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november 2009 doc id 13955 rev 6 1/41 41 a5970d up to 1 a step down switching regulator for automotive applications features qualified following the aec-q100 requirements (see ppap for more details) 1 a dc output current operating input voltage from 4 v to 36 v 3.3 v / (2%) reference voltage output voltage adjustable from 1.235 v to v in low dropout op eration: 100% duty cycle 250 khz internally fixed frequency voltage feedforward zero load current operation internal current limiting inhibit for zero current consumption synchronization protection against feedback disconnection thermal shutdown application dedicated to automotive applications description the a5970d is a step down monolithic power switching regulator with a minimum switch current limit of 1.35 a so it is able to deliver up to 1 a dc current to the load depending on the application conditions. the output voltage can be set from 1.235 v to v in . the device uses an internal p-channel dmos transistor (with a typical r ds(on) of 250 m ) as switching element to minimize the size of the external components. an internal oscillator fixe s the switching frequency at 250 khz. having a minimum input voltage of 4 v only it fits the automotive applications requiring the device operation even in cold crank conditions. pulse by puls e current limit with the internal frequency modulation offers an effective constant current short circuit protection. so8 figure 1. application schematic v in =4 to 36v v out =3.3v www.st.com
contents a5970d 2/41 doc id 13955 rev 6 contents 1 pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2 pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4 datasheet parameters over the temperature range . . . . . . . . . . . . . . . . 8 5 functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.1 power supply and voltage reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5.2 voltages monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.3 oscillator and synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.4 current protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.5 error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.6 pwm comparator and power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.7 inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.8 thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 6 additional features and protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.1 feedback disconnection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.2 output overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.3 zero load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7 closing the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 7.1 error amplifier and compensation network . . . . . . . . . . . . . . . . . . . . . . . . 17 7.2 lc filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 7.3 pwm comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 8 application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
a5970d contents doc id 13955 rev 6 3/41 8.1 component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 8.2 layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8.3 thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 8.4 short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 8.5 application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 8.6 positive buck-boost regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 8.7 negative buck-boost regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 8.8 synchronization example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 8.9 compensation network with mlcc at the out put . . . . . . . . . . . . . . . . . . . 33 8.10 external soft_start network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 9 typical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 10 package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 11 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
pin settings a5970d 4/41 doc id 13955 rev 6 1 pin settings 1.1 pin connection figure 2. pin connection (top view) 1.2 pin description table 1. pin description n pin description 1 out regulator output. 2 synch master/slave synchronization. 3inh a logical signal (active high) disables the device. if inh not used the pin must be grounded. when it is open an internal pull-up disable the device. 4 comp e/a output for frequency compensation. 5fb feedback input. connecting directly to this pin results in an output voltage of 1.23 v. an external resist ive divider is required for higher output voltages. 6 vref 3.3 v v ref . no cap is requested for stability. 7 gnd ground. 8 vcc unregulated dc input voltage.
a5970d electrical data doc id 13955 rev 6 5/41 2 electrical data 2.1 maximum ratings 2.2 thermal data table 2. absolute maximum ratings symbol parameter value unit v 8 input voltage 40 v v 1 out pin dc voltage out pin peak voltage at t = 0.1 s -1 to 40 -5 to 40 v v i 1 maximum output current int. limit. v 4 , v 5 analog pins 4 v v 3 inh -0.3 to v cc v v 2 synch -0.3 to 4 v p tot power dissipation at t a 70 c 0.6 w t j operating 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 thja maximum thermal resistance junction-ambient 120 (1) 1. package mounted on evaluation board c/w
