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this is information on a product in full production. august 2013 docid022801 rev 5 1/42 42 A7986A 3 a step-down switching regulator for automotive applications datasheet - production data features ? qualified following aec-q100 requirements ? 3 a dc output current ? 4.5 v to 38 v input voltage ? output voltage adjustable from 0.6 v ? 250 khz switching frequency, programmable up to 1 mhz ? internal soft-start and enable ? low dropout operation: 100% duty cycle ? voltage feed-forward ? zero load current operation ? overcurrent and thermal protection ? hsop8 package applications ? dedicated to automotive applications ? automotive led driving description the A7986A is a step-down switching regulator with a 3.7 a (min.) current limited embedded power mosfet, so it is able to deliver up to 3 a current to the load depending on the application conditions. the input voltage can range from 4.5 v to 38 v, while the output voltage can be set starting from 0.6 v to v in . requiring a minimum set of external components, the device includes an internal 250 khz switching frequency oscillator that can be externally adjusted up to 1 mhz. the hsop8 package with exposed pad allows the reduction of r th(ja) down to 40 c/w. figure 1. application circuit hsop8 exposed pad www.st.com
contents A7986A 2/42 docid022801 rev 5 contents 1 pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4 electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5 functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5.1 oscillator and synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.2 soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 5.3 error amplifier and compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.4 overcurrent protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.5 enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5.6 hysteretic thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 6 application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.1 input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.2 inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6.3 output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 6.4 compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.4.1 type iii compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.4.2 type ii compensation network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 6.5 thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.6 layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 6.7 application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 7 application ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 7.1 positive buck-boost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 7.2 inverting buck-boost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
docid022801 rev 5 3/42 A7986A contents 8 package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 9 ordering information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 10 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
pin settings A7986A 4/42 docid022801 rev 5 1 pin settings 1.1 pin connection figure 2. pin connection (top view) 1.2 pin description table 1. pin description n. type description 1 out regulator output. 2 synch master/slave synchronization. when it is left floating, a signal with a phase shift of half a period in respect to the power turn-on is present at the pin. when connected to an external signal at a frequency higher than the internal one, the device is synchronized by the external signal, with zero phase shift. connecting together the synch pins of two devices, the one with the higher frequency works as master and the other as slave; so the two powers turn-ons have a phase shift of half a period. 3en a logical signal (active high) enables the device. with en higher than 1.2 v the device is on and with en lower than 0.3 v the device is off. 4 comp error amplifier output to be used for loop frequency compensation. 5fb feedback input. connecting the output voltage directly to this pin the output voltage is regulated at 0.6 v. to have higher regulated voltages an external resistor divider is required from v out to the fb pin. 6f sw the switching frequency can be increased connecting an external resistor from the fsw pin and ground. if this pin is left floating the device works at its free-running frequency of 250 khz. 7 gnd ground. 8v cc unregulated dc input voltage.
docid022801 rev 5 5/42 A7986A maximum ratings 2 maximum ratings 3 thermal data table 2. absolute maximum ratings symbol parameter value unit v cc input voltage 45 v out output dc voltage -0.3 to v cc f sw , comp, synch analog pin -0.3 to 4 en enable pin -0.3 to v cc fb feedback voltage -0.3 to 1.5 p tot power dissipation at t a < 60 c hsop8 2 w t j junction temperature range -40 to 150 c t stg storage temperature range -55 to 150 c table 3. thermal data symbol parameter value unit r th(ja) maximum thermal resistance junction-ambient (1) 1. package mounted on demonstration board. hsop8 40 c/w
electrical characteristics A7986A 6/42 docid022801 rev 5 4 electrical characteristics t j = - 40 c to 125 c, v cc = 12 v, unless otherwise specified. table 4. electrical characteristics symbol parameter test conditions values unit min. typ. max. v cc operating input voltage range 4.5 38 v v ccon turn-on v cc threshold 4.5 v cchys v cc uvlo hysteresis 0.1 0.4 r ds(on) mosfet on-resistance 200 400 m i lim maximum limiting current t j = 25 c 3.7 5.2 a 3.5 5.2 oscillator f sw switching frequency 210 250 275 khz v fsw fsw pin voltage 1.254 v d duty cycle 0 100 % f adj adjustable switching frequency r fsw = 33 k 1000 khz dynamic characteristics v fb feedback voltage 4.5 v< v cc < 38 v 0.588 0.6 0.612 v dc characteristics i q quiescent current duty cycle = 0, v fb = 0.8 v 2.4 ma i qst-by total standby quiescent current 20 30 a enable v en en threshold voltage device off level 0.3 v device on level 1.2 i en en current en = v cc 7.5 10 a soft-start t ss soft-start duration fsw pin floating 7.3 8.2 9.8 ms f sw = 1 mhz, r fsw = 33 k 2 error amplifier
docid022801 rev 5 7/42 A7986A electrical characteristics v ch high level output voltage v fb < 0.6 v 3 v v cl low level output voltage v fb > 0.6 v 0.1 i o source source comp pin v fb = 0.5 v, v comp = 1 v 19 ma i o sink sink comp pin v fb = 0.7 v, v comp = 0.75 v 30 ma g v open loop voltage gain (1) 100 db synchronization function v s_in,hi high input voltage 2 3.3 v v s_in,lo low input voltage 1 t s_in_pw input pulse width v s_in,hi = 3 v, v s_in,lo = 0 v 100 ns v s_in,hi = 2 v, v s_in,lo =1 v 300 i synch,lo slave sink current v synch = 2.9 v 0.7 1 ma v s_out,hi master output amplitude i source = 4.5 ma 2 v t s_out_pw output pulse width synch floating 110 ns protection t shdn thermal shutdown 150 c hysteresis 30 1. guaranteed by design. table 4. electrical characteristics (continued) symbol parameter test conditions values unit min. typ. max.
