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january 2008 rev 4 1/30 AN1518 application note designing with the 2.5 a dc-dc converter l5973d introduction the l5973d is a step-down monolithic power switching regulator capable of delivering up to 2.5 a at output voltages from 1.235 v to 35 v. the operating input voltage ranges from 4.4 v to 36 v. it has been designed using bcdv te chnology and the power switching element is implemented through a p-channel d-mos transistor. it does not require a bootstrap capacitor, and the duty cycle can range up to 100%. an internal oscillator fixes the switching frequency at 250 khz. this minimizes the lc out put filter. a synchronization pin is available for cases where a higher frequency (up to 500 khz) is required. pulse-by-pulse and frequency foldback overcurrent protection offer effective short circuit protection. other features are voltage feed-forward, protection against feedback disconnection, inhibit and thermal shutdown. the device is housed in an hsop8 package with exposed pad that helps to reduce the thermal resistance junction-to-ambient (r thj-a ) down to approximately 40 c/w. figure 1. evaluation board l5973d (hsop8) board dimensions: 23x20 mm figure 2. package figure 3. pin connection hsop8 - exposed pad out sync inh comp 1 3 2 4 vcc vref gnd fb 8 7 6 5 d98in955 www.st.com
contents AN1518 2/30 contents 1 pin functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 functional description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1 power supply & voltage reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.2 voltages monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.3 oscillator & synchronizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.4 current protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.5 error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.6 pwm comparator and power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.7 inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.8 thermal shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4 additional features and protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.1 feedback disconnection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.2 output overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4.3 zero load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5 closing the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6 error amplifier and com pensation network . . . . . . . . . . . . . . . . . . . . . 11 7 lc filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 8 pwm comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 9 application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 9.1 component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 9.2 layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 9.3 thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 9.4 short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
AN1518 contents 3/30 9.5 application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 10 application ideas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 10.1 positive buck-boost regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 10.2 buck-boost regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 10.3 dual output voltage with auxiliary winding . . . . . . . . . . . . . . . . . . . . . . . . 26 10.4 synchronization example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 10.5 compensation network with mlcc (multiple layer ceramic capacitor) at the output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 10.6 external soft_start network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 11 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
list of figures AN1518 4/30 list of figures figure 1. evaluation board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 2. package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 3. pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 figure 4. block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 figure 5. internal circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 6. oscillator circuit block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 7. current limitation circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 figure 8. driving circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 9. block diagram of the loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 10. error amplifier equivalent circuit and compensation network . . . . . . . . . . . . . . . . . . . . . . . 12 figure 11. module plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 12. phase plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 13. layout example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 14. short-circuit current v in = 25 v. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 figure 15. short-circuit current v in = 30 v. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 16. evaluation board application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 figure 17. pcb layout (component side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 18. pcb layout (bottom side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 figure 19. pcb layout (front side) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 20. junction temperature vs. output current (v cc = 5 v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 21. efficiency vs. output current (v cc = 5 v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 22. junction temperature vs. output current (v cc = 12 v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 figure 23. efficiency vs. output current (v cc =12 v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 figure 24. positive buck-boost regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 figure 25. buck-boost regulator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 26. dual output voltage wi th auxiliary winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 27. synchronization example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 figure 28. mlcc compensation network example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 figure 29. soft start network example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
