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this is information on a product in full production. january 2016 docid026362 rev 3 1/41 L7987L 61 v, 2 a asynchronous step -down switching regulator with adjustable current limitation datasheet - production data features ? 2 a dc output current ? 4.5 v to 61 v operating input voltage ? r ds,on = 300 m typ. ? adjustable f sw (250 khz - 1.5 mhz) ? low iq-shd (11 a typ. from v in ) ? low iq (1 ma typ. - v in 24 v - v out 3.3 v) ? output voltage adjustable from 0.8 v to v in ? synchronization ? adjustable soft-start time ? adjustable curr ent limitation ? advanced bootstrap capacitor management for ldo operation ? v bias improves efficiency at light load ? pgood open collector output ? output voltage sequencing ? digital frequency foldback in short-circuit ? peak current foldba ck in short-circuit ? auto-recovery thermal shutdown applications ? designed for 24 v bus ? fail safe tolerant systems ? programmable logic controllers (plcs) description the L7987L device is a step-down monolithic switching regulator able to deliver up to 2 a dc. the output volta ge adjustability ranges from 0.8 v to almost v in . the embedded switchover feature on the v bias pin maximizes the efficiency at light load. the adjustable current limitation, designed to select the inductor rms current accordingly with the nominal output current, and the high switching frequency capab ility make the size of the application compact. pulse-by-pulse current sensing with digital frequency foldback implements an effective constant current protection over the different application conditions. the peak current foldback decreases the stress of the power components in heavy short-circuit condition. the pgood open collector output can also implement output voltage sequencing during the power-up phase. multiple devices can be synchronized sharing the synch pin to prevent beating noise in low noise applications like sensors with a/d conversion. htssop16 (r th = 40 c/w) www.st.com
contents L7987L 2/41 docid026362 rev 3 contents 1 application schematic and block diagram . . . . . . . . . . . . . . . . . . . . . . . 4 2 pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3 maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4 thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.5 esd protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4 functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.1 oscillator and synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 4.2 soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 4.3 error amplifier and light-load management . . . . . . . . . . . . . . . . . . . . . . . 16 4.4 low vin operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.5 overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.6 overtemperature protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 5 application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.1 input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 5.2 output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5.3 inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 5.4 compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5.4.1 type ii compensation networ k . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 5.4.2 type iii compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5.5 thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.6 layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
docid026362 rev 3 3/41 L7987L contents 41 7 application ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 7.1 positive buck-boost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 7.2 negative buck-boost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 8 package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 8.1 htssop16 package information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 9 ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 10 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
application schematic and block diagram L7987L 4/41 docid026362 rev 3 1 application schematic and block diagram figure 1. application schematic figure 2. block diagram 9,1 287 *1' )% %227 3*22' (1 66 /2*,& dqg &21752/ 89/2 95(* (1$%/( 5() dqg %,$6 62)7 67$57 ($ 7(03 021,725 +6 '5,9(5 /6 '5,9(5 &855(17 6(16,1* 3* &203 3:0 &203 9&& 9%,$6 %227 &+$5*( &203 )6: 6<1&+ ,/,0 , 66 9 5$03 ,/,0 )81&7,21 )% )% ,616 26&,//$725 $1'5$03 9 5$03 0$67(5 6/$9( (1 (1 $0
docid026362 rev 3 5/41 L7987L pin settings 41 2 pin settings 2.1 pin connection figure 3. pin connection (top view)
pin settings L7987L 6/41 docid026362 rev 3 2.2 pin description table 1. pin description number pin description 1 vbias auxiliary input that can be used to supply part of the analog circuitry to increase the efficiency at light load. typically connected to the regulate d output voltage or to an external voltage rail higher than 3 v. connect to the signal gnd if not used or bypass with a 1 ? f ceramic capacitor if supplied by the output vo ltage or by an auxiliary rail. 2 vin dc input voltage 3 vin dc input voltage 4vcc filtered dc input voltage to the internal circuitry. bypass to the signal gnd by a 1 ? f ceramic capacitor. 5 en active high enable pin. connect to the vcc pin if not used. 6ss an internal current generator (5 a typ.) charge s the external capacitor to implement the soft- start. 7 synch master / slave synchronization 8 comp output of the error amplifier. the designed compensation network is connected at this pin. 9 fb inverting input of the error amplifier. 10 fsw a pull-down resistor to g nd selects the switching frequency. 11 ilim a pull-down resistor to gnd selects the peak current limitation. 12 pgood the pgood open collector output is driven low when the output voltage, sensed on the fb pin, is out of regulation. 13 lx switching node 14 lx switching node 15 boot connect an external capacitor (100 nf typ.) between boot and lx pins. the gate charge required to drive the internal n-dmos is recove red by an internal regulator during the off-time 16 gnd signal gnd - e.p. exposed pad must be connected to signal gnd.