functional description A7986A 8/42 docid022801 rev 5 5 functional description the A7986A is based on a ?voltage mode?, constant frequency control. the output voltage v out is sensed by the feedback pin (fb) compared to an internal reference (0.6 v) providing an error signal that, compared to a fixed frequency sawtooth, controls the on and off time of the power switch. the main internal blocks are shown in the block diagram in figure 3. they are: ? a fully integrated oscillator that provides sawtooth to modulate the duty cycle and the synchronization signal. its switching frequency can be adjusted by an external resistor. the voltage and frequency feed-forward are implemented ? the soft-start circuitry to limit inrush current during the startup 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 p-channel power mosfet switch ? the peak current limit sensing block, to handle overload and short-circuit conditions ? a voltage regulator and internal reference. it supplies internal circuitry and provides a fixed internal reference ? a voltage monitor circuitry (uvlo) that checks the input and internal voltages ? a thermal shutdown block, to prevent thermal run-away. figure 3. block diagram peak current limit oscillator s r q thermal shutdown soft- start en trimming uvlo 0.6v regulator & bandgap 1.254v 3.3v synch & phase shift en fb comp fsw gnd synch out vcc driver e/a pwm peak current limit oscillator s r q thermal shutdown soft- start en trimming uvlo uvlo 0.6v regulator & bandgap regulator & bandgap 1.254v 3.3v synch & phase shift en fb comp fsw gnd synch out vcc driver e/a pwm
docid022801 rev 5 9/42 A7986A functional description 5.1 oscillator and synchronization figure 4 shows the block diagram of the oscillator circuit. the internal oscillator provides a constant frequency clock. its frequency depends on the resistor externally connected to the fsw pin. if the fsw pin is left floating, the frequency is 250 khz; it can be increased as shown in figure 6 by an external resistor connected to ground. to improve the line transient performance, keeping the pwm gain constant versus the input voltage, the voltage feed-forward is implemented by changing the slope of the sawtooth according to the input voltage change (see figure 5 .a). the slope of the sawtooth also changes if the oscillator frequency is increased by the external resistor. in this way a frequency feed-forward is implemented ( figure 5 .b) in order to keep the pwm gain constant versus the switching frequency (see section 6.4 for pwm gain expression). the synchronization signal is generated on the synch pin. 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 the synch pins are connected, the device with 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 minimizes the rms current flowing through the input capacitor (see the l5988d datasheet). figure 4. oscillator circuit block diagram the device can be synchronized to work at higher frequency feeding an external clock signal. the synchronization changes the sawtooth amplitude, changing the pwm gain ( figure 5 .c). this change must be taken into account when the loop stability is studied. to minimize the change of the pwm gain, the free-running frequency should be set (with a resistor on the fsw pin) only slightly lower than the external clock frequency. this pre- adjusting of the frequency changes the sawtooth slope in order to render the truncation of sawtooth negligible, due to the external synchronization. clock generator ramp generator fsw sawtooth clock synchronization synch clock generator ramp generator fsw sawtooth clock clock synchronization synch
functional description A7986A 10/42 docid022801 rev 5 figure 5. sawtooth: voltage and frequency feed-forward; external synchronization figure 6. oscillator frequency vs. fsw pin resistor
docid022801 rev 5 11/42 A7986A functional description 5.2 soft-start 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 monothonically. soft-start is performed by a staircase ramp on the non-inverting input (v ref ) of the error amplifier. so the output voltage slew rate is: equation 1 where sr vref is the slew rate of the non-inverting input, while r1and r2 is the resistor divider to regulate the output voltage (see figure 7 ). the soft-start staircase consists of 64 steps of 9.5 mv each, from 0 v to 0.6 v. the time base of one step is of 32 clock cycles. so the soft-start time and then the output voltage slew rate depend on the switching frequency. figure 7. soft-start scheme soft-start time results: equation 2 for example, with a switching frequency of 250 khz, the ss time is 8 ms. sr out sr vref 1 r1 r2 ------- - + ?? ?? ? = ss time 32 64 ? fsw ----------------- =