AN1518 pin functions 5/30 1 pin functions 1.1 pin description 2 block diagram figure 4. block diagram table 1. pin functions n. name description 1 out regulator output 2 sync master/slave synchronization. when open, a si gnal synchronous with the turn-off of the internal power is present. when connected to an external signal at a frequency higher than the internal one, the device is synchronized by the external signal. connecting the sync pins of two devices, the one with the higher frequency works as master and the other one works as slave. 3inh a logical signal (active high) disables the device. with an ihn higher than 2.2 v the device is off and with an inh lower than 0.8 v, the device is on. if inh is not used, the pin must be grounded. wh en it is open, an internal pull-up disables the device. 4 comp e/a output to be used for frequency compensation 5fb step-down feedback input. connec ting the output voltage directly to this pin results in an output voltage of 1.235 v. an external resistor divider is required for higher output voltages (the typical value for the resistor connecte d between this pin and ground is 4.7 k). 6v ref reference voltage of 3.3 v. no filt er capacitor is needed for stability 7 gnd ground 8v cc unregulated dc input voltage
functional description AN1518 6/30 3 functional description the main internal blocks are shown in the device block diagram in figure 4 . 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 sawtooth osc illator with a frequen cy 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. 3.1 power supply & voltage reference the internal regulator circuit (shown in figure 5 ) consists of a start-up circuit, an internal voltage preregulator, 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 preregulator block supplies the bandgap cell with a preregulated voltage v reg that has a very low supply voltage noise sensitivity. 3.2 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 5. internal circuit
AN1518 functional description 7/30 3.3 oscillator & synchronizer figure 6 shows the block diagram of the oscillator circuit. the clock generator provides the switching frequency 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 internal voltage feed-forward, while the synchronizer circuit generates the synchronization signal. the device also has a synchronization pin which can work both as master and slave. as master, it serves to synchronize external devices to the internal switching frequency, and as slave to synchronize itself using an external signal up to 500 khz. in particular, when connecting together two devices the one with the lower switching frequency works as a slave and the other as master. to synchronize the device, the sync pin must pass from a low level to a level higher than the synchronization threshold with a duty cycl e that can vary from approximately 10% to 90%, depending also on the signal frequency and amplitude. the frequency of the synchronization signal must be, at a minimum, higher than the internal switching frequency of the device (250 khz). figure 6. oscillator circuit block diagram 3.4 current protection the l5973d 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 avoid false a overcurrent signal) is too short to obtain a sufficiently low duty cycle at 250 khz, the output current, in strong overcurrent or short circuit conditions, could increase
functional description AN1518 8/30 again. for this reason the switching frequency is also reduced, thus keeping the inductor current under its maximum threshold. the frequency shifter ( figure 6 ) functions based on the feedback voltage. as the feedback voltage decreases (due to the reduced duty cycle), the switching freque ncy decreases also. figure 7. current limitation circuitry 3.5 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. 3.6 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. table 2. uncompensated error amplifier characteristics description values transconductance 2300 s low frequency gain 65db minimum sink/source voltage 1500 a/300 a output voltage swing 0.4 v/3.65 v input bias current 2.5 a
AN1518 functional description 9/30 however, there is a limit introduced by the recovery time of the recirculation diode. 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 parasitics. 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 parasitics 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 3.7 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.
additional features and protection AN1518 10/30 3.8 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 (150 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. 4 additional features and protection 4.1 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. 4.2 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 14 ), 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. 4.3 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 ? =
AN1518 closing the loop 11/30 5 closing the loop figure 9. block diagram of the loop 6 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 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 + ? + ? ? ? ------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------ - =