docid026362 rev 3 7/41 L7987L pin settings 41 2.3 maximum ratings 2.4 thermal data 2.5 esd protection table 2. absolute maximum ratings symbol description min. max. unit v in -0.3 61 v v cc -0.3 61 v boot v boot - gnd -0.3 65 v v boot - v lx -0.3 4 v v bias -0.3 v cc v en -0.3 v cc v pgood -0.3 v cc v lx -0.3 v in + 0.3 v synch -0.3 5.5 v ss -0.3 3.6 v fsw -0.3 3.6 v comp -0.3 3.6 v i lim -0.3 3.6 v fb -0.3 3.6 v t j operating temperature range -40 150 c t stg storage temperature range -65 150 c t lead lead temperature (soldering 10 sec.) 260 c i hs high-side rms current 2 a table 3. thermal data symbol parameter value unit r thja thermal resistance junction-ambient (device soldered on the stmicroelectronics ? demonstration board) 40 c/w table 4. esd protection symbol test condition value unit esd hbm 2 kv cdm 500 v
electrical characteristics L7987L 8/41 docid026362 rev 3 3 electrical characteristics all the population tested at t j = 25 c, v in = v cc = 24 v and v en = 3 v unless otherwise specified. table 5. electrical characteristics symbol parameter test condition min. typ. max. unit v in operating input voltage range (1) 4.5 61 v r dson hs high-side rdson i sw = 0.5 a 0.30 0.48 ? i sw = 0.5 a (1) 0.30 0.57 ? f sw switching frequency fsw floating 233 250 267 khz fsw floating (1) 225 250 275 khz selected switching frequency r fsw = 10 k ? 1350 1500 1650 khz i pk peak current limit r ilim = 27 k ? ; v fb = 0.6 v (2) 2.65 3.05 3.45 a selected peak current limit r ilim = 100 k ? ; v fb = 0.6 v (2) 0.68 0.85 1.01 a i skip pulse skipping peak current (2) 0.5 a v fold feedback foldback level (3) 400 mv t onmax maximum on time 12 s t onmin minimum on time 120 150 ns t offmin minimum off time (3) 360 ns vcc / vbias v cch v cc uvlo rising threshold (1) 3.85 4.10 4.30 v v cchyst v cc uvlo hysteresis (1) 160 250 340 mv swo v bias threshold switch internal supply from v cc to v bias . v bias ramping up from 0 v. (1) 2.84 2.90 2.96 v hysteresis (3) 80 mv v cc - v bias threshold switch internal supply from v cc to v bias . v in = v cc = 24 v, v bias falling from 24 v to gnd. (1) 3.35 4.05 4.90 v hysteresis (3) 750 mv power consumption i shtdwn shutdown current from v in v en = gnd 11 16 a i quiesc quiescent current from v in and v cc lx floating, v fb = 1 v, v bias = gnd, fsw floating 2.5 3.4 ma i qopvin quiescent current from v in and v cc lx floating, v fb = 1 v, v bias = 3.3 v, fsw floating 1.0 1.4 ma i qopvbias quiescent current from v bias 1.6 2.4 ma
docid026362 rev 3 9/41 L7987L electrical characteristics 41 enable v en device off level 0.06 0.30 v device on level 0.35 0.90 v soft-start t sssetup soft start setup time delay from uvlo rising to switching activity (3) 640 s i ss ch c ss charging current v ss = gnd 4.3 5.0 5.7 a error amplifier v fb voltage feedback 0.792 0.800 0.808 v v fb voltage feedback (1) 0.788 0.800 0.812 v v comph v fb = gnd; v ss = 3.2 v 3.2 3.35 3.5 v v compl v fb = 1 v; v ss = 3.2 v 0.1 v i fb fb biasing current v fb = 3.6 v 5 50 na i osource v fb = gnd; ss pin floating; v comp = 2 v (3) 3.1 ma i osink output stage sinking capability unity gain buffer configuration (fb connected to comp).comp voltage variation due to i osink injection lower than 0.1 v fb . (3) 5ma a v0 error amplifier gain (3) 100 db gbwp unity gain buffer configuration (fb connected to comp). no load on comp pin. (3) 23 mhz synchronization (fan out: 5 slave devices max.) f syn min synchronization frequency fsw floating 280 khz v synout master output amplitude i load = 4 ma 2.45 v i load = 0 a; pin synch floating 3.8 v synow output pulse width i load = 0 a; pin synch floating 155 225 275 ns v synih synch slave high level input threshold 2.0 v v synil synch slave low level input threshold 1.0 v i syn slave synch pull-down current v synch = 5 v 400 650 900 a v syniw input pulse width 200 ns pgood v pgdth pgood rising threshold v fb rising 0.67 0.70 0.73 v v pgdhyst pgood hysteresis v fb falling (3) 30 mv table 5. electrical ch aracteristics (continued) symbol parameter test condition min. typ. max. unit
electrical characteristics L7987L 10/41 docid026362 rev 3 vp gdlow pgood low level ipgd = 1 ma, v fb = gnd 30 mv i pgdlkg pgood leakage current v pgood = 61 v; v fb = 0.8 v 0.1 a thermal shutdown t shdwn thermal shutdown temperature (3) 170 c t hys thermal shutdown hysteresis (3) 15 c 1. specifications referred to t j from -40 to +125 c. specifications in the - 40 to +125 c temperature range are assured by design, characterization and statistical correlation. 2. parameter tested in static condition during testing phase. parameter value may change over dynamic application condition. 3. not tested in production. table 5. electrical ch aracteristics (continued) symbol parameter test condition min. typ. max. unit
docid026362 rev 3 11/41 L7987L functional description 41 4 functional description the L7987L device is based on a voltage mode, constant frequency control loop. the output voltage vout, sensed by the feedback pin (fb) , is compared to an internal reference (0.8 v) providing an error signal on the comp pin. the comp voltage level is then compared to a fixed frequency sawtooth ramp, which finally controls the on- and off-time of the power switch. the main internal blocks are shown in the block diagram in figure 2 on page 4 and can be summarized as follow. ? the fully integrated oscillator that provides the sawtooth ramp to modulate the duty cycle and the synchronization signal. its sw itching frequency can be adjusted by an external resistor. the input volt age feed-forward is implemented. ? the soft-start circuitry to limit inrush current during the start-up phase. ? the voltage mode error amplifier. ? the pulse width modulator and the relative logic circuitry necessary to drive the internal power switch. ? the high-side driver for embedded n-channel power mosfet switch and bootstrap circuitry. a dedicated high resistance low-side mosfet, for anti-boot discharge management purposes, is also present. ? the peak current limit sensing block, with programmable threshold, to handle overload and short-circuit conditions including current foldback and a thermal shutdown block, to prevent thermal runaway. ? the voltage regulator and internal reference, to supply the internal circuitry and provide a fixed internal reference. the switchover function from vcc to vbias can be implemented for higher efficiency. this block also implements a voltage monitor circuitry (uvlo) that checks th e input and internal voltages. ? the output voltage monitor circuitry which releases the pgood signal if the sensed output voltage is above 87% of the target value. 4.1 oscillator and synchronization figure 4 shows the block diag ram of the oscillator circuit. t he internal oscillator provides a constant frequency clock, whose frequency d epends on the resistor externally connected between the fsw pin and ground. figure 4. oscillator and synchronization
functional description L7987L 12/41 docid026362 rev 3 if the fsw pin is left floating, the programmed frequency is 250 khz (typ.); if the fsw pin is connected to an external resistor the programmed switching frequency can be increased up to 1.5 mhz, as shown in figure 5 . the required r fsw value (expressed in k ? ) is estimated by equation 1 : equation 1 figure 5. switching frequency programmability to improve the line transient performance, ke eping the pwm gain const ant versus the input voltage, the input voltage feed-forward is implemented by changing the slope of the sawtooth ramp, according to the input voltage change ( figure 6 a). the slope of the sawtooth also changes if the oscillator fr equency is prog rammed by the external resistor. in this way a freq uency feed-forward is implemented ( figure 6 b) in order to keep the pwm modulator gain constant versus the switching frequency. on the synch pin the synchronization signal is generated. this signal has a phase shift of 180 with respect to the clock. this delay is useful when two devices are synchronized connecting the synch pins together. when synch pins are connec ted, the device with a higher oscillator frequency works as master, so the slave device switches at the frequency of the master but with a delay of half a period. this helps reducing the rms current flowing through the input capacitor. up to five l7987 ls can be connected to the same synch pin; however, the clock phase shift from master switching frequency to slaves input clock is 180. the L7987L device can be synchronized to work at a higher frequency, in the range 250 khz - 1500 khz, providing an external clock si gnal on the synch pin. the synchronization changes the sawtooth amplitude, also affecting the pwm gain ( figure 6 c). this change must be taken into account when the loop stabi lity is studied. in order to minimize the change of pwm gain, the free running frequency should be set (with a resistor on the fsw pin) only slightly lower than the external clock frequency. f sw 250khz 12500 r fsw ---------------- + =
docid026362 rev 3 13/41 L7987L functional description 41 this pre-adjusting of the slave ic switching frequency keeps the truncation of the ramp sawtooth negligible. in case two or more (up to five) L7987L synch pins are tied together, the L7987L ic with higher programmed switching frequency is typically the master device; however, the synch circuit is also able to synchronize with a slightly lower external frequency, so the frequency pre-adjustment with the same resistor on the fsw pin, as suggested above, is required for a proper operation.