functional description A7986A 12/42 docid022801 rev 5 5.3 error amplifier and compensation the error amplifier (ea) 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 0.6 v voltage reference, while 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 so with high dc gain and low output impedance. the uncompensated error amplifier characteristics are the following: in continuous conduction mode (ccm), the transfer function of the power section has two poles due to the lc filter and one zero due to the esr of the output capacitor. different kinds of compensation networks can be used depending on the esr value of the output capacitor. if the zero introduced by the output capacitor helps 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 6.4 for details of the compensation network selection). anyway, the methodology to compensate the loop is to introduce zeros to obtain a safe phase margin. 5.4 overcurrent protection the A7986A implements the overcurrent protection sensing current flowing through the power mosfet. due to the noise created by the switching activity of the power mosfet, the current sensing 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 time?. the masking time is about 200 ns. if the overcurrent limit is reached, the power mosfet is turned off implementing the pulse- by-pulse overcurrent protection. under the overcurrent condition, the device can skip turn- on pulses in order to keep the output current constant and equal to the current limit. if, at the end of the ?masking time?, the current 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, the current is still higher than the overcurrent threshold, the device skips two pulses. this mechanism is repeated and the device can skip up to seven pulses. while, 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 (see figure 8 ). so the overcurrent/short-circuit protection acts by switching off the power mosfet and reducing the switching frequency down to one eighth of the default switching frequency, in order to keep constant the output current around the current limit. table 5.uncompensated error amplifier characteristics parameter value low frequency gain 100 db gbwp 4.5 mhz slew rate 7 v/ s output voltage swing 0 to 3.3 v maximum source/sink current 17 ma/25 ma
docid022801 rev 5 13/42 A7986A functional description this kind of overcurrent protection is effective if the output current is limited. to prevent the current from diverging, the current ripple in the inductor during the on-time must not be higher than the current ripple during the off-time. that is: equation 3 if the output voltage is shorted, v out ? 0, i out = i lim , d/f sw = t on_min , (1-d)/f sw ? 1/f sw . so, from the above equation, the maximum switching frequency that guarantees to limit the current results: equation 4 with r ds(on) = 300 m , drc = 0.08 , the worst condition is with v in = 38 v, i lim = 3.7 a; the maximum frequency to keep the output current limited during the short-circuit results 88 khz. based on the pulse-by-pulse mechanism, that reduces the switching frequency down to one eighth, the maximum f sw , adjusted by the fsw pin, which assures that a full effective output current limitation is 88 khz*8 = 706 khz. if, with v in = 38 v, the switching frequency is set higher than 706 khz, during short-circuit condition the system finds a different equilibrium with higher current. for example, with f sw = 800 khz and the output shorted to ground, the output current is limited around: equation 5 where f sw * is 800 khz divided by eight. v in v ? out r dson i out ? dcr i out ? ? ? lf sw ? ------------------------------------------------------------------------------------------------------------ d ? v out v f r dson i out ? dcr i out ? ++ + lf sw ? ----------------------------------------------------------------------------------------------------------- 1d ? () ? = f sw * v f dcr i ? + lim () v in r dson dcr + () i lim ? ? () ------------------------------------------------------------------------------ - 1 t on_min ---------------------- ? = i out v in f sw * ? v f t on_min ? ? drc t on_min ? () r dson dcr + () f sw * ? + --------------------------------------------------------------------------------------------------------------- - 4.2a ==
functional description A7986A 14/42 docid022801 rev 5 figure 8. overcurrent protection 5.5 enable function the enable feature allows the device to be put into standby mode. with the en pin is lower than 0.3 v the device is disabled and the power consumption is reduced to less than 30 a. with the en pin is lower than 1.2 v, the device is enabled. if the en pin is left floating, an internal pull-down ensures that the voltage at the pin reaches the inhibit threshold and the device is disabled. the pin is also v cc compatible. 5.6 hysteretic thermal shutdown the thermal shutdown block generates a signal that turns off the power stage if the junction temperature goes above 150 c. once the junction temperature goes back to about 120 c, the device restarts in normal operation. the sensing element is very close to the pdmos area, so ensuring an accurate and fast temperature detection.