lc filter AN1518 12/30 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 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 lc filter the transfer function of the l-c filter is given by: equation 6 f p1 1 2 ? r 0 ? c c ? -------------------------------------- - = f p2 1 2 ? r c ? c 0 c p + () ? --------------------------------------------------------- = f z1 1 2 ? r c ? c c ? -------------------------------------- - = a lc s () r load 1esrc out s ? ? + () ? s 2 lc out esr r load + () sesrc out ? r load l + ? () r load + ? + ? ? ? ------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------ - =
AN1518 pwm comparator 13/30 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 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 8 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 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 + ? + f plc 1 2 ? lc out ? ? ----------------------------------------------- - = g pwm s () v cc v oscmax v oscmin ? () ------------------------------------------------------------- = v oscmax v oscmin ? kv cc ? =
pwm comparator AN1518 14/30 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 example : considering r c = 2.7 k ? , c c = 22 nf and c p = 220 pf, the poles and zeroes of a 0 are: f p1 = 9 hz f p2 = 256 khz f z1 = 2.68 khz if l = 22 h, c out = 100 f and esr = 80 m ? , the poles and zeroes of a lc become: f plc = 3.39 khz f 0 = 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 g pwm s () 1 k --- - const == gs () g pwm s () r 2 r 1 r 2 + -------------------- ? a o s () ? a lc ? s () =
AN1518 application information 15/30 figure 12. phase plot the cut-off frequency and the phase margin are: equation 16 9 application information 9.1 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 f c 22.8khz = phase margin = 39.8 i rms i o d 2d 2 ? ---------------- ? d 2 ------ - + ? = d max v out v f + v inmin v sw ? ------------------------------------ - =
application information AN1518 16/30 and equation 19 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 price and their availability 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 4.: output capacitor selection . table 3. list of ceramic capacitors for the l597xd manufacturer series capacitor value ( ) rated voltage (v) tdk c3225 10 25 murata grm32 10 25 grm55 10 50 d min v out v f + v inmax v sw ? -------------------------------------- =
AN1518 application information 17/30 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 0.4 - 0.8 a with i o max= 2 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 = 2 v and ? i o =0.6 a, the inductor value is about 17 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 5.: inductor selection , some inductor manufacturers are listed. 9.2 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. table 4. output capacitor selection manufacturer series cap value ( f) rated voltage (v) esr (m ? ) avx tps 100 to 470 4 to 35 50 to 200 kemet t494/5 100 to 470 4 to 20 30 to 200 sanyo poscap (1) 1. poscap capacitors have some characterist ics which are very similar to tantalum. tpa/b/c 100 to 470 4 to 16 40 to 80 sprague 595d 220 to 390 4 to 20 160 to 650 table 5. inductor selection manufacturer series inductor value ( h) saturation current (a) coilcraft do3316 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 AN1518 18/30 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. another important issu e is the groundplane of the board. since the package has an exposed pad, it is very important to connect it to an extended groundplane in order to reduce the thermal resistance junction-to-ambient. figure 13. layout example 9.3 thermal considerations the dissipated power of the device is tied to three different sources: switching 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 substantially higher than this value to compensate for the losses in the overall application. for this reason, the switching losses related to the r dson increase compared to an ideal case. switching losses due to turning on and off. these are derived using the following equation: equation 23 l5973d cin d cout l inhibit signal to output voltage vin vout gnd r1 r2 1 4 5 8 very small high current circulating path to minimize radiation and high frequency resonance problems output capacitor directly connected to heavy ground compensation network far from high current paths minimun size of feedback pin connections to avoid pickup connection to groundplane through vias extended groundplane on the bottom side p on r dson i out () ? 2 d ? = p sw v in i out t on t off + () 2 ----------------------------------- - ? ? f sw v in = ? i out t sw ? f sw ? ? =
AN1518 application information 19/30 where t on and t off are the overlap times of the voltage across the power switch and the current flowing into it during the turn on and turn off phases. t sw is the equivalent switching time. quiescent current losses. equation 24 where i q is the quiescent current. example: ?v in = 5 v ?v out = 3.3 v ?i out = 2 a r dson 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 groundplane, that it has a thermal resistance junction-to-ambient (rth j-a ) of about 42 c/w, and an ambient temperature of about 70 c: equation 27 p q v in i q ? = p tot r dson i out () ? 2 dv in i out ? t sw ? f sw v in i q = ? + ? + ? = 0.4 2 2 ? 0.7 5 2 ? 70 ? 10 9 ? ? 250 ? 10 3 ? 52.5 ? 10 3 ? ? + ? + ? 13w , ? = t j t a rth ja ? p tot ? + = t j 70 1.3 42 125 c ? ? + =
application information AN1518 20/30 9.4 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 (see section 3.4: 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 coul d reach values higher than ilim. this can be understood considering the inductor curren t 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 and dcr l is the series resistance of the inductor. in short-circuit conditions, v out is negligible. so during t off , the voltage applied to the inductor is very small and it could occur that the current ripple in this phase does not compensate for the current ripple during t on . the maximum current peak can be easily measured through the inductor with v o = 0 v (short-circuit) and v cc =v inmax . in cases where 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. figure 14. short-circuit current v in = 25 v i l ? v in v out ? dcr l i ? ? () l ------------------------------------------------------------- - t on = = i l ? v d v out ? dcr l i ? ? () l ----------------------------------------------------------- - t off = =