functional description L7987L 14/41 docid026362 rev 3 figure 6. feed-forward
docid026362 rev 3 15/41 L7987L functional description 41 4.2 soft-start the soft-start is essential to assure a correct and safe startup of the step-down converter. it avoids inrush current surge and makes the output voltage increase monotonically. the soft-start is performed by charging an ex ternal capacitor, connected between the ss pin and ground, with a constant current (5 a typ.). the ss voltage is used as reference of the switching regulator and the output voltage of the converter tracks the ramp of the ss voltage. when the ss pin voltage reaches 0.8 v level, the error amp lifier switches to the internal 0.8 v 1% reference to regulate the output voltage. figure 7. soft-start during the soft-start period the current limit is set to the nominal value. the dv ss /dt slope is programmed in agreement with equation 2 : equation 2 before starting the c ss capacitor charge, the soft-start circuitry turns-on the discharge switch shown in figure 7 for t ssdisch minimum time, in order to completely discharge the c ss capacitor. as a consequence, the maximum value for the soft -start capacitor, whic h assures an almost complete discharge in case of en signal toggle, is provided by: equation 3 given t ssdisch = 530 ? s and r ssdisch = 380 ? typical values. c ss i ss t ss ? v ref ------------------------ 5 ? at ss ? 0.8v --------------------------- == c ss max ? t ssdisch 5r ssdisch ? ------------------------------------ 270nf ? ?
functional description L7987L 16/41 docid026362 rev 3 the enable feature allows to put the device in to standby mode. with the en pin lower than the device off level the device is disabled and the power consumption is reduced to less than 11 ? a (typ.). with the en pin higher than the de vice on level the device is enabled. if the en pin is left floating, an internal pull-do wn current ensures that the voltage at the pin reaches the inhibit threshold and the device is disabled. the pin is also vcc compatible. 4.3 error amplifier an d light-load management the error amplifier (e/a) provides the error signal to be compared with the sawtooth to perform the pulse width modulation. its non inverting input is internally connected to a 0.8 v voltage reference and its inverting input (fb) and output (comp) are externally available for feedback and frequency compensation. in this device the error amplifier is a voltage mode operational amplifier, therefore, with high dc gain and low output impedance. the uncompensated error amplifier characteristics are summarized in table 6 . in continuous conduction working mode (ccm), the transfer function of the power section has two poles due to the lc filter and one ze ro due to the esr of the output capacitor. different kinds of compensation networks c an be used depending on the esr value of the output capacitor. if the zero introduced by the output capacitor he lps to compensate the double pole of the lc filter, a type ii compensation network can be used. otherwise, a type iii compensation network must be used (see section 5.4 on page 23 for details on the compensation network design). in case of light load (i.e.: if the output current is lower than the half of the inductor current ripple) the L7987L device enters pulse-skipping working mode. the hs mos is kept off if the comp level is below 200 mv (typ.); when this bottom level is reached the integrated switch is turned on until th e inductor current reaches i skip value. so, in discontinuous conduction working mode (dcm), the hs mos on- time is only related to the time necessary to charge the inductor up to i skip level. due to current sensing comparator delay, the actual inductor charge current is slightly impacted by v in and inductance level. in order to let the bootstrap capacitor recharge, in case of extremely light load the L7987L is able to pull-down the lx net through an integrated small ls mos. in this way the bootstrap recharge current can flow from v in through c boot , lx and the ls mos. this mechanism is activated if the hs mos has been kept tu rned-off for more than 3 ms (typ.). table 6. error amplifier characteristics parameters value low frequency gain (a0) 100 db gbwp 23 mhz output voltage swing 0 to 3.5 v source/sink current capability 2 ma / 5 ma
docid026362 rev 3 17/41 L7987L functional description 41 4.4 low vin operation in normal operation (i.e.: vout programmed lower than input voltage) when the hs mos is turned off, a minimum off time (t offmin ) interval is performed. in case the input voltage falls close or below the programmed output voltage (low dropout, ldo) the L7987L control loop is able to keep the boot capacitor properly charged by limiting the hs mos on time to t onmax . when this limit is reached th e hs mos is turned-off and a pull-down resistor between lx and gnd is turned on until one of the following conditions is met: ? a negative current limit (300 ma typ.) is reached ? a timeout (1 ? s typ.) is reached. so doing the L7987L device is able to work in low dropout operation, due to the advanced boot capacitor management. 4.5 overcurrent protection the L7987L device implements an overcurrent protection by sensing the current flowing through the power mosfet. due to the noise creat ed by the switching activity of the power mosfet, the current sensing circuitry is disabled during the initial phase of the conduction time. this avoids an erroneous detection of a fault condition. this interval is generally known as ?masking time? or ?blanking ti me?. the masking time is about 120 ns. if the overcurrent limit is reached, the powe r mosfet is turned off implementing pulse-by- pulse overcurrent protection. in the overcurrent condition, the device can skip turn-on pulses in order to keep the inductor current constant and equal to the current limit, assuming only a slight drift due to input and output voltage variation. if, at the end of the ?masking time?, the curren t is higher than the overcurrent threshold, the power mosfet is turned off and one pulse is skipped. if, at the following switching on, when the ?masking time? ends, th e current is still higher than t he overcurrent threshold, the device skips two pulses. this mechanism is r epeated and the device can skip up to seven pulses (refer to figure 8 ). if at the end of the ?masking time? the current is lower than the overcurrent threshold, the number of skipped cycles is decreased by one unit. as a consequence, the overcurrent/short-circuit protection acts by switching off the power mosfet and reducing the switching frequency dow n to one eighth of the default switching frequency, in order to keep constant the output current close to the current limit.
