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2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page1 pt1203_ds rev en1.0 pt1203 general description the pt1203 is a monolithic pulse-width-modulated (pwm) synchronous step-down switch mode regulator with two internal power mosfets. it achieves 2a continuous output current over a wide input supply range with excellent load and line regulation. current mode operation provides fast transient response and eases loop stabilization. fault condition protection includes cycle-by-cycle current limit and thermal shutdown. the pt1203 requires a minimum number of external components, available in so p-8 and esop-8 package. features z 2a output current z 0.13 ? internal power mosfet switch z stable with low esr output ceramic capacitors z up to 93% efficiency z fixed 350khz frequency z thermal shutdown z cycle-by-cycle over current protection z wide 4.75v to 18v operating input range z output adjustable from 0.923v to 15v z available in 8-pin sop8 and esop8 packages application z pc monitors z lcd tv z distributed power systems z network systems z fpga, dsp, asic power supplies z notebook computers ordering information package temperature range ordering part number transport media marking sop-8 -40 to 85 pt1203esoh tape and reel 2500 units pt1203 xxxxxx esop-8 -40 to 85 PT1203EESH tape and reel 2500 units pt1203 xxxxxx note: typical application circuit xxxxxx assembly factory code lot number figure1 pt1203 typical application circuit
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page2 pt1203_ds rev en1.0 pt1203 pin assignment pin descriptions sop8 pin no. pin names description 1 bs high-side gate driver boost input. bs supplies the drive for the high-side n-channel mosfet switch. connect a 0.01uf or greater capacitor from sw to bs to power the high side switch. 2 in supply voltage. the pt1203 operates from a +4.75v to +18v unregulated input. c1 is needed to prevent large voltage spikes from appearing at the input. 3 sw power switching output. this pin is connected to in through m1 or to gnd through m2. 4 gnd ground 5 fb feedback input. fb senses the output voltage through an external resistive divider which is connected from the output to gnd. 6 comp compensation node. comp compensates the control loop by connecting a serial rc network to ground. in some cases, an additional capacitor from comp to gnd is required. 7 en enable input. en is digital input that turns the regulator on or off. drive en high to turn on the regulator, drive it low to turn it off. pull up to in with 100k ? resistor for automatic startup. 8 ss soft-start. connect a capacitor from ss to gnd to program the soft-start period. leave it open if unused. bs in sw gnd ss en comp fb top view sop8 bs in sw gnd ss en comp fb top view exposed pad on backside connect to gnd pin esop8
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page3 pt1203_ds rev en1.0 pt1203 absolute maximum ratings (note 1) symbol items value unit in supply voltage -0.3~24 v sw switch node voltage -0.3~24 v bs bootstrap voltage vsw-0.3~vsw+6 v fb feedback voltage -0.3~6 v en enable/uvlo voltage -0.3~6 v comp comp voltage -0.3~6 v tstg storage temperature range -65~+150 recommended operating range symbol items value unit vin supply voltage 4.75~18 v topt operating temperature -40~+85 note 1: absolute maximum ratings indicate limits beyond which damage to the device may occur. recommended operating range indicates conditions for which the device is functional, but do not guarantee specific performance limits. electrical characteristics state dc and ac electrical specifications und er particular test conditions which guarantee specific performance limits. this assumes that the device is within the operatin g range. specifications are not guaranteed for parameters where no limit is given, however, the typical value is a good indication of device performance. electrical characteristics the following specifications apply for v in =12v, v out =3.3v, load=0.5a, t a =25 o c, unless specified otherwise. symbol items conditions min. typ. max. unit iq supply current en=2v; vfb=1v 1.5 2.0 ma vfb feedback voltage 0.900 0.923 0.946 v av error amplifier voltage gain 400 v/v gea error amplifier trans-conductance ? icomp=+/-10ua 800 ua/v r_dson1 high side switch on resistance 0.13 ? r_dson2 low side switch on resistance 0.13 ? ipd shutdown supply current ven = 0v 13 20 ua i_leakage high side switch leakage current (2) ven=0v; vsw=0v 0 10 ua i_clh high side switch current limit (2) 2.4 3.5 a i_cll low side switch current limit (2) 1.0 a gcomp comp to current sense trans-conductance 3.2 a/v fosc1 oscillation frequency 350 khz
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page4 pt1203_ds rev en1.0 pt1203 electrical character istics (continued) symbol items conditions min. typ. max. unit fosc2 short circuit oscillation frequency vfb=0.3v 90 khz dmax maximum duty cycle vfb=0.8v 90 % ven en threshold voltage 1.1 1.5 2.0 v ven_hyst en threshold voltage hysteresis 0.2 v tsd thermal shutdown 160 oc note 2: guaranteed by design. simplified block diagram q q set clr s r pwm comparator regulator oscilator 90k/350k + - 1.2v 6ua scp comparator current sense amplifier + - 5v 2 vin 4 gnd fb 5 ss 8 en 7 3 sw 1 bs 6 comp ovp comparator ovp + - + - thermal shutdown 0.4v 1.1v 0.923v 5v uvlo tsd ovp !en m1 0.13 ? m2 0.13 ? error comparator + - figure 2?functional block diagram