electrical characteristics a5970d 6/41 doc id 13955 rev 6 3 electrical characteristics t j = -40 c to 125 c, vcc = 12 v, unless otherwise specified. table 4. electrical characteristics symbol parameter test co ndition min. typ. max. unit v cc operating input voltage range 436v r ds(on) mosfet on resistance 0.250 0.5 i l maximum limiting current (1) v cc = 5 v 1.35 1.87 2.25 a v cc = 5 v, t j = 25 c 1.5 1.87 2.25 f sw switching frequency 212 250 280 khz duty cycle 0 100 % dynamic characteristics (see test circuit). v 5 voltage feedback 4.4 v < v cc < 36 v, 1.198 1.235 1.272 v efficiency v 0 = 5 v, v cc = 12 v 90 % dc characteristics i qop total operating quiescent current 35ma i q quiescent current duty cycle=0; v fb =1.5 v 2.5 ma i qst-by total stand-by quiescent current v inh > 2.2 v 50 100 a v cc = 36 v; v inh > 2.2 v 80 150 a inhibit inh threshold voltage device on 0.8 v device off 2.2 v error amplifier v oh high level output voltage v fb = 1 v 3.5 v v ol low level output voltage v fb = 1.5 v 0.4 v i o source source output current v comp = 1.9 v; v fb = 1 v 190 300 a i o sink sink output current v comp = 1.9 v; v fb =1.5 v 11.5 ma i b source bias current 2.5 4 a dc open loop gain r l =5065db gm transconductance i comp = -0.1 ma to 0.1 ma; v comp = 1.9 v 2.3 ms
a5970d electrical characteristics doc id 13955 rev 6 7/41 synch function high input voltage v cc = 4.4 to 36 v; 2.5 v ref v low input voltage v cc = 4.4 to 36 v; 0.74 v slave synch current v synch = 0.74 v (2) v synch = 2.33 v 0.11 0.21 0.25 0.45 ma master output amplitude i source = 3 ma 2.75 3 v output pulse width no load, v synch = 1.65 v 0.20 0.35 s reference section reference voltage i ref = 0 to 5 ma v cc = 4.4 v to 36 v 3.2 3.3 3.399 v line regulation i ref = 0ma v cc = 4.4 v to 36 v 510mv load regulation i ref = 0 ma 8 15 mv short circuit current 5 18 35 ma 1. with t j = 85 c, i lim_min = 1.5 a, assured by design, charac terization and statistical correlation. 2. guaranteed by design table 4. electrical characteristics (continued) symbol parameter test co ndition min. typ. max. unit
datasheet parameters over the temperature range a5970d 8/41 doc id 13955 rev 6 4 datasheet parameters o ver the temperature range the 100% of the population in the production flow is tested at three different ambient temperatures (-40 c; +25 c, +125 c) to guarantee the datasheet parameters inside the junction temperature range (-40 c; +125 c). the device operation is so guaranteed when the junction temperature is inside the (-40 c; +150 c) temperature range. the designer can estimate the s ilicon temperature increase respect to the ambient temperature evaluating the internal power losses generated during the device operation (please refer to the chapter 2.2 ). however the embedded thermal protection disables the switching activity to protect the device in case the junction temperature reaches the t shtdwn (+150 c10 c) temperature. all the datasheet parameters can be guaranteed to a maximum junction temperature of +125 c to avoid triggering the thermal shutdown protection during the testing phase because of self heating.
a5970d functional description doc id 13955 rev 6 9/41 5 functional description the main internal blocks are shown in the device block diagram in figure 3 . they are: a voltage regulator supplying the internal circuitry. from this regulator, a 3.3 v reference voltage is externally available. a voltage monitor circuit which checks the input and the internal voltages. a fully integrated sawt ooth oscillator with a frequency of 250 khz 15%, including also the voltage feed forward function and an input/output synchronization pin. two embedded current limitation circuits which control the current that flows through the power switch. the pulse-by-pulse current limit forces the power switch off cycle by cycle if the current reaches an internal threshold, while the frequency shifter reduces the switching frequency in order to significantly reduce the duty cycle. a transconductance error amplifier. a pulse width modulator (pwm) comparator and the relative logic circuitry necessary to drive the internal power. a high side driver for the internal p-mos switch. an inhibit block for stand-by operation. a circuit to implement the thermal protection function. figure 3. block diagram 5.1 power supply and voltage reference the internal regulator circuit (shown in figure 4 ) consists of a start-up circuit, an internal voltage pre-regulator, the bandgap voltage reference and the bias block that provides current to all the blocks. the starter supplies the start-up currents to the entire device when the input voltage goes high and the device is enabled (inhibit pin connected to ground). the pre-regulator block supplies the bandgap cell with a pre-regulated voltage v reg that has a very low supply voltage noise sensitivity.