functional description L7987L 18/41 docid026362 rev 3 figure 8. ocp and frequency scaling if the sensed output voltage, monitored through fb pin, falls below the v fold threshold (400 mv typ.) the peak current limit threshold is reduced to 1/3 of the nominal value. this additional feature helps to reduce the ic stress in case of output short-circuit. as soon as the fb pin increases above the v fold threshold, the fu ll peak current limit threshold is restored. this foldback protec tion is disabled during the soft-start. this kind of overcurrent protection is effective if the inductor can be completely discharged during hs mos turn-off time, in order to avoid the inductor current to run away. in case of output short-circuit the maximum swit ching frequency can be computed by equation 4 . equation 4 assuming v f = 0.6 v the freewheeling diode direct voltage, r dcr = 70 m ? the inductor parasitic resistance, i lim = i pk = 0.9 a the peak current limit during foldback protection, r on = 0.30 ? the hs mos resistance and t on,min = 120 ns the minimum hs mos on duration, the maximum fsw frequency which avoids the inductor current run away in case of output short-circuit and v in = 61 v is 728 khz. if the programmed switching frequency is higher than the above computed limit, an estimation of the inductor current in case of output short-circuit fault is provided by equation 5 : equation 5 f sw max ? 8 ? v f r dcr i ? lim + ?? v in r on r dcr + ?? ? ? i lim ---------------------------------------------------------------------- 1 t on min ? ---------------------- ? ? i lim f sw t on v in 8v f ? ? ? ? 8r dcr f sw t on min ? ? r on r dcr + ?? ? + ? ---------------------------------------------------------------------------------------------------------------------- =
docid026362 rev 3 19/41 L7987L functional description 41 the peak current limit threshold (i lim ) can be programmed in the range 0.85 a - 3.0 a by selecting the proper r ilim resistor, as suggested in equation 6 : equation 6 in any case, the maximum high-side mos rms current must not be allowed to exceed the value shown in table 2 on page 7 . figure 9. current limit and programming resistor the minimum programmed current limit can't be lower than i skip = 0.5 a (typical), also in case of foldba ck detection. 4.6 overtemperature protection it is recommended that the device never exceeds the maximum allowable junction temperature. this temperature increase is main ly caused by the total power dissipated from the integrated power mosfet. to avoid any damage to the device when reaching high temperature, the L7987L device implements a thermal shutdown feature: when the junction temperature reaches 170 c (typ.) the device turns off the power mosfet and shuts down. when the junction temperature drops to 155 c (typ.), the device restarts with a new soft- start sequence. r ilim 20k ? i pk i lim --------- - ? =
application information L7987L 20/41 docid026362 rev 3 5 application information 5.1 input capacitor selection the input capacitor must be rated for the maximum input operating voltage and the maximum rms input current. since the step-down converters input current is a sequence of pulses from 0 a to i out , the input capacitor must absorb the equivalent rms current which can be up to the load current divided by two (worst case, with duty cycle of 50%). for this reason, the quality of these capacitors must be very high to minimize th e power dissipation generated by the internal esr, thereby improving system reliability and efficiency. the rms input current (flowing through the input capacitor) is roughly estimated by: equation 7 actual dc/dc conversion duty cycle, d = v out /v in , is influenced by a few parameters: equation 8 where v f is the freewheeling diode forward voltage and v sw the voltage drop across the internal high-side mosfet. considering the range d min to d max it is possible to determine the maximum i cin,rms flowing through the input capacitor. the input capacitor value must be dimensioned to safely handle the input rms current and to limit the vin and vcc ramp-up slew-rate to 0.5 v/ ? s maximum, in order to avoid the device active esd protections turn-on. different capacitors can be considered: ? electrolytic capacitors these are the most commonly used due to their low cost and wide range of operative voltage. the only drawback is that, considerin g ripple current rating requirements, they are physically larger than other capacitors. ? ceramic capacitors if available for the required value and volt age rating, these capacitors usually have a higher rms current rating for a given physical dimension (due to the very low esr). the drawback is their high cost. ? tantalum capacitors small, good quality tantalum capacitors with very low esr are becoming more available. however, th ey can occasionally burn if subj ected to very hi gh current, for example when they are connected to the power supply. i cin rms ? i out d1d ? ?? ? ? ? + v in min ? v sw max ? ? --------------------------------------------------- - = d min v out v f + v in max ? v sw min ? ? --------------------------------------------------- - =
docid026362 rev 3 21/41 L7987L application information 41 the amount of the input voltage ripple can be roughly overestimated by equation 9 . equation 9 in case of mlcc ceramic in put capacitors, the equivalent series resistance (r es,in ) is negligible. in addition to the above considerations, a ceramic capacitor with an appropriate voltage rating and with a value 1 ? f or higher should always be placed across vin and power ground and across vcc and the ic gnd pins, as close as possible to the L7987L device. this solution is necessary for spike filtering purposes. 5.2 output capacitor selection the output capacitor is very important in order to satisfy the output voltage ripple requirement. using a small inductor value is useful to reduce the size of the choke but increases the current ripple. so , to reduce the output voltage ripple, a low esr capacitor is required. nevertheless, the esr of the output capacitor introduces a zero in the open loop gain, which helps to increase the phase margin of the system. if the zero goes to very high frequency, typical drawback in case of cera mic output capacitor application, a type iii compensation network must be designed. the current in the output capacitor has a triangular waveform which generates a voltage ripple across it. this ripple is due to the capacitive component (charge and discharge of the output capacitor) and the resistive component ( due to the voltage drop across its esr). so the output capacitor must be selected in order to have a voltage ripple compliant with the application requirements. the amount of the voltage ripple can be estimated starting from the current ripple obtained by the inductor selection. assuming ? i l the inductor current ripple, the output voltage ripple is roughly overestimated by equation 10 . equation 10 usually the resistive component of the ripple is much higher than the capacitive one, if the output capacitor adopted is not a multi-layer ceramic capacitor (mlcc) with very low esr value. the output capacitor is important also for loop st ability: it fixes the double lc filter pole and the zero due to its esr. the output capacitor is also the key component that provides the current to the load during a load transient which exceeds the system ban dwidth. so, if the high slew rate load transient is required by the application, the ou tput capacitor must be designed in order to sustain the load transient or absorbs the energy stored in the inductor until the converter reacts. in fact, even if the controller detects immediatel y the load variation and sets the duty cycle at 100% or 0%, the output current slope is limited by the inductor value, the input and output voltage. v in pp ? d1d ? ?? i out ? ? c in f sw ? ---------------------------------------------- r es in ? i out ? + = ? v out pp ? ? i l r es out ? ? i l 8f sw c ? out ? ----------------------------------------- + ? ?
