to this featur e is the possibility to realize extremely low conversion ratios. figure 15 shows a conversion from 14 v to 0.3 v at 500 khz with a t on of about 50 ns. the on-time is limited by the turn-on and turn-off times of the mosfets. figure 15. 14 v -> 0.3 v @ 500 khz, 5 a 50ns vphase i l v out
l6732 device description 21/37 5.12 bootstrap anti-discharging system this built-in system avoids that the voltage across the bootstrap capacitor becomes less than 3.3 v. an internal comparator senses the voltage across the external bootstrap capacitor keeping it charged, eventually turning-on the low-side mosfet for approximately 200 ns. if the bootstrap capacitor is not enough charged the high-side mosfet cannot be effectively turned-on and it will present a higher r ds(on) . in some cases the ocp can be also triggered. the bootstrap capacitor can be discharged during the soft-start in case of very long soft-start time and light loads. it's also possible to mention one application condition during which the bootstrap capacitor can be discharged: 5.12.1 fan's power supply in many applications the fan is a dc motor driven by a voltage-mode dc/dc converter. often only the speed of the motor is controlled by varying the voltage applied to the input terminal and there's no control on the torque because the current is not directly controlled. in order to vary the motor speed the output voltage of the converter must be varied. the l6732 has a dedicated pin called earef (see the related section) that allows providing an external reference to the non-inverting input of the error-amplifier. in these applications the duty cycle depend s on the motor's speed and sometimes 100 % has to be set in order to go at the maximum speed. unfortunately in these conditions the bootstrap capacitor can not be recharged and the system cannot work properly. some pwm controller limits the maximum duty-cycle to 80-90 % in order to keep the bootstrap cap charged but this make worse the performance during the load transient. thanks to the ?bootstrap anti-discharging system? the l6732 can work at 100 % without any problem. the following picture shows the device behavior when input voltage is 5 v and 100 % is set by the external reference. figure 16. 100 % duty cycle operation
application details l6732 22/37 6 application details 6.1 inductor design the inductance value is defined by a compromise between the transient response time, the efficiency, the cost and the size. the inductor has to be calculated to sustain the output and the input voltage variation to maintain the ripple current ( ? i l ) between 20 % and 30 % of the maximum output current. the inductance value can be calculated with the following relationship: equation 6 where f sw is the switching frequency, v in is the input voltage and v out is the output voltage. figure 17. shows the ripple current vs. the output voltage for different values of the inductor, with v in = 5 v and v in = 12 v at a switching frequency of 500 khz. increasing the value of the inductance reduces the ripple current but, at the same time, increases the converter response time to a load transient. if the compensation network is well designed, during a load transient the device is able to set the duty cycle to 100 % or to 0 %. when one of these conditions is reached, the response time is limited by the time required to change the inductor current. during this time the output current is supplied by the output capacitors. minimizing the response time can minimize the output capacitor size. figure 17. inductor current ripple vin vout i fsw vout vin l l ? ? ? ? ? 0 1 2 3 4 5 6 7 8 01234 output voltage (v) inductor current rippl v in=5v , l=500nh v in=5v, l=1.5uh v in=12v, l=2uh v in=12v, l=1uh