functional description a5970d 10/41 doc id 13955 rev 6 5.2 voltages monitor an internal block continuously senses the v cc , v ref and v bg . if the voltages go higher than their thresholds, the regulator begins operat ing. there is also a hysteresis on the v cc (uvlo). figure 4. internal circuit 5.3 oscillator and synchronization figure 5 shows the block diagram of the oscillator circuit. the clock generator provides the switching frequen cy of the device, which is internally fixed at 250 khz. the frequency shifter block acts to reduce the switching frequency in case of strong overcurrent or short circuit. the clock signal is then used in the internal logic circuitry and is the input of the ramp generator and synchronizer blocks. the ramp generator circuit provides the sawtooth signal, used for pwm control and the internal voltage feed-forward, while the synchronizer circuit generates the synchronization signal. the device also has a synchronization pin which can wo rk both as master and slave. beating frequency noise is an issue when more than one voltage rail is on the same board. a simple way to avoid this issue is to operate all the regulators at the same switching frequency. the synchronization feature of a set of the a597 0d is simply get connecting together their synch pin. the device with highest swit ching frequency will be the master and it provides the synchronization signal to the others. therefore the synch is a i/o pin to deliver or recognize a frequency signal. the synchronization circuitry is powered by the internal reference (v ref ) so a small filtering capacitor ( 100 nf) connected between v ref pin and the signal ground of the master device is suggested for its proper operation. however when a set of synchronized devices populates a board it is not possible to know in advance the one working as master, so the filtering capacitor have to be designed for whole set of devices. when one or more devices are synchronized to an external signal, its amplitude have to be in comply with specific ations given in the ta bl e 4 . the frequency of the synchronization signal must be, at a minimum, higher than the maximum guaranteed natural switching frequency of the device (275 khz, see ta bl e 4 ) while the duty cycle of the synchronization signal can vary from approximately 10% to 90%. the small capacitor under v ref pin is required for this operation.
a5970d functional description doc id 13955 rev 6 11/41 figure 5. oscillator circuit block diagram figure 6. synchronization example 5.4 current protection the a5970d features two types of current limit protection: pulse-by-pulse and frequency foldback. the schematic of the current limitation circuitry for the pulse-by-pulse protection is shown in figure 7 . the output power pdmos transistor is split into two parallel pdmos transistors. the smallest one includes a resistor in series, r sense . the current is sensed through r sense and if it reaches the threshold, the mi rror becomes unbalanced and the pdmos is switched off until the next falling ed ge of the internal clock pulse. due to this reduction of the on time, the output voltage decreases. since the minimum switch on time necessary to sense the current in order to avoid a false overcurrent signal is too short to obtain a sufficiently low duty cycle at 250 khz (see chapter 8.4 ), the output current in strong overcurrent or short circuit conditions could be not properly limited. for this reason the switching frequency is also reduced, thus keeping the inductor current under its maximum out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch out gnd comp fb ss/inh a5973d synch a5970d a5970d a5970d a5970d
functional description a5970d 12/41 doc id 13955 rev 6 threshold. the frequency shifter ( figure 5 ) functions based on the feedback voltage. as the feedback voltage decreases (due to the reduced duty cycle), the switching frequency decreases also. figure 7. current limitation circuitry 5.5 error amplifier the voltage error amplifier is the core of the loop regulation. it is a transconductance operational amplifier whose non inverting input is connected to the internal voltage reference (1.235 v), while the inverting input (fb) is connected to the external divider or directly to the output voltage. the output (comp) is connected to the external compensation network. the uncompensated error amplifier has the following characteristics: the error amplifier out put is compared to the oscillato r sawtooth to perform pwm control. 5.6 pwm comparator and power stage this block compares the oscillator sawtooth and the error amplifier output signals to generate the pwm signal for the driving stage. the power stage is a highly critical block, as it functions to guarantee a correct turn on and turn off of the pdmos. the turn on of the power element, or more accurately, the rise time of the current at turn on, is a very critical parameter. at a first approach, it appears that the faster the rise time, the lower the turn on losses. however, there is a limit introduced by the recovery time of the recirculation diode. table 5. uncompensated error amplifier characteristics description values transconductance 2300 s low frequency gain 65 db minimum sink/source voltage 1500 a/300 a output voltage swing 0.4 v/3.65 v input bias current 2.5 a
a5970d functional description doc id 13955 rev 6 13/41 in fact, when the current of the power element is equal to the inductor current, the diode turns off and the drain of the power is able to go high. but during its recovery time, the diode can be considered a high value capacitor an d this produces a very high peak current, responsible for numerous problems: spikes on the device supply voltage that ca use oscillations (and thus noise) due to the board parasites. turn on overcurrent leads to a decrease in the efficiency and system reliability. major emi problems. shorter freewheeling diode life. the fall time of the current during turn off is also critical, as it produces voltage spikes (due to the parasites elements of the board) that increase the voltage drop across the pdmos. in order to minimize these problems, a new driving circuit topology has been used and the block diagram is shown in figure 8 . the basic idea is to change the current levels used to turn the power switch on and off, based on the pdmos and the gate clamp status. this circuitry allows the power switch to be turned off and on quickly and addresses the freewheeling diode recovery time problem. the gate clamp is necessary to ensure that v gs of the internal switch does not go higher than v gs max. the on/off control block protects against any cross conduction between the supply line and ground. figure 8. driving circuitry 5.7 inhibit function the inhibit feature is used to put the device in standby mode. with the inh pin higher than 2.2 v the device is disabled and the power consumption is reduced to less than 100 a. with the inh pin lower than 0.8 v, the device is enabled. if the inh pin is left floating, an internal pull up ensures that the voltage at the pin reaches the inhibit threshold and the device is disabled. the pin is also v cc compatible.