application information L7987L 22/41 docid026362 rev 3 the output voltage has a drop or overshoot that depends on the esr and capacitive charge/discharge, as roughly estimated in equation 11 : equation 11 where ? v l is the voltage applied to the inductor during the load appliance or load release. equation 12 mlcc capacitors have typically low esr to minimize the ripple but also have low capacitance that does not minimize the volta ge deviation during dynamic load variations. electrolytic capacitors, on the other hand, have a large capacitance which minimizes voltage deviation during load transients whereas they do not show the same esr values as the mlccs, resulting then in higher ripple voltages. a mix between an electrolytic and mlcc capacito r can be used to minimize ripple as well as reducing voltage deviation in dynamic mode. the high bandwidth error amplifier of the L7987L and external compensation feature let design a wide range of output filter configur ations (including all mlcc solutions) and perform fast transient response. 5.3 inductor selection the inductance value fixes the current ripple flowing through the output capacitor. so the minimum inductance value, in order to have the expected current ripple, must be selected. the rule to fix the current ripple value is to have a ripple at 20% - 40% of the output current. in the continuous conduction mode (ccm), the required inductance value can be calculated by equation 13 : equation 13 in order to guarantee a maximum current ripple in every condition, equation 13 must be evaluated in case of maximum input voltage, assuming v out fixed. increasing the value of the inductance help to reduce the current ripple but, at the same time, strongly impacts the conv erter response time to a dynamic load change. the response time is the time required by the inductor to change its current from the initial to the final value. until the inductor has finished its charging (or discharging) time, the output current is supplied (or recovered) by the output capacitors. further, if the compensation network is proper ly designed, during a load variation the device is able to properly change the duty cycle so improving the control loop transient response. when this condition is reached the response ti me is only limited by the time required to change the inductor current, basically by v in , v out and l. minimizing the response time, at the end, can he lp to decrease the output filter total cost and to reduce the application area. ? v out lt ? ? i out r es out ? ? i out l ? i out ? 2c out ? v l ? ? ---------------------------------------- ? + ? ? ? v l d max v in v out ? ?? ? v out ? ? ? ? ? ? = l v out 1 v out v in -------------- ? ?? ?? ? ? l l f sw ? ------------------------------------------------- - =
docid026362 rev 3 23/41 L7987L application information 41 5.4 compensation network the compensation network must assure stabilit y and good dynamic pe rformance. the loop of the L7987L device is based on the voltage mode control. the error amplifier is an operational amplifier with high bandwidth. so , by selecting the compensation network the e/a is considered as ideal, that is, its ba ndwidth is much larger than the system one. figure 10. switching regulator control loop simplified model the transfer function of the pwm modulator, from the error amplifier output (comp pin) to the lx pin results in an almost constant gain, due to the voltage feed-forward which generates a sawtooth with amplitude v s directly proportional to the input voltage: equation 14 the synchronization of the device with an external cl ock provided through the synch pin can modify the pwm modulator gain (see section 4.1 on page 11 to understand how this gain changes and how to keep it constant in spite of the external synchronization). 9 6 3:0 ($ 9 ,1 / & 2 5 (6 5 2 5 '& = ) = 6 5 8 5 ' &203 9 5() $0 ,qwhuqdo vzlwfk 3rzhu vhfwlrq &rpshqvdwlrq qhwzrun g pwo v in v s -------- - = 1 k ff -------- - 30 ==
application information L7987L 24/41 docid026362 rev 3 the transfer function of the power section (i.e.: the l-c o filters and the output load) is the ratio of the parallel of c o and r o and the sum of l and the parallel of c o and r o , including l and c o parasitics: equation 15 given l, r dc , c o , r es and r o the parameters shown in figure 10 . the power section transfer function can be rewritten as follows: equation 16 equation 17 equation 18 with the assumption that the in ductor parasitic resistance, r dc , is negligible compared to r o . the closed loop gain is then given by: equation 19 g loop (s) = g lc (s) ? g pwo (s) ? g comp (s) as noted in section 5.2 on page 21 , two different kinds of network can compensate the loop, depending on the value of f zesr , lower or higher than the regulator required bandwidth. in section 5.4.1 and section 5.4.2 the guidelines to select the type ii and type iii compensation network are illustrated. ?? ??? ? 2 ' & 2 ( 6 2 ' & ( 6 2 ' & 2 2 ( 6 2 2 ( 6 2 2 ' & 2 ( 6 2 2 ( 6 2 / & 5 5 5 5 & 5 5 & 5 5 & / v 5 5 / & v 5 v & 5 5 v / v & 5 5 v & 5 5 v * ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 2 1 1 1 "! g lc s ?? g lco 1 s 2 ? f zesr ? --------------------------- + 1 s 2 ? q ? f lc ? ------------------------------ - s 2 ? f lc ? -------------------- ?? ?? 2 ++ ---------------------------------------------------------------------------- - ? ; = g lco r o r o r dc + ------------------------- - 1 ? = f zesr 1 2 ? c o r es ? ------------------------------- -; = f lc 1 2 ? lc o r o r es + r o r dc + ------------------------- - ----------------------------------------------------- - 1 2 ? lc o r o r es + r o ------------------------- ----------------------------------------------------- ? = q lc o r o r dc + ? r o r es + ? lc o ? r o r dc r o r es r es r dc ++ ?? + ---------------------------------------------------------------------------------------------------------- - lc o r o ? r o r es + ?? ? lc o r o r es + ------------------------------------------------------------------------- ? =
