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a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 1 - de s cri p t i o n f eat u r es A7431 is a wide input voltage, high efficiency cc/cv step - down dc - dc converter that operates in either cv (constant output voltage) mode or cc (constant output current) mode. A7431 provides up to 3a output current at 125khz switching frequency. A7431 eliminates the expensive, high accuracy current sense resistor, making it ideal for battery charging applications and adaptors with accurate current limit. the A7431 achieves higher efficiency than traditional constant current switching re gulators by eliminating its associated power loss on the additional current sensing resistor. A7431 provides ovp pin for output over voltage protection. A7431 integrates adaptive gate drive to achieve excellent emi performance passing en55022 class b emc s tandard without adding additional emi components while maintaining high conversion efficiency. protection features include cycle - by - cycle current limit, thermal shutdown, and frequency foldback at short circuit. A7431 are available in a p sop8 package and require very few external devices for operation. the A7431 is available in psop8 package. o rde ri ng i nf o rm a t i o n p ac k ag e t y pe p ar t n um ber psop8 spq: 4,000pcs/reel mp8 A7431 mp8 r A7431 mp8vr note v: halogen free package r: tape & reel ait provides all rohs products ? 42v input voltage surge ? 36 v steady state operation ? up to 3a output current ? output voltage up to 12v ? 125khz switching frequency ? up to 91% efficiency ? stable with low - esr ceramic capacitors to allow low - profile designs ? integrated over voltage pr otection ? excellent emi performance ? constant current control without additional current sensing resistor improves efficiency and lowers cost. ? resistor programmable current limit from 1.5a to 3.5a ? up to 0.5v excellent cable voltage drop compensation ? 7.5% cc accuracy ? 2% feedback voltage accuracy ? advanced feature set integrated soft start thermal shutdown cycle - by - cycle current limit ? available in psop8 package appl i c at i o n ? car charger/ adaptor ? rechargeable portable devices ? general - purpose cc/cv supply ty p i c al ap pl i c at i o n
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 2 - p i n de s cri p t i o n top view p i n # s y mb ol f u nc t i on 1 bs high side bias pin. this provides power to the internal high - side mosfet gate driver. connect a 22nf capacitor from bs pin to sw pin. 2 v in power supply input. bypass this pin with a minimum 10f ceramic capacitor to gnd, placed as close to the ic as possible. 3 sw power switching output to external inductor. 4 gnd ground. connect this pin to a large pcb copper area for best heat dissipation . return fb, comp, and i set to this gnd, and connect this gnd to power gnd at a single point for best noise immunity. 5 fb feedback input. the voltage at this pin is regulated to 0.808v. connect to the resistor divider between output and gnd to set the ou tput voltage. 6 comp error amplifier output. this pin is used to compensate the converter. 7 ovp ovp input. if the voltage at this pin exceeds 0.8v, the ic shuts down high - side switch. 8 i set output current setting pin. connect a resistor from i set to g nd to program the output current. 9 exposed pad heat dissipation pad. connect this exposed pad to large ground copper area with copper and vias.
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 3 - abso l u t e m ax i m u m r at i n g s input supply voltage - 0.3v ~ 42v sw voltage - 1v ~ v in + 1v boost voltage v sw - 0 .3v ~ v sw + 7v all other pins voltage - 0.3v ~ 6v junction to ambient thermal resistance 46c/w operating junction temperature - 40c ~160c storage temperature - 55c ~ 150c operating temperature - 40c ~ 85c lead temperature (soldering 10 sec.) 300 stress beyond above listed absolute maximum ratings may lead permanent damage to the device. these are stress ratings only and operations of the device at these or any other conditions beyond those indicated in the operational sections of the specifications are not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability.