l6732 application details 23/37 6.2 output capacitors the output capacitors are basic components for the fast transient response of the power supply. they depend on the output voltage ripple requirements, as well as any output voltage deviation requirement during a load transient. during a load transient, the output capacitors supply the current to the load or absorb the current stored in the inductor until the converter reacts. in fact, even if the controlle r recognizes immediately the load transient and sets the duty cycle at 100% or 0%, the current slope is limited by the inductor value. the output voltage has a first drop due to the current variation inside the capacitor (neglecting the effect of the esl): equation 7 moreover, there is an additional drop due to the effective capacitor discharge or charge that is given by the following formulas: equation 8 equation 9 formula (8) is valid in case of positive load tr ansient while the formula (9) is valid in case of negative load transient. d max is the maximum duty cycle value that in the l6732 is 100%. for a given inductor value, minimum input voltage, output voltage and maximum load transient, a maximum esr and a minimum c out value can be set. the esr and c out values also affect the static output voltage ripple. in the worst case the output voltage ripple can be calculated with the following formula: equation 10 usually the voltage drop due to the esr is the biggest one while the drop due to the capacitor discharge is almost negligible. 6.3 input capacitors the input capacitors have to sustain the rms current flowing through them, that is: equation 11 esr iout vout esr ? ? = ? ) max min , ( 2 2 vout d vin cout l iout vout cout ? ? ? ? ? ? = ? vout cout l iout vout cout ? ? ? ? = ? 2 2 ) 8 1 ( fsw cout esr i vout l ? ? + ? ? = ? ) 1 ( d d iout irms ? ? ? =
application details l6732 24/37 where d is the duty cycle. the equation reaches its maximum value, i out /2 with d = 0.5. the losses in worst case are: equation 12 6.4 compensation network the loop is based on a voltage mode control ( figure 18. ). the output voltage is regulated to the internal/external reference voltage and scaled by the external resistor divider. the error amplifier output v comp is then compared with the oscillato r triangular wave to provide a pulse-width modulated (pwm) with an amplitude of v in at the phase node. this waveform is filtered by the output filter. the modulator transfer function is the small signal transfer function of v out /v comp . this function has a double pole at frequency f lc depending on the l-c out resonance and a zero at fesr depending on the output capacitor's esr. the dc gain of the modulator is simply the input voltage v in divided by the peak-to-peak oscillator voltage: v osc . the compensation network consists in the internal error amplifier, the impedance networks z in (r3, r4 and c20) and z fb (r5, c18 and c19). the compensation network has to provide a closed loop transfer function with the highest 0db crossing frequency to have fastest transient response (but always lower than fsw/10) and the highest gain in dc conditions to minimize the load regulation error. a stable control loop has a gain crossing the 0db axis with -20db/decade slope and a phase margin greater than 45 . to locate poles and zeroes of the compensation networks, the following suggestions may be used: modulator singularity frequencies: equation 13 2 ) 5 . 0 ( iout esr p ? ? = figure 18. compensation network cout l lc ? = 1
l6732 application details 25/37 equation 14 compensation network singularity frequencies: equation 15 equation 16 equation 17 equation 18 compensation network design: ? put the gain r 5 /r 3 in order to obtain the desired converter bandwidth equation 19 ? place z1 before the output filter resonance lc ; ? place z2 at the output filter resonance lc ; ? place p1 at the output capacitor esr zero esr ; ? place p2 at one half of the switching frequency; ? check the loop gain considering the error amplifier open loop gain. cout esr esr ? = 1 ? ? ? ? ? ? ? ? + ? ? = 19 18 19 18 5 1 1 c c c c r p 20 4 2 1 c r p ? = 19 5 1 1 c r z ? = () 4 3 20 2 1 r r c z + ? = lc c vosc vin r r ? ? ? ? ? = 3 5
application details l6732 26/37 figure 19. asymptotic bode plot of converter's open loop gain