docid022801 rev 5 15/42 A7986A application information 6 application information 6.1 input capacitor selection the capacitor connected to the input must be capable of supporting the maximum input operating voltage and the maximum rms input current required by the device. the input capacitor is subject to a pulsed current, the rms value of which is dissipated over its esr, affecting the overall system efficiency. so the input capacitor must have an rms current rating higher than the maximum rms input current and an esr value compliant with the expected efficiency. the maximum rms input current flowing through the capacitor can be calculated as: equation 6 where io is the maximum dc output current, d is the duty cycle, and is the efficiency. considering =1 , this function has a maximum at d= 0.5 and is equal to io/2. in a specific application the range of possible duty cycles must be considered in order to find out the maximum rms input current. the maximum and minimum duty cycles can be calculated as: equation 7 and: equation 8 where v f is the forward voltage on the freewheeling diode and v sw is voltage drop across the internal pdmos. the peak-to-peak voltage across the input capacitor can be calculated as: equation 9 where esr is the equivalent series resistance of the capacitor. given the physical dimension, ceramic capacitors can well meet the requirements of the input filter sustaining a higher input rms current than electrolytic/tantalum types. in this case the equation of c in as a function of the target v pp can be written as follows: i rms i o d 2d 2 ? -------------- - ? d 2 2 ------ - + ? = d max v out v f + v inmin v sw ? ------------------------------------ - = d min v out v f + v inmax v sw ? -------------------------------------- = v pp i o c in f sw ? ------------------------- 1 d --- - ? ?? ?? d d --- - 1d ? () ? + ? esr i o ? + ? =
application information A7986A 16/42 docid022801 rev 5 equation 10 neglecting the small esr of ceramic capacitors. considering = 1, this function has its maximum in d = 0.5, therefore, given the maximum peak-to-peak input voltage (v pp_max ), the minimum input capacitor (c in_min ) value is: equation 11 typically, c in is dimensioned to keep the maximum peak-to-peak voltage in the order of 1% of v inmax . in the below table some multi-layer ceramic capacitors suitable for this device are reported: a ceramic bypass capacitor, as close to the vcc and gnd pins as possible, so that additional parasitic esr and esl are minimized, is recommended in order to prevent instability on the output voltage due to noise. the value of the bypass capacitor can go from 100 nf to 1 f. 6.2 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 continuous current mode (ccm), the inductance value can be calculated by the following equation: equation 12 where t on is the conduction time of the internal high-side switch and t off is the conduction time of the external diode (in ccm, f sw = 1/(t on + t off )). the maximum current ripple, at fixed v out , is obtained at maximum t off , that is at minimum duty cycle (see section 6.1 to calculate minimum duty). so, fixing i l = 20% to 30% of the maximum output current, the minimum inductance value can be calculated as: table 6. input mlcc capacitors manufacture series cap value ( f) rated voltage (v) taiyo yuden umk325bj106mm-t 10 50 gmk325bj106mn-t 10 35 murata grm32er71h475k 4.7 50 c in i o v pp f sw ? -------------------------- - 1 d --- - ? ?? ?? d d --- - 1d ? () ? + ? ? = c in_min i o 2v pp_max f sw ?? ----------------------------------------------- - = i l v in v out ? l ----------------------------- - t on ? v out v f + l --------------------------- - t off ? ==
docid022801 rev 5 17/42 A7986A application information equation 13 where f sw is the switching frequency, 1/(t on + t off ). for example, for v out = 5 v, v in = 24 v, i o = 3 a and f sw = 250 khz, the minimum inductance value to have i l = 30% of i o is about 18 h. the peak current through the inductor is given by: equation 14 therefore, if the inductor value decreases, then the peak current (that must be lower than the minimum current limit of the device) increases. according to the maximum dc output current for this product family (3 a), the higher the inductor value, the higher the average output current that can be delivered, without triggering the overcurrent protection. in tab le 7 below, some inductor part numbers are listed. 6.3 output capacitor selection the current in the capacitor has a triangular waveform which generates a voltage ripple across it. this ripple is due to the capacitive component (charge or 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 calculated starting from the current ripple obtained by the inductor selection. equation 15 table 7. inductors manufacturer series inductor value ( h) saturation current (a) coilcraft mss1038 3.8 to 10 3.9 to 6.5 mss1048 12 to 22 3.84 to 5.34 wurth pd type l 8.2 to 15 3.75 to 6.25 pd type m 2.2 to 4.7 4 to 6 sumida cdrh6d226/hp 1.5 to 3.3 3.6 to 5.2 cdr10d48mn 6.6 to 12 4.1 to 5.7 l min v out v f + i max --------------------------- - 1d min ? f sw ---------------------- - ? = i lpk , i o i l 2 ------- - + = v out esr i max ? i max 8c out f sw ?? ------------------------------------ - + =
application information A7986A 18/42 docid022801 rev 5 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 stability: it fixes the double lc filter pole and the zero due to its esr. section 6.4 illustrates how to consider its effect in the system stability. for example, with v out = 5 v, v in = 24 v, i l = 0.9 a (resulting by the inductor value), in order to have a v out = 0.01v out , if the multi-layer ceramic capacitor is adopted, 10 f are needed and the esr effect on the output voltage ripple can be neglected. in the case of not negligible esr (electrolytic or tantalum capacitors), the capacitor is chosen taking into account its esr value. so, in the case of 330 f with esr = 730 m , the resistive component of the drop dominates and the voltage ripple is 28 mv . the output capacitor is also important to sustain the output voltage when a load transient with high slew rate is required by the load. when the load transient slew rate exceeds the system bandwidth, the output capacitor provides the current to the load. so, if the high slew rate load transient is required by the application, the output capacitor and system bandwidth must be chosen in order to sustain the load transient. in tab le 8 below, some capacitor series are listed. 