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page5 pt1203_ds rev en1.0 pt1203 operation description the pt1203 is a synchronous rectified current mode step-down regulator. it regulates input voltage from 4.75v to 18v down to a voltage as low as 0.923v, and supplies up to 2a of load current. on the rising edge of internal oscillator, the low-side mosfet will be turned off and the high-side mosfet will be turned on if the pwm comparator output is low. the output voltage is measured at fb pin through a resistive voltage divider and amplified through the internal error amplifier, the voltage at the comp pin is compared with the switch current. as the switch current ramp up, the pwm comparator will output high, the high-side mosfet will be turned off and the low-side mosfet will be turned on until the next rising edge of the internal oscillator. when the output voltage exceeds 20% of the nominal regulated voltage, the over-voltage comparator will be tripped and turn off the high-side mosfet by discharging the ss pin to gnd. when the fb voltage is less than 0.4v, the frequency of the internal oscillator will be set to 90khz to decease the short circuit current. application information setting the output voltage the output voltage is set using a resistive voltage divider from the output to fb pin. the voltage divider divides the output voltage down to the feedback voltage by the ratio: 2 1 2 r r r v v out fb + = , where the fb v is the feedback voltage and out v is the output voltage. thus the output voltage is : 2 2 1 * 923 . 0 r r r v out + = . the typical value of r2 is 10k ? . inductor selection the inductor is required to supply constant current to the output load while being driven by the switched input voltage. a larger value inductor will result in less ripple current that will result in lower output ripple voltage. a good rule for choosing the inductor is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. the inductance value can be calculated by: ? ? ? ? ? ? ? ? ? = in out l sw out v v i f v l 1 * * , where out v is the output voltage, in v is the input voltage, sw f is the switching frequency, and l i is the peak-to-peak inductor ripple current. choose an inductor that will not saturate under the maximum inductor peak current. the peak inductor current can be calculated by: ? ? ? ? ? ? ? ? ? + = in out sw out load lp v v l f v i i 1 * * * 2 , where load i is the load current. input capacitor selection the input current to the converter is discontinuous, therefore a capacitor is required to supply the ac current to the converter while maintaining the dc input voltage. use the low esr capacitors for the best performance. ceramic capacitors are preferred, but tantalum or low-esr electrolytic capacitors may also suffice. since the input capacitor absorbs the input switching current it requires an adequate ripple current rating. the rms current in the input capacitor can be estimated by:
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page6 pt1203_ds rev en1.0 pt1203 ? ? ? ? ? ? ? ? ? = in out in out load c v v v v i i in 1 * * . choose the input capacitor whose rms current rating greater than the calculated value. output capacitor selection the output capacitor is required to maintain the dc output voltage. ceramic, tantalum, or low esr electrolytic capacitors are recommended. low esr capacitors are preferred to keep the output voltage ripple low. the output voltage ripple can be estimated by: ? ? ? ? ? ? ? ? + ? ? ? ? ? ? ? ? ? = 2 * * 8 1 * 1 * * c f r v v l f v v sw esr in out sw out out , where 2 c is the output capacitance value and esr r is the equivalent series resistance value of the output capacitor. compensation components selection the system stability and transient response are controlled through the comp pin. a series capacitor-resistor combination sets a pole-zero combination to control the characteristics the system. the dc gain of the voltage feedback loop is: out fb ea cs load dc v v a g r a * * * = , where ea a is the error amplifier voltage gain, cs g is the current sense transconductance and load r is the load resistor value. the voltage feedback loop has two poles of importance. one is due to the compensation capacitor and the output resistor of the error amplifier, and the other is due to the output capacitor and the load resistor, these poles are located at: load p ea ea p r c f a c g f * * 2 1 * * 2 2 2 3 1 = = , where ea g is the error amplifier transconductance. the system has one zero of importance, due to the compensation capacitor (c3) and the compensation resistor (r3). this zero is located at: 3 3 1 * * 2 1 r c f z = . the system may have another zero of importance, if the output capacitor has a large capacitance and/or a high esr value. the zero due to the esr and capacitance of the output capacitor, is located at: esr esr r c f * * 2 1 2 = , in this case, a third pole set by the compensation capacitor (c6) and the compensation resistor (r3) is used to compensate the effect of the esr zero on th e loop gain. the pole is located at: 3 6 3 * * 2 1 r c f p = . the goal of compensation is to shape the converter transfer function to get the desired loop gain. lower crossover frequencies result in slower load transient and line responses, wh ile higher crossover frequencies could cause system instability. a good rule of thumb is to set the crossover frequency blow one-tenth of the switching frequency. the following procedure can be used to optimize the compensation components. 1. choose the compensation re sistor (r3) to set the desired crossover frequency.