functional description a5970d 14/41 doc id 13955 rev 6 5.8 thermal shutdown the shutdown block generates a signal that turns off the power stage if the temperature of the chip goes higher than a fixed internal threshold (15010 c). the sensing element of the chip is very close to the pdmos area, ensuring fast and accurate temperature detection. a hysteresis of approximately 20 c keeps the device from turning on and off continuously.
a5970d additional features and protection doc id 13955 rev 6 15/41 6 additional features and protection 6.1 feedback disconnection if the feedback is disconnected, the duty c ycle increases towards the maximum allowed value, bringing the output voltage close to the input supply. this condition could destroy the load. to avoid this hazardous condition, the device is turned off if the feedback pin is left floating. 6.2 output overvoltage protection overvoltage protection, or ovp, is achieved by using an internal comparator connected to the feedback, which turns off the power stage when the ovp threshold is reached. this threshold is typically 30% higher than the feedback voltage. when a voltage divider is required to adjust the output voltage ( figure 15 ), the ovp intervention will be set at: equation 1 where r 1 is the resistor connected between the output voltage and the feedback pin, and r 2 is between the feedback pin and ground. 6.3 zero load due to the fact that the internal power is a pdmos, no boostrap capacitor is required and so the device works properly even with no load at the output. in this case it works in burst mode, with a random burst repetition rate. v ovp 1.3 r 1 r 2 + r 2 -------------------- ? v fb ? =
closing the loop a5970d 16/41 doc id 13955 rev 6 7 closing the loop figure 9. block diagram of the loop
a5970d closing the loop doc id 13955 rev 6 17/41 7.1 error amplifier and compensation network the output l-c filter of a step-down converter contributes with 180 degrees phase shift in the control loop. for this reason a compensation network between the comp pin and ground is added. the simplest compensation network together with the equivalent circuit of the error amplifier are shown in figure 10 . r c and c c introduce a pole and a zero in the open loop gain. cp does not significantly affect system stability but it is useful to reduce the noise of the comp pin. the transfer function of the error amplifier and its compensation network is: equation 2 where a vo = g m r o figure 10. error amplifier equivalent circuit and compensation network the poles of this transfer function are (if c c >> c 0 +c p ): equation 3 equation 4 a 0 s () a v0 1s + r c c c ? ? () ? s 2 r 0 ? c 0 c p + () r c c c sr 0 c c ? r 0 c 0 c p + () r c c c ? + ? + () 1 + ? + ? ? ? ------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------- = f p1 1 2 ? r 0 ? c c ? ------------------------------------- = f p2 1 2 ? r c ? c 0 c p + () ? ------------------------------------------------------- - =
closing the loop a5970d 18/41 doc id 13955 rev 6 whereas the zero is defined as: equation 5 f p1 is the low frequency which sets the bandwidth, while the zero f z1 is usually put near to the frequency of the double pole of the l-c filter (see below). f p2 is usually at a very high frequency. 7.2 lc filter the transfer function of the l-c filter is given by: equation 6 where r load is defined as the ratio between v out and i out . if r load >>esr, the previous expression of a lc can be simplified and becomes: equation 7 the zero of this transfer function is given by: equation 8 f 0 is the zero introduced by the esr of the output capacitor and it is very important to increase the phase margin of the loop. the poles of the transfer function can be calculated through the following expression: equation 9 in the denominator of a lc the typical second order system equation can be recognized: equation 10 f z1 1 2 ? r c ? c c ? ------------------------------------- = a lc s () r load 1 esr c out s ? ? + () ? s 2 lc out esr r load + () s esr c out ? r load l + ? () r load + ? + ? ? ? ------------------------------------------------------------------------------------------------------------------------------- --------------------------------------------------------------------- = a lc s () 1 esr c out ? s ? + lc out ? s 2 esr c out ? s1 + ? + ? ---------------------------------------------------------------------------------------------- = f o 1 2 ? esr ? c out ? --------------------------------------------------- - = f plc1 2 , esr c out esr c out ? () 2 4l ? c out ? ? ? ? 2l ? c out ? ------------------------------------------------------------------------------------------------------------------------------- ----------- = s 2 2 ? n ? s 2 n + ? +