docid026362 rev 3 25/41 L7987L application information 41 5.4.1 type ii co mpensation network if the equivalent series resistance (r es ) of the output capacitor introduces a zero with a frequency lower than the desired bandwidth (that is: 2 ?? ? r es ? c o > 1 / bw), this zero helps stabilize the loop. elec trolytic capacitors show non-negligible esr (> 30 m ? typically), so with this kind of output capacitor the type ii network combined with the zero of the esr allows to stabilize the loop. figure 11. type ii compensation network the type ii compensation network transfer function, from v out to comp, is computed in equation 20 . equation 20 equation 21 the following suggestions can be followed for a quite common compensation strategy, assuming that c p << c f . ? starting from equation 19 , in case of type ii compensation network and electrolytic output capacitors the control loop gain module at s = 2 ? ? f bw allows to fix the r f /r u ratio: equation 22 ($ = ) 5 8 5 ' &203 9 5() 9 287 )% & 3 & ) 5 ) $0 ?? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 1 0 1 2 1 2 2 1 1 ) ( 1 1 ) ( ) ( p p z f p f p f f f u u f compii f s f s f s r c sc c c s r sc r r s z s g ? ? ? ?? f p f p u p f p f f z r c c f r c c f r c f ? ? ? ? ? ? ? ? ? ? ? ? ? ? 2 1 ; 2 1 ; 2 1 1 0 1 ?? 1 1 1 ) 2 ( 2 , ? ? ? ? ? ? ? bw u f zesr lc ff bw ii loop f r r f f k f s g ?
application information L7987L 26/41 docid026362 rev 3 after choosing the regulator bandwidth (typically f bw < 0.2 ? f sw ) and a value for r u , usually between 1 k ? and 50 k ? , in order to achieve c f and c p not comparable with parasitic capacitance of the board, the r f required value is computed by equation 22 . ? select c f in order to place f z1 below f lc (typically 0.1 ? f lc ) ? select c p in order to place f p1 at 0.5 ? f sw equation 23 the resultant control loop and other transfer functions gain are shown in figure 12 . figure 12. type ii compensation - bode plot c f 1 2 ? r f 0.1 f lc ? ? ? ----------------------------------------------- - c p ; 1 2 ? r f 0.5 f sw ? ? ? ------------------------------------------------- ==
docid026362 rev 3 27/41 L7987L application information 41 5.4.2 type iii compensation network if f zesr is higher than the target loop bandwidth, as usually happens if the output filter is based on mlcc ceramic capacitors, a type iii compensation network must be designed. figure 13. type iii compensation network the type iii compensation network transfer fu nction, from vout to comp, is computed in equation 24 . equation 24 in addition to what shown in equation 21 , two more singularities are proper of this compensation network: equation 25 ($ = ) 5 8 5 ' &203 9 5() 9 287 )% & 3 & ) 5 ) = 6 5 6 & 6 $0 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 2 1 0 2 1 2 1 2 1 2 2 1 2 1 1 ) ( // ) ( ) ( p p p z z s u f compiii f s f s f s f s f s s z r s z s g ? ? ? ? ? f z2 1 2 ? c s ? r u r s + ?? ? ------------------------------------------------------ -; = f p2 1 2 ? c s r s ? --------------------------- - =
application information L7987L 28/41 docid026362 rev 3 the following suggestions can be followed for a quite common compensation strategy, assuming that c p << c f and r s << r u . ? starting from equation 19 on page 24 , in case of type iii compensation network and mlcc ceramic output capacitors the control loop gain module at s = 2 ?? ? f bw allows to fix the r f /r u ratio: equation 26 after choosing the regulator bandwidth (typically f bw < 0.2 ? f sw ) and a value for r u , usually between 1 k ? and 50 k ? , in order to achieve c f and c p not comparable with parasitic capacitance of the board, the r f required value is computed by equation 26 . ? select c f in order to place f z1 below f lc (typically 0.1 ? f lc ) ? select c p in order to place f p1 at 0.5 ? f sw equation 27 ? select c s in order to place f z2 at f lc ? select r s in order to place f p2 at 0.5 ? f sw equation 28 1 1 ) 2 ( , ? ? ? ? ? ? u f bw lc ff bw iii loop r r f f k f s g ? c f 1 2 ? r f 0.1 f lc ? ? ? ----------------------------------------------- -; = c p 1 2 ? r f 0.5 f sw ? ? ? ------------------------------------------------- = c s 1 2 ? r u f lc ? ? ---------------------------------- ; = r s 1 2 ? c s 0.5 f sw ? ? ? ------------------------------------------------- - =
docid026362 rev 3 29/41 L7987L application information 41 the resultant control loop and other transfer functions gain are shown in figure 14 . figure 14. type iii compensation - bode plot 5.5 thermal considerations the thermal design is important to prevent the thermal shutdown of the device if junction temperature goes above 170 c (typ.). the three different sources of losses within the device are: ? conduction losses due to the non-negligible rdson of the power switch; these are equal to: equation 29 p hs,on = r hs,on ? d ? (i out ) 2 where d is the duty cycle of the applicat ion and the maximum rdson in the full temperature range is 570 m ? . note that the duty cycle is theoretically given by the ratio between v out and v in , but actually it is quite higher in order to compensate the losses of the regulator. so the conduction losses increase compared with the ideal case; ? switching losses due to power mosfet turn on and off; these ca n be calculated as: equation 30 where t rise and t fall are the overlap times of the voltage across the power switch (v ds ) and the current flowing into it during turn on and turn off phases. $0 ($ jdlq * /223,,, * &203,,, * /&,,, i = i /& i = i ](65 i 3 i 3 i %: p hs sw ? v in i out t rise t fall + ?? 2 ------------------------------------------ - ? ? f sw v in i out t tr f sw ? ? ? ? ? =
