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a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 1 - de s cri p t i o n f eat u r es a7431a 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. a7431a provides up to 3a output current at 230khz switchin g frequency. a7431a eliminates the expensive, high accuracy current sense resistor, making it ideal for battery charging applications and adaptors with accurate current limit. the a7431a achieves higher efficiency than traditional constant current switchin g regulators by eliminating its associated power loss on the additional current sensing resistor. protection features include cycle - by - cycle current limit, thermal shutdown, and frequency foldback at short circuit. a7431a are available in a p sop8 package and require very few external devices for operation. the a7431 a 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 mp8 a7431 a mp8 r a7431 a mp8vr note v: halogen free package r: tape & reel spq: 4 ,000pcs/reel ait provides all rohs products suffix v means halogen free package ? 42v input voltage surge ? 36v steady state operation ? up to 3a output current ? output voltage up to 12v ? 230khz switching frequency ? up to 91% efficiency ? stable with low - esr ceramic capacitors to allow low - profile designs ? 230khz switching frequency eases emi design ? 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 accurac y ? 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 t ypi 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 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 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 10 f 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 iset 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 output voltage. 6 comp error amplifier output. this pin is used to compensate the converter. 7 en enable input. en is pulled up to 5v with a 10 a current, and contains a precise 1.6v logic threshold. drive this pin to a logic - high or leave unconnected to enable the ic. drive to a logic - low to disable the ic and enter shutdown mode. 8 i set output current setting pin. connect a resistor from i set to gnd 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 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 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 - 40 c ~ 85 c 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 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 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 voltag e 10 36 v input voltage surge 42 v v in uvlo turn - on voltage input voltage rising 6 7 8 v v in uvlo hysteresis input voltage falling 0.1 v standby supply current v en = 3v, v fb = 1v 2.5 ma v en = 3v, v out = 5v, no load 3 ma shutdown supply current v en = 0v 10 a feedback voltage 792 808 824 mv internal soft - start time 3 ms error 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 230 khz foldback switching fre quency v fb = 0v 50 khz maximum duty cycle 98 % minimum on - time 200 ns comp to current limit transconductance v comp = 1.2v 4 .2 a/v secondary cycle - by - cycle current limit v out =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 en shutdown threshold voltage en pin rising 1.2 v en shutdown threshold voltage hysteresis en pin falling 20 mv en lockout threshold volt age en pin rising 1.47 1.6 1.73 v en lockout hysteresis en pin falling 125 mv en internal pull - up current 10 a high - side switch on - resistance 0.12 sw off leakage current v en = v sw = 0v 1 10 a thermal shutdown temperature temperature risin g 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 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 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 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 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 a 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 hi gher 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 load. the oscillator normally switches at 230khz. however, if fb voltage is less than 0.6v, then the switching frequency decreases to 50khz gradually. e na bl e p i n the a7431a has an enable input en for turning the ic on or off. the en pin contains a precision 1.6v comparator with 125mv hysteresis and a 10a pull - up current source. the comparator can be used with a resistor divider from v in to program a startup voltage higher than the normal uvlo value. it can be used with a resistor divider from v out to disable charging of a deeply discharged batt ery, or it can be used with a resistor divider containing a thermistor to provide a temperature - dependent shutoff protection for over temperature battery. the thermistor should be thermally coupled to the battery pack for this usage. if left floating, the en pin will be pulled up to roughly 5v by the internal 10 a current source. it can be driven from standard logic signals greater than 1.6v, or driven with open - drain logic to provide digital on/off control. t h er ma l s h ut d ow n the a7431a 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 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 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 shows the connections for setting the output voltage. select the proper ratio of the two feedback resistors r fb1 and r fb2 b ased 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 a7431a constant current value is set by a resistor connected between the i set pin and gnd. the cc output current is approximating linearly proportio nal 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 : cu r 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 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 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: high er 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 = fx x v l ) v (v x v sw in out in out ? 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 i lim i s the internal current limit, which is typically 4.5a, as shown in electrica l characteristics table. e x t er na l h i g h v o l t a ge b i as d i o de
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 9 - 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 efficiency. the input capacitance needs to be higher than 10 f. 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 capacitor 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 i t or the output capacitor also needs to have low esr to keep low output voltage ripple. the output ripple voltage is: v ripple = i ou tmax k ripple r esr + out 2 sw in lc f x 28 v where i outmax is the maximum output current, k ripple is the ripple factor, r esr is the esr of the output capacitor, f sw is the switchi ng 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 capacitor, typically choose a capacitance of about 22f. f or tantalum or electrolytic capacitors, choose a capacitor with less than 50m esr. 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
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 10 - 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 comp onents 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 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 :
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 11 - 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 requ ired. 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 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.
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 12 - 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 a 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 feed back 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 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 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 connecting 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 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 14 - p a c k a ge in f or m a t ion dimension in psop8 (unit: mm) s y mb ol m in m ax a 1.400 1.7 00 a1 0.050 0.150 a2 1.350 1.550 b 0.330 0.510 c 0.170 0.250 d 4.700 5.1 00 d1 3.202 3.402 e 3.800 4.000 e1 5.800 6.200 e2 2.313 2.513 e 1.270(bsc) l 0.400 1.270 0 8
a i t s em i c ond u ctor i n c . www.ait - ic.com a 74 31 a dc - dc converter buck (step - down) 42v 3a cc/cv rev1. 0 - sep 201 6 released - - 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 bein g 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 potential risks of death, personal injury, or servere 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 suppo rt. ait warrants the performance of its products of the specifications applicable at the time of sa le.

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