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 4 - e l e ct ri ca l cha ra ct e ri s t i cs v in = 20v, t a = +25 , unless otherwise noted. p ar a met er c ond i t i ons mi n. t y p. m ax . u n it input voltage 10 36 v input voltage surge 42 v v in uvlo turn - on voltage input voltage rising 6.7 v v in uvlo hysteresis input voltage falling 0.1 v standby supply current v fb = 1v 2.5 ma feedback voltage 792 808 824 mv internal soft - start time 3 ms err or amplifier transconductance v fb = v comp = 0.8v, i comp = 10a 500 a/v error amplifier dc gain 4000 v/v switching frequency v fb = 0.808v 125 khz foldback switching frequency v fb = 0v 50 khz maximum duty cycle 98 % minimum on - time 200 ns comp to current limit transconductance v comp = 1.2v 4 a/v secondary cycle - by - cycle current limit vout=3.5v 4.5 a slope compensation duty = d max 1.2 a i set voltage 1 v i set to i out dc room temp current gain i out / i set , r iset = 11.5k 27500 a/a cc controller dc accuracy r iset = 22 k, v in = 14v, v out = 3.5v open - loop dc tes t 1250 ma ovp pin voltage ovp pin rising 0.8 v ovp pin voltage ovp pin voltage falling 0.57 v high - side switch on - resistance 0.12 sw off leakage current v in = v sw = 0v 1.0 10 a thermal shutdown temp erature temperature rising 160 c thermal shutdown temperature hysteresis temperature falling 40 c
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 5 - bl o c k d i ag r am
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 6 - d e t a il e d in f or m a t ion f unc t i onal d es c r i pt i on c v / c c lo op r eg ul at i on as seen in functional block diagram, the A7431 is a peak current mode pulse width modulation (pwm) converter with cc and cv control. the converter operates as follows: a switching cycle starts when the rising edge of the oscillator clock output causes the high - side power switch to turn on and the low - side power switch to turn off. with the sw side of the inductor now connected to v in , the inductor current ramps up to store energy in the magnetic field. the inductor current level is measured by the current sense amplifier and added to the oscillator ramp signal. if the resulting summation is higher than the comp voltage, the output of the pwm comparator goes high. when this happens or when oscillator clock output goes low, the high - side power switch turns off. at this point, the sw side of the inductor swings to a diode voltage below ground, causing the inductor current to decrease and magnetic energy to be transferred to output. this state continues until the cycle starts again. the high - side power switch is driven by logic using bs as the positive rail. this pin is charged to v sw + 5v when the low - side power switch turns on. the comp voltage is the integration of the error between fb input and the internal 0.808v reference. if fb is lower than the reference voltage, comp tends to go higher to increase current to the output. output current will increase until it reaches the cc limit set by the i set resistor. at this point, the device will transition from regulating output voltage to regulating output current, and the output voltage will drop with increasing lo ad. the oscillator normally switches at 125khz. however, if fb voltage is less than 0.3v, then the switching frequency decreases to 50khz. o v er v o l t a ge p r ot ec t i o n the A7431 has an ovp pin. if the voltage at this pin exceeds 0.8v, the ic shuts down high side switch. t h er ma l s h ut d ow n the A7431 disables switching when its junction temperature exceeds 160c and resumes when the temperature has dropped by 40c.
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 7 - a p p l ic a t ion s in f or m a t ion o ut put v ol t ag e s et t i n g f i gur e 1 : o ut p ut v o l t a ge s e t t i ng figure 1 sho ws the connections for setting the output voltage. select the proper ratio of the two feedback resistors r fb1 and r fb2 based on the output voltage. adding a capacitor in parallel with r fb1 helps the system stability. typically, use r fb2 10k and determine r fb1 from the following equation: r fb1 = r fb2 ?? ? ?? ? ? 1 0.808v v out c c c ur r e nt s et t i n g A7431 constant current value is set by a resistor connected between the i set pin and gnd. the cc output current is approximating linearly proportional to the current flowing out of the i set pin. the voltage at i set is roughly 1v and the current gain from i set to output is roughly 27500 (27.5ma/1a). to determine the proper resistor for a desired current, please refer to figure 2 below. f i gur e 2 : c ur v e f or p r ogr am mi ng o ut p ut c c c ur r e nt
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 8 - f i gur e 3 : cc/ c v c u r v e r 3 = 1 1. 5k , r 8 = 5 2. 3k , r 2 = 10k i nduc t or s el ec t i o n the inductor maintains a continuous current to the output load. this inductor current has a ripple that is dependent on the inductance value: hi gher inductance reduces the peak - to - peak ripple current. the trade off for high inductance value is the increase in inductor core size and series resistance, and the reduction in current handling capability. in general, select an inductance value l based on ripple current requirement: l = ripple loadmax sw in out in out k i f v ) v (v x v ? where v in is the input voltage, v out is the output voltage, f sw is the switching frequency, i loadmax is the maximum load current, and k ripple is the ripple factor. typically, choose k ripple = 30% to correspond to the peak - to - peak ripple current being 30% of the maximum load current. with a selected inductor value the peak - to - peak inductor current is estimated as: i lpk -pk = sw x in out in out f x v l ) v (v x v ? the peak inductor current is estimated as: i lpk =i loadmax + 2 1 i lpk -pk the selected inductor should not saturate at i lpk . the maximum output current is calculated as: i outmax = i lim - 2 1 i lpk -pk l lim is the internal current limit, which is typically 4.5a, as shown in electrical characteristics table.