application information L7987L 30/41 docid026362 rev 3 t tr is the equivalent switching time. for this device the typi cal value for the equivalent switching time is 20 ns. ? quiescent current losses, calculated as equation 31 p q = v in ? i qopvin + v bias ? i qopvbias where i qopvin and i qopvbias are the L7987L quiescent current in case of separate bias supply. if the switchover feature is not used, the ic quiescent current is the only one from vin, i quiesc , as summarized in table 5 on page 8 . the junction temperature t j can be calculated as: equation 32 t j = t a + r th,ja ? p tot where t a is the ambient temperature and p tot is the sum of the power losses just seen. r thja is the equivalent thermal resistance juncti on to ambient of the device; it can be calculated as the parallel of many paths of heat conduction from the junction to the ambient. for this device the path through the exposed pa d is the one conducting the largest amount of heat. the r thja , measured on the demonstration board described in section 5.6 , is about 40 c/w for the htssop16 package. 5.6 layout considerations the pcb layout of the switchi ng dc/dc regulators is very im portant to minimize the noise injected in high impedance nodes and interference generated by the high switching current loops. two separated ground areas must be considered: the signal ground and the power ground. in a step-down converter the input loop (including the input capacitor, the power mosfet and the freewheeling diode) is the most critical one. this is due to t he fact that high value pulsed currents are flowing through it. in order to minimize the emi, this loop must be as short as possible. the input loop, including also the output capacitor, must be referred to the power ground. all the other components are referred to the signal ground. the feedback pin (fb) connection to the external resistor divider is a high impedance node, so the interference can be minimized by placin g the routing of the feedback node as far as possible from the high current paths. to reduc e the pick-up noise, the resistor divider must be placed very close to the device. to filter the high frequency noise, a small bypass capacitor (1 ? f or higher) must be added as close as possible to the input voltage pi n of the device for both vin and vcc pins. thanks to the exposed pad of the device, the ground plane helps to reduce the junction to ambient thermal resistance; so a wide ground plane enhances the thermal performance of the converter, allowing high power conversion. the exposed pad must be connected to the sign al gnd pin. the connection to the ground plane must be achieved by taking care of the above mentioned input loop, in order to avoid high current flowing through the signal gnd. refer to section 6 for the L7987L layout example.
docid026362 rev 3 31/41 L7987L demonstration board 41 6 demonstration board in this section the L7987L demonstration board is described. the default settings are: ? programmed v out = 5 v ? max. i out = 2 a ? f sw = 500 khz ? v bias = v out ? soft-start 5.3 ms figure 15. L7987L demonstration board schematic table 7. L7987L demonstration board component list reference part package note manufacturer p/n c1, c2 4.7 ? f 1210 x7s/100 v/10% tdk c3225x7s2a475k c3, c4 1 ? f 0805 x7s/100 v/10% tdk c2012x7s2a105k c5 47 ? f 1210 x5r/16 v/20% tdk c3225x7s0j476m c6, c12 n. m. c7 100 nf 0603 x7r/16 v c8 680 pf 0603 c0g/50 v c9 6.8 nf 0603 x7r/50 v c10 33 nf 0603 x7r/16 v c11 68 pf 0603 c0g/50 v r1, r10 n. m. r2 100 k ? 0603 r4 0 ? 0603 r5 910 ? 0603 1% tolerance r6 10 k ? 0603 1% tolerance r7 68 k ? 0603 1% tolerance
demonstration board L7987L 32/41 docid026362 rev 3 figure 16. L7987L demonstration board layout (top and bottom) r8 47 k ? 0603 1% tolerance r9 27 k ? 0603 1% tolerance r11 13 k ? 0603 1% tolerance l1 15 ? h 10 x 10 3.86 a sat./50 m ? coilcraft mss1038-153 d1 stps2l60 smb flat 60 v - 2 a schottky rectifier stmicroelectronics stps2l60uf u1 L7987L htssop16 stmicroelectronics L7987L table 7. L7987L demonstration board component list (continued) reference part package note manufacturer p/n
docid026362 rev 3 33/41 L7987L demonstration board 41 figure 17. efficiency vs. output current v out = 5 v, f sw = 500 khz figure 18. junction temperature increase vs. output current. t amb = 25 c figure 19. input quiescent current vs. input voltage. no load figure 20. input shutdown current vs. input voltage figure 21. load regulation, v out = 5v , f sw = 500 khz figure 22. line regulation, v out = 5 v, f sw = 500 khz
application ideas L7987L 34/41 docid026362 rev 3 7 application ideas 7.1 positive buck-boost the L7987L device can implement the step-up/ down conversion with a positive output voltage. figure 23 shows the complete schematic: one power mosfet and one schottky diode are added to the standard buck topology to provide 24 v output voltage, with input voltage from 12 v to 60 v. in this desi gn example, the programmed switching frequency is 570 khz and the maximum expected load is 0.5 a. figure 23. L7987L - positive buck-boost schematic example in this topology the relationship between input and output voltage is: equation 33 so the duty cycle is given by: equation 34 the output voltage isn't limited by the maxi mum operating voltage of the device, because the output voltage is sensed only through the resistor divider. the external power mosfet maximum drain to source voltage must be higher than the output voltage and also the additional diode, d2, must be rated for the same maximum voltage. in figure 23 a clamping network has been added to limit the q1 gate to source voltage (c10, r15 and d5) and to speed up q1 turn-off time (d4). 3 3 3 51 &/ $ o' $ ?' 51 4:/$) $ ?' 6 -- 1(00% 7*/@ 44 4: /$) '48 *-*. $0.1 &/ '# (/% 7$$ -9@ -9@ 7*/@ 7#*"4 #005 51 51 1(00% 51 7*/ $ o' 51 (/% $ ?' $ q' 3 $ o' 51 7065 $ o' 3 $ o' 3 51 (/% % 4514- 2 45-/--) $ ' % 4514- % #"4+ 3 % #;9# $ ?' - ?) ". ' ' y 9 9 , 1 2 8 7 - , 1 2 8 7 2 8 7 9 9 9 '
docid026362 rev 3 35/41 L7987L application ideas 41 the current flowing through th e internal power mosfet is transferred to the load only during the off time, so according to the maximum allowed L7987L dc switch current (2.0 a), the maximum output current for the buck-boost topology can be calculated from equation 35 . equation 35 where i sw is the average current in the embedded power mosfet during the on time. in addition to these constraints, the thermal considerations summarized in section 5.5 on page 29 must also be evaluated. figure 24. buck-boost pcb layout (top and bottom) the transfer function of the power section for buck-boost topology is summarized below: equation 36 with below singularities and parameters: equation 37 $ ' , , 2 8 7 6 : - 2 2 0 1 1 1 ) ( lc lc rhpz z lc s q s s s g s g ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? o lc o rhpz es o z c l d l d d r r c ? ? ? ? ? ? ? ? ? 1 1 1 2 ? ? ?