a5970d closing the loop doc id 13955 rev 6 19/41 if the damping coefficient is very close to zero, the roots of the equation become a double root whose value is n . similarly for a lc the poles can usually be defined as a double pole whose value is: equation 11 7.3 pwm comparator the pwm gain is given by the following formula: equation 12 where v oscmax is the maximum value of a sawtooth waveform and v oscmin is the minimum value. a voltage feed forward is implemented to ensure a constant gpwm. this is obtained by generating a sawtooth waveform directly proportional to the input voltage v cc . equation 13 where k is equal to 0.076. therefore the pwm gain is also equal to: equation 14 this means that even if the input voltage changes, the error amplifier does not change its value to keep the loop in regulation, thus ensuring a better line regulation and line transient response. in summary, the open loop gain can be expressed as: equation 15 f plc 1 2 ? lc out ? ? ---------------------------------------------- = g pwm s () v cc v oscmax v oscmin ? () ------------------------------------------------------------- = v oscmax v oscmin ? kv cc ? = g pwm s () 1 k --- - const == gs () g pwm s () r 2 r 1 r 2 + -------------------- ? a o s () ? a lc ? s () =
closing the loop a5970d 20/41 doc id 13955 rev 6 example: considering r c = 4.7 k , c c = 22 nf and c p = 220 pf, the poles and zeroes of a 0 are: f p1 = 9 hz f p2 = 150 khz f z1 = 1.5 khz if l = 33 h, c out = 100 f and esr = 80 m , the poles and zeroes of a lc become: f plc = 2.7 khz f zesr = 19.89 khz finally r 1 = 5.6 k and r 2 = 3.3 k . the gain and phase bode diagrams are plotted respectively in figure 11 and figure 12 . figure 11. module plot figure 12. phase plot the cut-off frequency and the phase margin are: equation 16 f c 25khz = phase margin = 40
a5970d application information doc id 13955 rev 6 21/41 8 application information 8.1 component selection input capacitor the input capacitor must be able to support the maximum input operating voltage and the maximum rms input current. since step-down converters draw current from the input in pulses, the input current is squared and the height of each pulse is equal to the output current. the input capacitor has to absorb all this switching 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 has to be very high to minimize the power dissipation generated by the internal esr, thereby improving system reliability and efficiency. the critical parameter is usually th e rms current rating, which must be higher than the rms input current. the maximum rms input current (flowing through the input capacitor) is: equation 17 where is the expected system efficiency, d is the duty cycle and i o is the output dc current. this function reaches its maximum value at d = 0.5 and the equivalent rms current is equal to i o divided by 2 (considering = 1). the maximum and minimum duty cycles are: equation 18 and equation 19 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 ? -------------------------------------- =
application information a5970d 22/41 doc id 13955 rev 6 where v f is the freewheeling diode forward voltage and v sw the voltage drop across the internal pdmos. considering the range d min to d max , it is possible to determine the max irms going through the input capacitor. capacitors that can be considered are: electrolytic capacitors: these are widely used due to their low pric e and their availabilit y in a wide range of rms current ratings. the only drawback is that, considering ripple current rating requirements, they are physically larger than other capacitors. ceramic capacitors: if available for the required value and voltage rating, these capacitors usually have a higher rms current rating for a given physical dimension (due to very low esr). the drawback is the considerably high cost. tantalum capacitors: very good, small tantalum capacitors with very low esr are becoming more available. however, they can occasionally burn if subjected to very high current during charge. therefore, it is better to avoid this type of capacitor for the input filter of the device. they can, however, be subjected to high surge current when connected to the power supply. output capacitor the output capacitor is very important to meet the output voltage ripple requirement. using a small inductor value is useful to reduce the size of the choke but it 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 a very high frequency, its effect is negligible. for this reason, ceramic capacitors and very low esr capacitors in general should be avoided. tantalum and electrolytic capacitors are usua lly a good choice for this purpose. a list of some tantalum capacitor manufacturers is provided in table 7.: output capacitor selection . table 6. list of ceramic capacitors for the a5970d manufacturer series capacitor value ( ) rated voltage (v) taiyo yuden umk325bj106mm-t 10 50 murata grm42-2 x7r 475k 50 4.7 50