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 9 - e x t er na l h i g h v o l t a ge b i as d i o de it is recommended that an external high voltage bias diode be added when the system has a 5v fixed input or the power supply generates a 5v output. this helps improve the efficiency of the regulator. the high voltage bias diode can be a low cost one such as in4148 or bat54. f i gur e 4 : e x t er nal h i gh v o l t a ge b i as d i od e this diode is also recommended for high duty cycle operation and high output voltage applications. i npu t c a pac i t or the input capacitor needs to be carefully selected to maintain sufficiently low ripple at the supply input of the converter. a low esr capacitor is highly recommended. since large current flows in and out of this capacitor during switching, its esr also affects eff iciency. the input capacitance needs to be higher than 10f. the best choice is the ceramic type, however, low esr tantalum or electrolytic types may also be used provided that the rms ripple current rating is higher than 50% of the output current. the input capaci tor should be placed close to the in and gnd pins of the ic, with the shortest traces possible. in the case of tantalum or electrolytic types, they can be further away if a small parallel 10f ceramic capacitor is placed right next to the ic. o ut put c a pac it o r the output capacitor also needs to have low esr to keep low output voltage ripple. the output ripple voltage is: v ripple = i outmax k ripple r esr + out x in lc f 28 v 2 fsw where i outmax is the maximum output current, k ripple is the ripple factor, r esr is t he esr of the output capacitor, f sw is the switching frequency, l is the inductor value, and c out is the output capacitance. in the case of ceramic output capacitors, r esr is very small and does not contribute to the ripple. therefore, a lower capacitance value can be used for ceramic type. in the case of tantalum or electrolytic capacitors, the ripple is dominated by r esr multiplied by the ripple current. in that case, the output capacitor is chosen to have sufficiently low esr. for ceramic output capacito r, typically choose a capacitance of about 22f. for tantalum or electrolytic capacitors, choose a capacitor with less than 50m esr.
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 10 - r e c t if ie r d io d e use a schottky diode as the rectifier to conduct current when the high - side power switch is off. the schottky diode must have current rating higher than the maximum output current and a reverse voltage rating higher than the maximum input voltage. st abi l i t y c o m pen sat i o n f i gur e 5 : s t ab i l i t y c o mp ens at i o n c comp2 is needed only for high esr output capacitor the feedback loop of the ic is stabilized by the components at the comp pin, as shown in figure 5. the dc loop gain of the system is determined by the following equation: a vdc = out i 0.808v a vea g comp the dominant pole p1 is due to c comp : p1 = comp vea ea c a 2 g the second pole p2 is the output pole: p2 = out out out c v 2 i the first zero z1 is due to r comp and c comp : z1 = comp comp c r 2 1 and finally, the third pole is due to r comp and c comp2 (if c comp2 is used): p3 = 2 comp comp c r 2 1
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 11 - the following steps should be used to compensate the ic: step 1. set the cross over frequency at 1/10 of the switching frequency via r comp : r comp = 0.808v x g 10g f c v 2 comp ea sw out out =5.12 x 10 7 v out c out ( ) step 2. set the zero f z1 at 1/4 of the cross over frequency. if r comp is less than 15k, the equation for c comp is: c comp = comp 5 r 10 x 2.83 (f) if r comp is limited to 15k, then the actual cross over frequency is 6.58 / (v out c out ). therefore: c comp = 6.45 x 10 -6 v out c out (f) step 3. if the output capacitors esr is high enough to cause a zero at lower than 4 times the cross over frequency, an additional compensation capacitor c comp2 is required. the condition for using c comp2 is: r esrcout ? ?? ? ? ?? ? ? out out 6 ,0.006xv c 1.77x10 min ( ) and the proper value for c comp2 is: c omp2 = comp esrcout out r r c though c comp2 is unnecessary when the output capacitor has sufficiently low esr, a small value c comp2 such as 100pf may improve stability against pcb layout parasitic effects. table 1 shows some calculated results based on the compensation method above. v out c o ut r c omp c c omp c c omp2 2.5v 47uf ceramic cap 5.6 k 2.2nf none 3.3v 47uf ceramic cap 6.2 k 2.2nf none 5.0v 47uf ceramic cap 12 k 2.2nf no ne 2.5v 220uf/10v/30m 20 k 2.2nf 47pf 3.3v 220uf/10v/30m 20 k 2.2nf 47pf 5.0v 220uf/10v/30m 20 k 2.2nf 47pf c comp2 is needed for high esr output capacitor. c comp2 47pf is recommended. t a bl e 1: t y pi c a l c om pens at i o n f or d i f f er en t o ut p ut v o l t ages and o ut p ut c ap ac i t or s