6.4 compensation network the compensation network must assure stability and good dynamic performance. the loop of the A7986A is based on the voltage mode control. the error amplifier is a voltage operational amplifier with high bandwidth. so by selecting the compensation network, the ea is considered as ideal, that is, its bandwidth is much larger than the system one. the transfer function of the pwm modulator, from the error amplifier output (comp pin) to the out pin (see figure 10 ) results: equation 16 where v s is the sawtooth amplitude. as seen in section 5.1 , the voltage feed-forward generates a sawtooth amplitude directly proportional to the input voltage, that is: table 8. output capacitors manufacturer series cap value ( f) rated voltage (v) esr (m ) murata grm32 22 to 100 6.3 to 25 < 5 grm31 10 to 47 6.3 to 25 < 5 panasonic ecj 10 to 22 6.3 < 5 eefcd 10 to 68 6.3 15 to 55 sanyo tpa/b/c 100 to 470 4 to 16 40 to 80 tdk c3225 22 to 100 6.3 < 5 g pw0 v in v s -------- - =
docid022801 rev 5 19/42 A7986A application information equation 17 in this way the pwm modulator gain results constant and equal to: equation 18 the synchronization of the device with an external clock provided trough the synch pin can modify the pwm modulator gain (see section 5.1 to understand how this gain changes and how to keep it constant in spite of the external synchronization). figure 9. the error amplifier, the pwm modulation and the lc output filter the transfer function on the lc filter is given by: equation 19 where: equation 20 v s kv in ? = g pw0 v in v s -------- - 1 k --- - 18 === fb comp v ref e/a pwm v s out v cc c out esr l g pw0 g lc fb comp v ref e/a pwm v s out v cc c out esr l g pw0 g lc g lc s () 1 s 2 f zesr ? ------------------------- - + 1 s 2 qf ? lc ? ---------------------------- s 2 f lc ? ------------------- ?? ?? 2 ++ ------------------------------------------------------------------------ - = f lc 1 2 lc out ? 1 esr r out -------------- - + ?? ----------------------------------------------------------------------- - = f zesr 1 2 esr c out ?? ------------------------------------------- - = ,
application information A7986A 20/42 docid022801 rev 5 equation 21 as seen in section 5.3 , two different kinds of network can compensate the loop. in the two following paragraphs, the guidelines to select the type ii and type iii compensation network are illustrated. 6.4.1 type iii compensation network the methodology to stabilize the loop consists of placing two zeros to compensate the effect of the lc double pole, therefore increasing phase margin; then to place one pole in the origin to minimize the dc error on regulated output voltage; finally, to place other poles far from the zero db frequency. if the equivalent series resistance (esr) of the output capacitor introduces a zero with a frequency higher than the desired bandwidth (that is: 2 ? esr ? cout<1/bw), the type iii compensation network is needed. multi-layer ceramic capacitors (mlcc) have very low esr (<1 m ), with very high frequency zero, so a type iii network is adopted to compensate the loop. in figure 10 , the type iii compensation network is shown. this network introduces two zeros (f z1 , f z2 ) and three poles (f p0 , f p1 , f p2 ). they are expressed as: equation 22 equation 23 q r out lc out r out esr + () ?? ? lc out r out esr ?? + ------------------------------------------------------------------------------------------ r out v out i out -------------- = , = f z1 1 2 c 3 r 1 r 3 + () ?? ------------------------------------------------ = f z2 1 2 r 4 c 4 ?? ----------------------------- - = , f p0 0 = f p1 1 2 r 3 c 3 ?? ----------------------------- - = f p2 1 2 r 4 c 4 c 5 ? c 4 c 5 + -------------------- ?? ------------------------------------------- - = ,,
docid022801 rev 5 21/42 A7986A application information figure 10. type iii compensation network in figure 11 , the bode diagram of the pwm and lc filter transfer function (g pw0 g lc (f)) and the open loop gain (g loop (f) = g pw0 g lc (f) g typeiii (f)) are drawn. figure 11. open loop gain: module bode diagram the guidelines for positioning the poles and the zeroes and for calculating the component values can be summarized as follows: 1. choose a value for r 1 , usually between 1 k and 5 k . 2. choose a gain (r 4 /r 1 ) in order to have the required bandwidth (bw), that means: equation 24 where k is the feed-forward constant and 1/k is equal to 18. 3. calculate c 4 by placing the zero at 50% of the output filter double pole frequency (f lc ): r 4 bw f lc --------- - kr 1 ?? =
application information A7986A 22/42 docid022801 rev 5 equation 25 4. calculate c 5 by placing the second pole at four times the system bandwidth (bw): equation 26 5. set also the first pole at four times the system bandwidth and also the second zero at the output filter double pole: equation 27 the suggested maximum system bandwidth is equal to the switching frequency divided by 3.5 (f sw /3.5), anyway lower than 100 khz if the f sw is set higher than 500 khz. for example, with v out = 5 v, v in = 24 v, i o = 3 a, l = 18 h, c out = 22 f, and esr<1 m , the type iii compensation network is: equation 28 in figure 12 , the module and phase of the open loop gain is shown. the bandwidth is about 32 khz and the phase margin is 51. c 4 1 r 4 f lc ?? --------------------------- = c 5 c 4 2 r 4 c 4 4bw ? 1 ? ??? ------------------------------------------------------------- - = r 3 r 1 4bw ? f lc ----------------- 1 ? -------------------------- - = c 3 1 2 r 3 4bw ??? ---------------------------------------- - = , 1 . = r 2 680 = r 3 200 = r 4 2k = c 3 3.3nf = c 4 22nf = c 5 220pf = ,,,, ,,
docid022801 rev 5 23/42 A7986A application information figure 12. open loop gain bode diagram with ceramic output capacitor 6.4.2 type ii compensation network if the equivalent series resistance (esr) of the output capacitor introduces a zero with a frequency lower than the desired bandwidth (that is: 2 ? esr ? cout>1/bw), this zero helps stabilize the loop. electrolytic capacitors show not negligible esr (>30 m ), so with this kind of output capacitor the type ii network combined with the zero of the esr allows the stabilization of the loop. in figure 13 , the type ii network is shown.