a5970d application information doc id 13955 rev 6 23/41 inductor the inductor value is very important as it fixes the ripple current flowing through the output capacitor. the ripple current is usually fixed at 20 - 40% of i omax , which is 0.2 - 0.4 a with i o max = 1 a. the approximate inductor value is obtained using the following formula: equation 20 where t on is the on time of the internal switch, given by d t. for example, with v out = 3.3 v, v in = 12 v and i o = 0.3 a, the inductor value is about 33 h. the peak current through the inductor is given by: equation 21 and it can be observed that if the inductor value decreases, the peak current (which must be lower than the current limit of the device) increases. so, when the peak current is fixed, a higher inductor value allows a higher value for the output current. in the table 8.: inductor selection , some inductor manufacturers are listed. table 7. output capacitor selection manufacturer series cap value ( f) rated voltage (v) esr (m ) sanyo poscap (1) 1. poscap capacitors have some characterist ics which are very similar to tantalum. tae 100 to 470 4 to 16 25 to 35 thb/c/e 100 to 470 4 to 16 25 to 55 avx tps 100 to 470 4 to 35 50 to 200 kemet t494/5 100 to 470 4 to 20 30 to 200 sprague 595d 220 to 390 4 to 20 160 to 650 table 8. inductor selection manufacturer series inductor value ( h) saturation current (a) coilcraft do3316t 15 to 33 2.0 to 3.0 coiltronics up1b 22 to 33 2.0 to 2.4 bi hm76-3 15 to 33 2.5 to 3.3 epcos b82476 15 to 33 2 to 3 wurth elektronik 74456115 15 to 33 2.5 to 3 l v in v out ? () i ---------------------------------- t on ? = i pk i o i 2 ----- + =
application information a5970d 24/41 doc id 13955 rev 6 8.2 layout considerations the layout of switching dc-dc converters is very important to minimize noise and interference. power-generating portions of the layout are the main cause of noise and so high switching current loop areas should be kept as small as possible and lead lengths as short as possible. high impedance paths (in particular the feedback connections) are susceptible to interference, so they should be as far as possible from the high current paths. an layout example is provided in figure 13 below. the input and output loops are minimized to avoid radiation and high frequency resonance problems. the feedback pin connections to the external divider are very close to the device to avoid pick-up noise. figure 13. layout example 8.3 thermal considerations the dissipated power of the device is tied to three different sources: conduction losses due to the not insignificant r dson , which are equal to: equation 22 where d is the duty cycle of the application. note that the duty cycle is theoretically given by the ratio between v out and v in , but in practice it is substant ially higher than this value to a5970d p on r dson i out () ? 2 d ? =
a5970d application information doc id 13955 rev 6 25/41 compensate for the losses in the overall application. for this reason, the switching losses related to the r dson increases compared to an ideal case. switching losses due to turning on and off. these are derived using the following equation: equation 23 where t rise and t fall represent the switching times of the power element that cause the switching losses when driving an inductive load (see figure 14 ). t sw is the equivalent switching time. figure 14. switching losses quiescent current losses. equation 24 where i q is the quiescent current. example: ?v in = 12 v ?v out = 3.3 v ?i out = 1 a p sw v in i out t on t off + () 2 ----------------------------------- - ? ? f sw v in = ? i out t sw ? f sw ? ? = p q v in i q ? =
application information a5970d 26/41 doc id 13955 rev 6 r ds(on) has a typical value of 0.25 @ 25 c and increases up to a maximum value of 0.5. @ 150 c. we can consider a value of 0.4 . t sw is approximately 70 ns. i q has a typical value of 2.5 ma @ v in = 12 v. the overall losses are: equation 25 the junction temperature of device will be: equation 26 where t a is the ambient temperature and rth j-a is the thermal resistance junction-to- ambient. considering that the device is mounted on board with a good ground plane, that it has a thermal resistance junction-to-ambient (rth j-a ) of about 120 c/w, and an ambient temperature of about 70 c: equation 27 8.4 short-circuit protection in overcurrent protection mode, when the peak current reaches the current limit, the device reduces the t on down to its minimum value (approximately 250 nsec) and the switching frequency to approximately one third of its nominal value even when synchronized to an external signal (see section 5.4: current protection ). in these conditions, the duty cycle is strongly reduced and, in most applications, this is enough to limit the current to ilim. in any event, in case of heavy short-circuit at the output (v o = 0 v) and depending on the application conditions (v cc value and parasitic effect of external components) the current peak could reach values higher than ilim. this can be understood considering the inductor current ripple during the on and off phases: on phase equation 28 off phase equation 29 where v d is the voltage drop across the diode, dcr l is the series resistance of the inductor. p tot r dson i out () ? 2 dv in i out ? t sw ? f sw v in i q = ? + ? + ? = 0.4 1 2 ? 0.3 12 1 ? 70 ? 10 9 ? ? 250 ? 10 3 12 2.5 ? 10 3 ? ? + ? + ? 0.36w ? = t j t a rth ja ? p tot ? + = t j 70 0.36 120 110 c ? ? + = i l ton v in v out ? dcr l r dson + () i ? ? l ------------------------------------------------------------------------------------ t on () = i l toff v d v out dcr l i ? ++ () ? l --------------------------------------------------------------- t off () =