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 12 - c c lo op s t ab i l i t y the constant - current control loop is internally compensated over the 1500ma - 3000ma output range. no additional external compensation is required to stabilize the cc current. o ut put c a bl e r es i s t anc e c o mp ens at i o n to compensate for resistive voltage drop across the charger's output cable, the A7431 integrates a simple, user - programmable cable voltage drop compensation using the impedance at the fb pin. use the curve in figure 6 to choose the proper feedb ack resistance values for cable compensation. r fb1 is the high side resistor of voltage divider. in the case of high r fb1 used, the frequency compensation needs to be adjusted correspondingly. as show in figure 7 , adding a capacitor in paralleled with r fb1 or increasing the compensation capacitance at comp pin helps the system stability. f i gur e 6 : c ab l e c o mpe ns at i on at v ar i ous r es i s t or d i v i d er v al ues f i gur e 7 : f r equ enc y c o mpe ns at i on f or h i g h r fb 1
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 13 - p c b oar d l ay out g ui danc e figure 8 showed the example of components placement and pcb layout. when laying out the printed circuit board, the following checklist should be used to ensure proper operation of the ic. 1) arrange the power components to reduce the ac loop size consisting of input ceramic capacitor c1, v in pin, sw pin and the schottky diode d1. 2) place input decoupling c eramic capacitor c1 as close to v in pin as possible. c1 is connected power gnd with vias or short and wide path. 3) return fb, comp and i set to signal gnd pin, and connect the signal gnd to power gnd at a single point for best noise immunity. connect exposed pad to power ground copper area with copper and vias. 4) use copper plane for power gnd for best heat dissipation and noise immunity. 5) place feedback resistor close to fb pin. 6) use short trace c onnecting bs - c5 - sw loop. f i gur e 8 : e x a mp l e of p c b l ay out f i gur e 9 . t yp i ca l e f f i c i e n cy c u r ve
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 14 - p a c k a ge in f or m a t ion dimension in psop8 (unit: mm) s y mb ol m i ll im e t e r s i nc hes m in m ax m in m ax a 1.400 1.700 0.055 0.067 a1 0.050 0.150 0.002 0.006 a2 1.350 1.550 0.053 0.061 b 0.330 0.510 0.013 0.020 c 0.170 0.250 0.007 0.010 d 4.700 5.100 0.185 0.200 d1 3.202 3.402 0.126 0.134 e 3.800 4.000 0.150 0.157 e1 5.800 6.200 0.228 0.244 e2 2.313 2.513 0.091 0.099 e 1.270(bsc) 0.050(bsc) l 0.400 1.270 0.016 0.050 l1 1.04(ref) 0.041(ref) l1 - l1 - 0.12 - 0.005 0 8 0 8
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 1 - ja n 201 6 released , sep 2016 updated - - 15 - i m p o rt a nt no t i ce ait semiconductor inc. (ait) reserves the right to make changes to any its product, specifications, to discontinue any integrated circuit product or service without notice, and advises its customers to obtain the latest version of relevant information to verify, before placing orders, that the information being relied on is current. ait semiconductor inc.'s integrated circuit products are not designed, i ntended, authorized, or warranted to be suitable for use in life support applications, devices or systems or other critical applications. use of ait products in such applications is understood to be fully at the risk of the customer. as used herei n may involve potentia l risks of death, personal injury, or server property, or environmental damage. in order to minimize risks associated with the customer's applications, the customer should provide adeq uate design and operating safeguards. ait semiconductor inc. assumes to no liability to customer product design or application support. ait warrants the performance of its products of the specifications applicable at the time of sa le.

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