AN1518 application information 21/30 figure 15. short-circuit current v in = 30 v in figure 14 and figure 15 , for example, it can be observed that when the input voltage increases for a given component list, the current peak increases also. the current limit is immediately triggered but the current peak increases until the current ripple during t off is equal to the current ripple during t on . 9.5 application circuit figure 16 shows the evaluation board application circuit, where the input supply voltage, v cc , can range from 4.4 v to 25 v due to the voltage rating of the input capacitor, and the output voltage is adjustable from 1.235 v to v cc . figure 16. evaluation board application circuit table 6. component list reference part number description manufacturer c1 grm32dr61e106ka12l 10 f, 25 v murata c2 poscap 6tpb330m 330 f, 6.3 v sanyo
application information AN1518 22/30 figure 17. pcb layout (component side) figure 18. pcb layout (bottom side) c3 c1206c221j5gac 220 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 4.7 k ? , 1%, 0.1 w 0603 neohm d1 stps2l25u 2 a, 25 v stmicroelectronics l1 do3316p-153 15 h, 3 a coilcraft table 6. component list (continued) reference part number description manufacturer gnd ref vin c1 r2 eval.board l5973d r1 www.st.com d1 u1 r3 c3 c4 l1 c2 vout gnd. syn inh r bottom 21-se p -2007 rigo ls
AN1518 application information 23/30 figure 19. pcb layout (front side) below, some graphs are provided which show the t j versus output current in different input and output voltage conditions, as well as some efficiency measurements. top 21-se p -2007 rigo lc figure 20. junction temperature vs. output current (v cc = 5 v) figure 21. efficiency vs. output current (v cc = 5 v)
application information AN1518 24/30 figure 22. junction temperature vs. output current (v cc = 12 v) figure 23. efficiency vs. output current (v cc =12 v)
AN1518 application ideas 25/30 10 application ideas 10.1 positive buck-boost regulator the device can be used to implement a step-up/down converter with a positive output voltage. the figure below shows the schematic circuit of this topology for a 12 v output voltage. the input voltage can range from 5 v and 35 v. the output voltage is given by v o =v in d/ (1-d), where d is the duty cycle. the maximum output current is given by i out =1 (1-d). the current capability is reduced by the term (1-d) and so, for example, with a duty cycle of 0.5, and considering on average current through the switch of 2 a, the maximum output current deliverable to the load is 1 a. 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. figure 24. positive buck-boost regulator 10.2 buck-boost regulator in figure 25 , the schematic circuit for a standard buck-boost topology is shown. the output voltage is given by v o =-v in d/(1-d). the maximum output current is equal to i out =1 (1-d), for the same reason as that of the up/down converter. 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).
application ideas AN1518 26/30 figure 25. buck-boost regulator 10.3 dual output voltage with auxiliary winding when two output voltages are required, it is possible to create a dual output voltage converter by using a coupled inductor. during the on phase, the current is delivered to v out while d2 is reverse-biased. during the off phase, the current is delivered, through the auxiliary winding, to the output voltage v out1 . this is possible only if the magnetic core has stored sufficient energy. so, to be certain that the application is working properly, the load related to the second output v out1 should be much lower than the load related to v out . figure 26. dual output voltage with auxiliary winding 10.4 synchronization example two or more devices (up to 6) can be synchronized simply by connecting the synchronization pins. in this case, the device with a slightly higher switching frequency value will work as a master and the ones with slight ly lower switching frequency values will work as slaves. the device can also be synchronized from an external source. in this case the logic signal must have a frequency higher than the internal switching frequency of the device (250 khz).
AN1518 application ideas 27/30 figure 27. synchronization example 10.5 compensation network with mlcc (multiple layer ceramic capacitor) at the output mlccs 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 5 ) and in order to keep the system stable, a more complicated compensation network may be required. the figure below shows an example of a compensation network stabilizing the system with ceramic capacitors at the output (the optimum component value depends on the application). figure 28. mlcc compensation network example
application ideas AN1518 28/30 10.6 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 29 . the capacitor c is charged up to an external reference through r and the b jt clamps the comp pin. this clamps the duty cycle, limiting the slew rate of the output voltage. figure 29. soft start network example
AN1518 revision history 29/30 11 revision history table 7. document revision history date revision changes 07-sep-2003 1 first release 05-oct-2006 2 ? new template ? ta b l e 3 added 22-may-2007 3 ? section 5: closing the loop modified 31-jan-2008 4 ? minor text changes ? document title modified
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