a5970d application information doc id 13955 rev 6 27/41 in short-circuit conditions v out is negligible so during t off the voltage across the inductor is very small as equal to the voltage drop across parasitic components (typically the dcr of the inductor and the v fw of the free wheeling diode) while during t on the voltage applied the inductor is instead maximized as approximately equal to v in . so the equation 28 and the equation 29 in overcurrent conditions can be simplified to: equation 30 considering t on that has been already reduced to its minimum. equation 31 considering that f sw has been already reduced to one third of the nominal. in case a short circuit at the output is applied and v in = 12 v the inductor current is controlled in most of the applications (see figure 15 ). when the application must sustain the short-circuit condition for an extended period, the external components (mainly the inductor and diode) must be selected based on this value. in case the v in is very high, it could occur that the ripple current during t off ( equation 31 ) does not compensate the current increase during t on ( equation 30 ). the figure 17 shows an example of a power up phase with v in = v in max = 36 v where il ton > il toff so the current escalates and the balance between equation 30 and equation 31 occurs at a current slightly higher than the current limit. this must be taken into account in particular to avoid the risk of an abrupt inductor saturation. figure 15. short-circuit current v in = 12 v i l ton v in dcr l r dson + () i ? ? l ---------------------------------------------------------------- t on min () v in l -------- - 250ns () ? = i l toff v d v out dcr l i ? ++ () ? l --------------------------------------------------------------- 3t ? sw () v d v out dcr l i ? ++ () ? l --------------------------------------------------------------- 12 s () ? =
application information a5970d 28/41 doc id 13955 rev 6 figure 16. short-circuit current v in = 24 v figure 17. short-circuit current v in = 36 v 8.5 application circuit figure 18 shows the evaluation board application circuit, where the input supply voltage, v cc , can range from 4 v to 36 v and the output voltage is adjustable from 1.235 v to 6.3 v due to the voltage rating of the output capacitor,.
a5970d application information doc id 13955 rev 6 29/41 figure 18. evaluation board application circuit table 9. component list reference part number description manufacturer c1 grm42-2 x7r 475k 50 4.7 f, 50 v murata c2 poscap 6tae330ml 330 f, 6.3 v sanyo c3 c1206c221j5gac 47 pf, 5%, 50 v kemet c4 c1206c223k5rac 22 nf, 10%, 50 v kemet r1 5.6 k , 1%, 0.1 w 0603 neohm r2 3.3 k , 1%, 0.1 w 0603 neohm r3 22 k , 1%, 0.1 w 0603 neohm d1 stps3l40u 2 a, 40 v stmicroelectronics l1 do3316t-333mld 33 h, 2.1 a coilcraft a5970d l1 33uh
application information a5970d 30/41 doc id 13955 rev 6 figure 19. pcb layout (component side) figure 20. pcb layout (bottom side) figure 21. pcb layout (front side)
a5970d application information doc id 13955 rev 6 31/41 8.6 positive buck-boost regulator the device can be used to implement a step-up/down converter with a positive output voltage. the output voltage is given by: equation 32 where the ideal duty cycle d fo r the buck boost converter is: equation 33 however, due to power losses in the passive elem ents, the real duty cycle is always higher than this. the real value (that can be measured in the application) should be used in the following formulas. the peak current flowing in the embedded switch is: equation 34 while its average cu rrent is equal to: equation 35 this is due to the fact that the current flowing through the internal power switch is delivered to the output only during the off phase. the switch peak current must be lower than the minimum current limit of the overcurrent protection (see ta b l e 4 for details) while the average current must be lower than the rated dc current of the device. as a consequence, the maximum output current is: equation 36 where i sw max represents the rated current of the device. the current capability is reduced by the term (1-d) and so, for example, with a duty cycle of 0.5, and considering an average current through the switch of 1 a, the maximum output current deliverable to the load is 0.5 a. the figure below shows the schematic circuit of this topology for a 12 v output voltage and 5 v input. v out v in d 1d ? ------------- ? = d v out v in v out + ----------------------------- - = i sw i load 1d ? --------------- i ripple 2 ------------------- - + i load 1d ? --------------- v in 2l ? ---------- - d f sw --------- ? + == i sw i load 1d ? --------------- = i out max i sw max 1d ? () ? ?