l6732 l6732 demonstration board 27/37 7 l6732 demonstration board 7.1 20 a board description and pcb layout l6732 20 a demonstration board realizes in a four layer pcb a step-down dc/dc converter and shows the operation of the device in a general purpose application. the input voltage can range from 4.5 v to 14 v and the output voltage is at 3.3 v. the module can deliver an output current in excess of 20 a. the switching frequency is set at 250 khz (controller free- running fsw) but it can be set to 500 khz acting on the earef pin. figure 20. demonstration board schematic l1 q1-3 q4-6 d3 9 u1 l6732 vin vout 12 och c11 gout r9 c9 13 14 10 11 hgate phase lgate pgnd 7 vcc 16 vcc 6 ss c4 boot d1 15 vccd r c10 5 vfb comp r3 r1 c1 r2 c2 c3 r4 r7 8 earef c7 r8 4 3 gnd gin ocl r10 r11 c12-c13 c16-c19 c5 r6 r5 c8 j1 r12 c15 ext ref j2 pgood synch 1 2 synch vccdr pgood r13
l6732 demonstration board l6732 28/37 table 7. demonstration board part list reference value manufacturer package supplier r1 1 k ? neohm smd 0603 ifarcad r2 1 k ? neohm smd 0603 ifarcad r3 4 k7 r4 2 k7 neohm smd 0603 ifarcad r5 0 ? neohm smd 0603 ifarcad r6 n.c. neohm smd 0603 ifarcad r7 2 k neohm smd 0603 ifarcad r8 10 ? neohm smd 0603 ifarcad r9 1 k5 neohm smd 0603 ifarcad r10 2.2 ? neohm smd 0603 ifarcad r11 2.2 ? neohm smd 0603 ifarcad r12 n.c. neohm smd 0603 ifarcad r13 10 k neohm smd 0603 ifarcad c1 4.7 nf kemet smd 0603 ifarcad c2 47 nf kemet smd 0603 ifarcad c3 1 nf kemet smd 0603 ifarcad c4 100 nf kemet smd 0603 ifarcad c5 100 nf kemet smd 0603 ifarcad c6 n.c. / / / c7 100 nf kemet smd 0603 ifarcad c8 4.7 f 20 v avx sma6032 ifarcad c9 1 nf kemet smd 0603 ifarcad c10 1 f kemet smd 0603 ifarcad c11 220 nf kemet smd 0603 ifarcad c12-13 3x 15 f / / st (tdk) c15 n.c. / / / c16-19 2x 330 f / / st (poscap) l1 1.8 h panasonic smd st d1 stps1l30m st do216aa st d3 stps1l30m st do216aa st q1-q2 sts12nh3ll st so8 st q4-q5 sts25nh3ll st so8 st u1 l6732 st htssop16 st
l6732 l6732 demonstration board 29/37 table 8. other inductor manufacturer manufacturer series inductor va lue (h) saturation current (a) wurth elektronic 744318180 1.8 20 sumida cdep134-2r7mc-h 2.7 15 epcos hpi_13 t640 1.4 22 tdk spm12550t-1r0m220 1 22 toko fda1254 2.2 14 coiltronics hcf1305-1r0 1.15 22 hc5-1r0 1.3 27 table 9. other capacitor manufacturer manufacturer series capacitor value (f) rated voltage (v) tdk c4532x5r1e156m 15 25 c3225x5r0j107m 100 6.3 nippon chemi-con 25ps100mj12 100 25 panasonic ecj4yb0j107m 100 6.3 figure 21. demonstration board efficiency fsw =400khz 75.00% 80.00% 85.00% 90.00% 95.00% 13 579111315 io u t (a ) efficien f sw = 500khz v in = 5v v in = 12v
l6732 demonstration board l6732 30/37 figure 22. top layer figure 23. power ground layer figure 24. signal-ground layer figure 25. bottom layer
l6732 l6732 demonstration board 31/37 7.2 5 a board description and pcb layout l6732 5 a demonstration board realizes in a two layer pcb a step-down dc/dc converter and shows the operation of the device in a general purpose application. the input voltage can range from 4.5 v to 14 v and the output voltage is at 3.3 v. the module can deliver an output current of up to 5 a. the switching frequency is set at 250 khz (controller free- running f sw ) but it can be set to 500 khz acting on the earef pin. compared to the 20 a version, the only difference of this board, compared to the first one, is the presence of a dual mosfet chip, for the high-side and low-side mosfets; besides r15 has been inserted between high side mosfet gate and phase pin; r14 has been inserted between low side mosfet gate and pgnd pin. table 10. demonstration board part list reference value manufacturer package supplier r1 1 k ? neohm smd 0603 ifarcad r2 1 k ? neohm smd 0603 ifarcad r3 4 k7 r4 2 k7 neohm smd 0603 ifarcad r5 0 ? neohm smd 0603 ifarcad r6 n.c. neohm smd 0603 ifarcad r7 4 k99 neohm smd 0603 ifarcad r8 10 ? neohm smd 0603 ifarcad r9 2 k49 neohm smd 0603 ifarcad r10 2.2 ? neohm smd 0603 ifarcad r11 2.2 ? neohm smd 0603 ifarcad r12 n.c. neohm smd 0603 ifarcad r13 10 k neohm smd 0603 ifarcad r14 n.c. neohm smd 0603 ifarcad r15 n.c. neohm smd 0603 ifarcad c1 4.7 nf kemet smd 0603 ifarcad c2 47 nf kemet smd 0603 ifarcad c3 1 nf kemet smd 0603 ifarcad c4 100 nf kemet smd 0603 ifarcad c5 100 nf kemet smd 0603 ifarcad c6 n.c. / / / c7 100 nf kemet smd 0603 ifarcad c8 4.7 f 20 v avx sma6032 ifarcad c9 1 nf kemet smd 0603 ifarcad c10 1 f kemet smd 0603 ifarcad c11 220 nf kemet smd 0603 ifarcad