application ideas L7987L 36/41 docid026362 rev 3 equation 38 the singularity ? rhpz = 2 ? f rhpz , computed at the maximum load and minimum input voltage, is the limitation in the loop bandwidth design. typically the maximum bandwidth, f bw , is designed to be lower than one fourth of the above described singularity, in order to achieve a good phase margin. in case ? z and ? lc are lower than the target bandwidth, a type ii compensation network is enough for loop stabilization, following the compensation strategy described in section 5.4.1 on page 25 . in case ceramic or very low esr elec trolytic output ca pacitors are used, ? z is typically higher than the target f bw so a type iii compensation network is necessary, as described in section 5.4.2 on page 27 . 7.2 negative buck-boost the L7987L device can implement the step-up/ down conversion with a negative output voltage. figure 25 shows the schematic to regulate -12 v at the 1 a maximum load, assuming vin falling in the range 12 v to 48 v. no further external compone nts are added to the standard buck topology. figure 25. L7987L - negative (or inverting) buck-boost schematic example ?? ?? d d v g l c r d q v v v d o o o in o o ? ? ? ? ? ? ? ? ? 1 1 0 51 1(00% $ ?'7 51 &/ 51 4: /$) 3 3 l 8 $ o' $ ? ' 7 3 l 51 7*/ $ o' $ ?' 7 51 (/% $ ? ' 7 $ q' 3 $ o ' 51 (/% 3 3 $ o ' 3 51 7065 % 4514- 6 -- 1(00% 7*/ 44 4: /$) '48 *-*. $0.1 &/ '# (/% 7$$ -9 -9 7*/ 7#*"4 #005 51 3 $ ? ' 7 (/%1-"/& *$(/% - ?) ".
docid026362 rev 3 37/41 L7987L application ideas 41 figure 26. negative (or inverting) buck-boost pcb layout (top and bottom) equation 33 and equation 34 for positive buck-boost can be used to program the output voltage and estimate the working duty cycle, assuming for vout a positive voltage. the other considerations summarized in section 7.1 are also applied to the inverting buck-boost. in this topology the device gnd is shorted to vout, so the resulting voltage stress on the integrated power mos is the sum of vin an d vout. consequently, the maximum input voltage must be lower than: equation 39 v in,max = 61 v is the maximum operating input vo ltage for the L7987L device, as shown in table 5: electrical characteristics on page 8 . therefore, if the output voltage is -12 v, the maximum operating input voltage is close to 49 v, if also the freewheeling diode has the same reverse voltage rating. for control loop stability analysis and comp ensation, the transfer function model and considerations summarized in section 7.1 are also applied to this topology. 2 8 7 0 $ ; , 1 , 1 9 9 7 9 -
package information L7987L 38/41 docid026362 rev 3 8 package information in order to meet environmental requirements, st offers these devices in different grades of ecopack ? packages, depending on their level of environmental compliance. ecopack specifications, grade definitions a nd product status are available at: www.st.com . ecopack is an st trademark.
docid026362 rev 3 39/41 L7987L package information 41 8.1 htssop16 package information figure 27. htssop16 package outline table 8. htssop16 package mechanical data symbol dimensions (mm) min. typ. max. a 1.20 a1 0.15 a2 0.80 1.00 1.05 b 0.19 0.30 c 0.09 0.20 d 4.90 5.00 5.10 d1 2.80 3.00 3.20 e 6.20 6.40 6.60 e1 4.30 4.40 4.50 e2 2.80 3.00 3.20 e0.65 l 0.45 0.60 0.75 l1 1.00 k 0.00 8.00 aaa 0.10
ordering information L7987L 40/41 docid026362 rev 3 9 ordering information 10 revision history table 9. order codes order code package packaging L7987L htssop16 tube L7987Ltr htssop16 tape and reel table 10. document revision history date revision changes 15-may-2014 1 initial release. 03-jun-2014 2 updated main title: 61 v, 2 a asynchronous step-down switching regulator with adjustable current limitation on page 1 (replaced ?3 a? by ?2 a?). 12-jan-2016 3 updated section : features on page 1 (replaced ?low dropout operation...? by ?advanced bootstrap capacitor management for ldo operation?) and section : description on page 1 (added ?almost?, removed ?the wide input voltage ... systems?). updated section 3: electrical characteristics on page 8 (replaced by new section). updated section 4.2: soft-start on page 15 (replaced ?0.32 v? by ?device off level? and ?1.16 v? by ?device on level?). updated section 4.4: low vin operation on page 17 (updated text, removed ?and the effective maximum duty cycle ... 92 % ?) . updated section 4.5: overcurrent protection on page 17 (replaced ?3.6 a? by ?3.0 a?, removed ?i pk = 3.6 a ... not mounted? sentence). updated figure 12 on page 26 (replaced by new figure). updated table 7 on page 31 (updated l1 component). added figure 17 to figure 22 to section 6: demonstration board on page 31 . added section 7: application ideas on page 34 . minor modifications throughout document.
docid026362 rev 3 41/41 L7987L 41 important notice ? please read carefully stmicroelectronics nv and its subsidiaries (?st?) reserve the right to make changes, corrections, enhancements, modifications, and improvements to st products and/or to this document at any time without notice. purchasers should obtain the latest relevant in formation on st products before placing orders. st products are sold pursuant to st?s terms and conditions of sale in place at the time of o rder acknowledgement. purchasers are solely responsible for the choice, selection, and use of st products and st assumes no liability for application assistance or the design of purchasers? products. no license, express or implied, to any intellectual property right is granted by st herein. resale of st products with provisions different from the information set forth herein shall void any warranty granted by st for such product. st and the st logo are trademarks of st. all other product or service names are the property of their respective owners. information in this document supersedes and replaces information previously supplied in any prior versions of this document. ? 2016 stmicroelectronics ? all rights reserved

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