application information A7986A 24/42 docid022801 rev 5 figure 13. type ii compensation network the singularities of the network are: equation 29 in figure 14 , the bode diagram of the pwm and lc filter transfer function (g pw0 g lc (f)) and the open loop gain (g loop (f) = g pw0 g lc (f) g typeii (f)) are drawn. figure 14. open loop gain: module bode diagram f z1 1 2 r 4 c 4 ?? ----------------------------- - = f p0 0 = f p1 1 2 r 4 c 4 c 5 ? c 4 c 5 + -------------------- ?? ------------------------------------------- - = ,,
docid022801 rev 5 25/42 A7986A application information the guidelines for positioning the poles and the zeroes and for calculating the component values can be summarized as follows: 1. choose a value for r 1 , usually between 1 k and 5 k , in order to have values of c4 and c5 not comparable with parasitic capacitance of the board. 2. choose a gain (r 4 /r 1 ) in order to have the required bandwidth (bw), that means: equation 30 where f esr is the esr zero: equation 31 and vs is the sawtooth amplitude. the voltage feed-forward keeps the ratio vs/vin constant. 3. calculate c 4 by placing the zero one decade below the output filter double pole: equation 32 4. then calculate c 3 in order to place the second pole at four times the system bandwidth (bw): equation 33 for example with v out = 5 v, v in = 24 v, i o = 3 a, l = 18 h, c out = 330 f, and esr = 35 m , the type ii compensation network is: in figure 15 , the module and phase of the open loop gain is shown. the bandwidth is about 21 khz and the phase margin is 45. r 4 f esr f lc ----------- - ?? ?? 2 bw f esr ----------- - v s v in -------- - r 1 ??? = f esr 1 2 esr c out ?? ------------------------------------------- - = c 4 10 2 r 4 f lc ?? ------------------------------ - = c 5 c 4 2 r 4 c 4 4bw ? 1 ? ??? ------------------------------------------------------------- - = r 1 1.1k = r 2 150 = r 4 4.99k = c 4 82nf = c 5 68pf = ,, ,,
application information A7986A 26/42 docid022801 rev 5 figure 15. open loop gain bode diagram with electrolytic/tantalum output capacitor
docid022801 rev 5 27/42 A7986A application information 6.5 thermal considerations the thermal design is important to prevent the thermal shutdown of the device if junction temperature goes above 150 c. the three different sources of losses within the device are: a) conduction losses due to the not negligible r ds(on) of the power switch; these are equal to: equation 34 where d is the duty cycle of the application and the maximum r ds(on) over temperature is 220 m . note that the duty cycle is theoretically given by the ratio between v out and v in , but actually it is quite higher to compensate the losses of the regulator. so the conduction losses increase compared with the ideal case. b) switching losses due to power mosfet turn on and off; these can be calculated as: equation 35 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, as shown in figure 16 . t sw is the equivalent switching time. for this device the typical value for the equivalent switching time is 40 ns. c) quiescent current losses, calculated as: equation 36 where i q is the quiescent current (i q = 2.4 ma). the junction temperature t j can be calculated as: equation 37 where t a is the ambient temperature and p tot is the sum of the power losses just seen. r th(ja) is the equivalent thermal resistance junction 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 pad is the one conducting the largest amount of heat. the r th(ja) measured on the demonstration board described in the following paragraph is about 40 c/w for the hsop8 package. p on r ds on () i out () 2 d ?? = p sw v in i out t rise t fall + () 2 ------------------------------------------ - fsw ?? ? v in i out t sw f sw ??? == p q v in i q ? = t j t a rth ja p tot ? + =
application information A7986A 28/42 docid022801 rev 5 figure 16. switching losses 6.6 layout considerations the pc board layout of switching dc-dc regulator is very important to minimize the noise injected in high impedance nodes and interference generated by the high switching current loops. 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 the fact that the high value pulsed current is flowing through it. in order to minimize the emi, this loop must be as short as possible. the feedback pin (fb) connection to the external resistor divider is a high impedance node, so the interference can be minimized by placing the routing of the feedback node as far as possible from the high current paths. to reduce 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 (220 nf - 1 f) can be added as close as possible to the input voltage pin of the device. thanks to the exposed pad of the device, the ground plane helps to reduce the thermal resistance junction to ambient; so a large ground plane enhances the thermal performance of the converter allowing high power conversion. in figure 17 , a layout example is shown.