l6732 demonstration board l6732 32/37 reference value manufacturer package supplier c12-13 3x 10 f/ /st (tdk) c15 n.c. / / / c16-19 2x 330 f/ /st (poscap) l1 2,7 h do3316p-272hc coilcraft smd st d1 stps1l30m st do216aa st d3 stps1l30m st do216aa st q1 sts8dnh3ll (dual mosfet) st so8 st u1 l6732 st htssop16 st figure 26. demonstration board efficiency table 10. demonstration board part list (continued)
l6732 l6732 demonstration board 33/37 figure 27. top layer figure 28. power ground layer
package mechanical data l6732 34/37 8 package mechanical data in order to meet environmental requirements, st offers these devices in ecopack ? packages. these packages have a lead-free second level interconnect. the category of second level interconnect is marked on the package and on the inner box label, in compliance with jedec standard jesd97. the maximum ratings related to soldering conditions are also marked on the inner box label. ecopack is an st trademark. ecopack specifications are available at: www.st.com.
l6732 package mechanical data 35/37 figure 29. htssop16 mechanical data dim. mm. inch min. typ max. min. typ. max. a 1.2 0.047 a1 0.15 0.004 0.006 a2 0.8 1 1.05 0.031 0.039 0.041 b 0.19 0.30 0.007 0.012 c 0.09 0.20 0.004 0.0089 d 4.9 5 5.1 0.193 0.197 0.201 d1 3.0 0.118 e 6.2 6.4 6.6 0.244 0.252 0.260 e1 4.3 4.4 4.5 0.169 0.173 0.177 e2 3.0 0.118 e 0.65 0.0256 k0 80 8 l 0.45 0.60 0.75 0.018 0.024 0.030 tssop16 exposed pad mechanical data 7419276a
revision history l6732 36/37 9 revision history table 11. document revision history date revision changes 20-dec-2005 1 initial release 24-jan-2006 2 improved description of soft-s tart, in case of pre-bias start-up 29-may-2006 3 new template, thermal data updated 26-jun-2006 4 note page 10 deleted 25-sep-2006 5 new demonstration boards section 7: l6732 demonstration board on page 27 04-jun-2008 6 updated: table 4 on page 7 , table 5 on page 9 , section 5.4 on page 12
l6732 37/37 please read carefully: information in this document is provided solely in connection with st products. stmicroelectronics nv and its subsidiaries (?st ?) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described he rein at any time, without notice. all st products are sold pursuant to st?s terms and conditions of sale. purchasers are solely responsible for the choice, selection and use of the st products and services described herein, and st as sumes no liability whatsoever relating to the choice, selection or use of the st products and services described herein. no license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. i f any part of this document refers to any third party products or services it shall not be deemed a license grant by st for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoev er of such third party products or services or any intellectual property contained therein. unless otherwise set forth in st?s terms and conditions of sale st disclaims any express or implied warranty with respect to the use and/or sale of st products including without limitation implied warranties of merchantability, fitness for a parti cular purpose (and their equivalents under the laws of any jurisdiction), or infringement of any patent, copyright or other intellectual property right. unless expressly approved in writing by an authorized st representative, st products are not recommended, authorized or warranted for use in milita ry, air craft, space, life saving, or life sustaining applications, nor in products or systems where failure or malfunction may result in personal injury, death, or severe property or environmental damage. st products which are not specified as "automotive grade" may only be used in automotive applications at user?s own risk. resale of st products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by st for the st product or service described herein and shall not create or extend in any manner whatsoev er, any liability of st. st and the st logo are trademarks or registered trademarks of st in various countries. information in this document supersedes and replaces all information previously supplied. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners. ? 2008 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com

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