application information a5970d 32/41 doc id 13955 rev 6 figure 22. positive buck-boost regulator 8.7 negative buck-boost regulator in figure 23 , the schematic circuit for a standard buck-boost topology is shown. the output voltage is: equation 37 where the ideal duty cycle d fo r the buck boost converter is: equation 38 the considerations given in section 8.7 for the real duty cycle are still valid here. also the equation 34 till equation 36 can be used to calculate the maximum output current. so, as an example, considering the conversion v in = 12 v to v out = -5 v, i load = 0.2 a: equation 39 equation 40 an important thing to take into account is that the ground pin of the device is connected to the negative output voltage. therefore, the device is subjected to a voltage equal to v in -v o , which must be lower than 36 v (the maximum operating input voltage). v out v in ? d 1d ? ------------- ? = d v ? out v in v out ? ----------------------------- - = d 5 512 + --------------- - 0.706 == i sw i load 1d ? --------------- 0.2 10.706 ? ----------------------- - 0.7a == =
a5970d application information doc id 13955 rev 6 33/41 figure 23. negative buck-boost regulator 8.8 synchronization example see chapter 5.3 for details. figure 24. synchronization example 8.9 compensation network with mlcc at the output mlccs (multiple layer ceramic capacitor) with values in the range of 10 f-22 f and rated voltages in the range of 10 v-25 v are available today at relatively low cost from many manufacturers. these capacitors have very low esr values (a few m ) and thus are occasionally used for the output filter in order to reduce the voltage ripple and the overall size of the application. however, a very low esr value affects the compensation of the loop (see section 7 ) and in order to keep the system stable, a more complicated compensation network may be required. however, due to the architecture of the internal error amplifier the bandwidth with this compensation is limited. that is why output capacitors with a not negligible esr are suggested. the selection of the output capacitor have to guarantee that the zero introduced by this component is inside the designed system bandwidth and close to the frequency of the double pole introduced by the lc filter. a general rule for the selection of this compound for the syst em stability is provided in equation 41 . -12v/0.3a a5970d a5970d a5970d
application information a5970d 34/41 doc id 13955 rev 6 equation 41 the figure below shows an example of a compen sation network stabiliz ing the system with ceramic capacitors at the output (the optimum component value depends on the application). figure 25. mlcc compensation network example f z esr 1 2 esr c out ?? ? ----------------------------------------------- - = bandwidth < f lc f < z esr 10 f lc ? <
a5970d application information doc id 13955 rev 6 35/41 8.10 external soft_start network at start-up the device can quickly increase the current up to the current limit in order to charge the output capacitor. if soft ramp-up of the output voltage is required, an external soft-start network can be implemented as shown in figure 26 . the capacitor c is charged up to an external reference through r and the bjt clamps the comp pin. this clamps the duty cycle, limiting the slew rate of the output voltage. figure 26. soft-start network example a5970d
typical characteristics a5970d 36/41 doc id 13955 rev 6 9 typical characteristics figure 27. line regulation f igure 28. load regulation figure 29. output voltage vs junction temperature figure 30. switching frequency vs junction temperature figure 31. quiescent current vs junction temperature figure 32. shutdown current vs junction temperature 3.276 3.28 3.284 3.288 3.292 3.296 3.3 3.304 3.308 3.312 00.511.5 io (a) vo (v) tj = 125c tj = 25c vcc = 12v vo = 3.3v
a5970d typical characteristics doc id 13955 rev 6 37/41 figure 33. junction temperature vs output current figure 34. junction temperature vs output current figure 35. efficiency vs output current figure 36. efficiency vs output current
package mechanical data a5970d 38/41 doc id 13955 rev 6 10 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.
a5970d package mechanical data doc id 13955 rev 6 39/41 table 10. so-8 mechanical data dim. mm. inch min typ max min typ max a 1.35 1.75 0.053 0.069 a1 0.10 0.25 0.004 0.010 a2 1.10 1.65 0.043 0.065 b 0.33 0.51 0.013 0.020 c 0.19 0.25 0.007 0.010 d (1) 1. dimensions d does not include mold flash, protrusions or gate burrs. mold flash, potrusions or gate burrs shall not exceed 0.15mm (.006inc h) in total (both side). 4.80 5.00 0.189 0.197 e 3.80 4.00 0.15 0.157 e 1.27 0.050 h 5.80 6.20 0.228 0.244 h 0.25 0.50 0.010 0.020 l 0.40 1.27 0.016 0.050 k 0 (min.), 8 (max.) ddd 0.10 0.004 figure 37. package dimensions
revision history a5970d 40/41 doc id 13955 rev 6 11 revision history table 11. document revision history date revision changes 06-aug-2007 1 initial release. 24-oct-2007 2 updated: table 4 on page 5 2-may-2008 3 updated cover page, table 4 on page 5 27-aug-2008 4 updated cover page and table 4 on page 5 22-apr-2009 5 updated first feature in coverpage 03-nov-2009 6 updated table 4 on page 6 and added figure 28 on page 36
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