docid022801 rev 5 29/42 A7986A application information figure 17. layout example
application information A7986A 30/42 docid022801 rev 5 6.7 application circuit in figure 18 the demonstration board application circuit is shown. figure 18. demonstration board application circuit table 9. component list reference part number description manufacturer c1 umk325bj106mm-t 10 f, 50 v taiyo yuden c2 grm32er61e226ke15 22 f, 25 v murata c3 3.3 nf, 50 v c4 33 nf, 50 v c5 100 pf, 50 v c6 470 nf, 50 v r1 4.99 k , 1%, 0.1 w 0603 r2 1.1 k , 1%, 0.1 w 0603 r3 330 , 1%, 0.1 w 0603 r4 1.5 k , 1%, 0.1 w 0603 r5 180 k , 1%, 0.1 w 0603 d1 stps3l40 3 a dc, 40 v stmicroelectronics l1 mss1038-103nl 10 h, 30%, 3.9 a, dcr max =35 m coilcraft
docid022801 rev 5 31/42 A7986A application information figure 19. pcb layout: A7986A (component side) figure 20. pcb layout: A7986A (bottom side) figure 21. pcb layout: A7986A (front side)
application information A7986A 32/42 docid022801 rev 5 figure 22. junction temperature vs. output current @ v in = 24 v figure 23. junction temperature vs. output current @ v in = 12 v v out =1.8v v out =3.3v v out =5v hsop vqfn v in =24v f sw =250khz t amb =25 c v out =1.8v v out =3.3v v out =5v hsop vqfn v in =12v f sw =250khz t amb =25 c figure 24. junction temperature vs. output current @ v in = 5 v figure 25. efficiency vs. output current @ v in = 12 v v out =1.2v v out =1.8v v out =3.3v hsop vqfn v in =5v f sw =250khz t amb =25 c 60 65 70 75 80 85 90 95 0.100 0.600 1.100 1.600 2.100 2.600 3.100 io [a] eff [%] vin=12v vin=18v vin=24v vo=5.0v fsw=250khz figure 26. efficiency vs.output current @ v in =5 v, v o = 3.3 v figure 27. efficiency vs.output current @ v in =5 v, v o = 1.8 v 50 55 60 65 70 75 80 85 90 95 0.100 0.600 1.100 1.600 2.100 2.600 3.100 io [a] eff [%] vin=5v vin=12v vin=24v vo=3.3v fsw=250khz 40 45 50 55 60 65 70 75 80 85 0.100 0.600 1.100 1.600 2.100 2.600 3.100 io [a] e ff [%] vin=5v vin=12v vin=24v vo=1.8v fsw=250khz
docid022801 rev 5 33/42 A7986A application information figure 28. load regulation figure 29. line regulation 3.305 3.310 3.315 3.320 3.325 3.330 3.335 3.340 3.345 3.350 0.00 0.50 1.00 1.50 2.00 2.50 3.00 v out [v] io [a] vin=5v vin=12v vin=24v 3.3200 3.3250 3.3300 3.3350 3.3400 3.3450 3.3500 3.3550 3.3600 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 v out [v] v in [v] io=1a io=2a io=3a figure 30. load transient: from 0.4 a to 2 a figure 31. soft-start v out 200mv/div ac coupled i l 1a/div time base 200us/div v in =24v v out =3.3v c out =47uf l=10uh f sw =520k i l 1a/div v out 1v/div time base 1ms/div i l 0.5a/div v out 0.5v/div time base 1ms/div i l 1a/div v out 1v/div time base 1ms/div i l 0.5a/div v out 0.5v/div time base 1ms/div figure 32. short-circuit behavior v in =12 v figure 33. short-circuit behavior v in =24 v out 5v/div i l 1a/div v out 1v/div synch 5v/div timebase 10us/div out 5v/div i l 1a/div v out 1v/div synch 5v/div timebase 10us/div out 5v/div i l 1a/div v out 1v/div synch 5v/div timebase 10us/div out 5v/div i l 1a/div v out 1v/div synch 5v/div out 5v/div i l 1a/div v out 1v/div synch 5v/div out 5v/div i l 1a/div v out 1v/div synch 5v/div timebase 10us/div
application ideas A7986A 34/42 docid022801 rev 5 7 application ideas 7.1 positive buck-boost the A7986A can implement the step-up/down converter with a positive output voltage. figure 34 shows the schematic: one power mosfet and one schottky diode are added to the standard buck topology to provide 12 v output voltage with input voltage from 4.5 v to 38 v. figure 34. positive buck-boost regulator the relationship between input and output voltage is: equation 38 so the duty cycle is: equation 39 the output voltage isn?t limited by the maximum operating voltage of the device (38 v), 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; the maximum gate to source voltage must be higher than the input voltage (in figure 34 , if v in is higher than 16 v, the gate must be protected through a zener diode and resistor). the current flowing through the internal power mosfet is transferred to the load only during the off time, so according to the maximum dc switch current (3.0 a), the maximum output current for the buck-boost topology can be calculated from the following equation. v out v in d 1d ? ------------- ? = d v out v out v in + ----------------------------- - =