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page7 pt1203_ds rev en1.0 pt1203 fb out cs ea c v v g g f c r * * * * 2 3 2 = , where c f is the desired crossover frequency which is typically below one tenth of the switching frequency. 2. choose the compensation capacitor (c3) to achieve the desired phase margin. for typical applications, setting the compensation zero below one-forth of the crossover frequency will provides sufficient phase margin. c f r c * * 2 4 3 3 > , where r3 is the compensation resistor. 3. if the esr zero of the output capacitor is located at less than half of the switching frequency, or the following relationship is valid: 2 * * 2 1 2 sw esr f r c < . if this is the case, then add the second compensation capacitor (c6) to set the pole at the location of the esr zero: 3 2 6 * r r c c esr = . optional schottky diode selection during the transition between high-side switch and low-side switch, the body diode of the low-side power mosfet conducts the inductor current. the forward voltage of the body diode is high. an optional schottky diode may be paralleled between the sw pin and gnd pin to improve overall efficiency.
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page8 pt1203_ds rev en1.0 pt1203 typicalperformance characteristics vin=12v vo = 3 . 3 v l=10uh c1=10uf c2=22uf ta=+2 5unless otherwise noted. efficiency vs load current osc frequency vs temperature output voltage vs temperature line regulation load regulation
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page9 pt1203_ds rev en1.0 pt1203 startup through enable vin=12v vout=3.3v iout=2a resistance load shutdown through enable vin=12v vout=3.3v iout=2a resistance load light load operation vin=12v vout=3.3v iout=0a iin=8ma medium load operation vin=12v vout=3.3v iout=1a heavy load operation vin=12v vout=3.3v iout=2a load transient vin=12v vout=3.3v iout=0a to 1a
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page10 pt1203_ds rev en1.0 pt1203 short circuit protection short circuit recovery
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page11 pt1203_ds rev en1.0 pt1203 package information sop8 l dimensions in millimeters dimensions in inch symbol min max min max a 1.350 1.750 0.053 0.069 a1 0.100 0.250 0.004 0.010 a2 1.350 1.550 0.053 0.061 b 0.330 0.510 0.013 0.020 c 0.170 0.250 0.006 0.010 d 4.700 5.100 0.185 0.200 e 3.800 4.000 0.150 0.157 e1 5.800 6.200 0.228 0.244 e 1.270(bsc) 0.050(bsc) l 0.400 1.270 0.016 0.050 0 8 0 8
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page12 pt1203_ds rev en1.0 pt1203 esop8 d1 e2 b b e 1 b e e 0.25 l l 1 c h 2 b b 1 c 1 c base metal with plating secion b-b
2a, 18v, 350khz synchronous step-down converter china resources powtech (shanghai) limited www.crpowtech.com page13 pt1203_ds rev en1.0 pt1203 millimeters symbol min nom max a 1.75 a1 0.05 0.15 a2 1.30 1.40 1.50 a3 0.60 0.65 0.70 b 0.39 0.48 b1 0.38 0.41 0.43 c 0.21 0.26 c1 0.19 0.20 0.21 d 4.70 4.90 5.10 e 5.80 6.00 6.20 e1 3.70 3.90 4.10 e 1.27(bsc) h 0.25 0.50 l 0.50 0.80 l1 1.05(bsc) 0 8 d1 2.13ref 2.90ref e2 2.13ref 2.00ref

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