docid022801 rev 5 35/42 A7986A application ideas equation 40 where i sw is the average current in the embedded power mosfet in the on time. to chose the right value of the inductor and to manage transient output current, which for a short time can exceed the maximum output current calculated by equation 40 , also the peak current in the power mosfet must be calculated. the peak current, shown in equation 41 , must be lower than the minimum current limit (3.7 a). equation 41 where r is defined as the ratio between the inductor current ripple and the inductor dc current: so, in the buck-boost topology the maximum output current depends on the application conditions (firstly input and output voltage, secondly switching frequency and inductor value). in figure 35 , the maximum output current for the above configuration is depicted varying the input voltage from 4.5 v to 38 v. the dashed line considers a more accurate estimation of the duty cycles given by equation 42 , where power losses across diodes, external power mosfet, and internal power mosfet are taken into account. figure 35. maximum output current according to max. dc switch current (3.0 a): v o =12 v i sw i out 1d ? ------------- 2 a < = i sw,pk i out 1d ? ------------- 1 r 2 -- - + 3.7a < ? = r v out i out lf sw ?? ------------------------------------ 1d ? () 2 ? =
application ideas A7986A 36/42 docid022801 rev 5 equation 42 where v d is the voltage drop across the diodes, v sw and v swe across the internal and external power mosfet. 7.2 inverting buck-boost the A7986A can implement the step-up/down converter with a negative output voltage. figure 34 shows the schematic to regulate -5 v: no further external components are added to the standard buck topology. the relationship between input and output voltage is: equation 43 so the duty cycle is: equation 44 as in the positive one, in the inverting buck-boost the current flowing through the power mosfet is transferred to the load only during the off time. so according to the maximum dc switch current (3.0 a), the maximum output current can be calculated from equation 40 , where the duty cycle is given by equation 44 . figure 36. inverting buck-boost regulator the gnd pin of the device is connected to the output voltage so, given the output voltage, the input voltage range is limited by the maximum voltage the device can withstand across vcc and gnd (38 v). therefore, if the output is -5 v, the input voltage can range from 4.5 v to 33 v. d v out 2v d ? + v in v sw v swe v out 2v ? d ++ ? ? ------------------------------------------------------------------------------------------- - = v out v in ? d 1d ? ------------- ? = d v out v out v in ? ----------------------------- - =
docid022801 rev 5 37/42 A7986A application ideas as in the positive buck-boost, the maximum output current according to application conditions is shown in figure 37 . the dashed line considers a more accurate estimation of the duty cycles given by equation 45 , where power losses across diodes and the internal power mosfet are taken into account. equation 45 figure 37. maximum output current according to switch max. peak current (3.0 a): v o = - 5 v d v out v d ? v ? in v sw v out v d ? + ? ---------------------------------------------------------------- - =
package mechanical data A7986A 38/42 docid022801 rev 5 8 package mechanical data 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 and product status are available at: www.st.com . ecopack ? is an st trademark. table 10. hsop8 mechanical data dim. mm min. typ. max. a 1.70 a1 0.00 0.150 a2 1.25 b 0.31 0.51 c 0.17 0.25 d 4.804.905.00 e 5.806.006.20 e1 3.80 3.90 4.00 e1.27 h 0.25 0.50 l 0.40 1.27 k 0.00 8.00 ccc 0.10
docid022801 rev 5 39/42 A7986A package mechanical data figure 38. hsop8 package dimensions ' pp7\s ( pp7\s $0y
ordering information A7986A 40/42 docid022801 rev 5 9 ordering information table 11. ordering information order code package packaging A7986A hsop8 tube A7986Atr hsop8 tape and reel
docid022801 rev 5 41/42 A7986A revision history 10 revision history table 12. document revision history date revision changes 13-feb-2012 1 initial release. 20-mar-2012 2 section 8: package mechanical data has been updated. 16-oct-2012 3 in section 5.6 changed temperature value from 130 to 120 c. 04-jul-2013 4 updated values for v fb parameter in table 4: electrical characteristics . 12-aug-2013 5 changed v fb parameter in table 4 from 0.594 to 0.588.
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