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?f2011fsemtechfcorporation 1 SC403B 6a ecospeed ? integrated fet regulator with programmable ldo features powerfsystem: inputfvoltageff3vftof28v internalforfexternalfbiasfvoltageff3vftof5.5v referencefvoltageff600mv integratedfbootstrapfswitch programmablefldofoutputff200ma 1%freferenceftolerancef f -40ftof+85fc ecospeed ? farchitecturefwithfpseudo-fxedffre - quencyfadaptivefon-timefcontrol logicfinput/outputfcontrol:f independentfcontrolfenfforffldofandfswitcherf programmablefv in fuvlofthreshold powerfgoodfoutput selectablef powerfsavefmode programmablefsoftfstartftime protections: over-voltagefandfunder-voltage tcfcompensatedfr ds(on)f fsensedfcurrentflimit thermalfshutdown anyfesrffsp,fposcap,foscon,fandfceramicfcapacitors package:f lead-freefpackageff5x5mm,f32-pinfmlpqf fullyfrohs/weeefcompliantfandfhalogen-free applications ofcefautomationfandfcomputing networkingfandftelecommunicationfequipment pointfoffloadfpowerfsuppliesfandfmodulefreplacement ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? description thefSC403Bfisfafstand-alonefsynchronousfecospeed ? fbuckf powerfsupplyfwhichfincorporatesfsemtechsfadvancedf patentedfadaptivefon-timefcontrolfarchitecture.fitfpro - videsfexcellentflight-loadfefficiencyfandffastftransientf response.fitffeaturesfintegratedfpowerfmosfets,fafboot - strapf switch,f andf af programmablef ldof inf af 5x5mmff package.ffthefdevicefisfhighlyfefcientfandfusesfminimalf pcbfarea.f thefSC403Bfsupportsfusingfstandardfcapacitorftypesfsuchf asfelectrolyticforfspecialfpolymer,finfadditionftofceramic,fatf switchingffrequenciesfupftof1mhz.fthefprogrammablef frequency,fsynchronousfoperation,fandfselectablefpower- savefprovidefhighfefciencyfoperationfoverfafwidefloadf range. additionalffeaturesfincludefafprogrammablefsoft-start,f programmablefcycle-by-cyclefover-currentflimitfprotec - tion,funderfandfover-voltagefprotections,fsoftfshutdown,f andfselectablefpower-save.ffthefdevicefalsofprovidesffsep - aratefenablefinputsfforfthefpwmfcontrollerfandfldofasf wellfasfafpowerfgoodfoutputfforfthefpwmfcontroller. thefwidefinputfandfbiasfvoltagefranges,fprogrammablef frequency,f andf programmablef ldof makef thef devicef extremelyfflexiblefandfeasyftofusefinfafbroadfrangefoff applications.ffthefSC403Bfcanfbefusedfinfserverfcomputersf andfsinglefcellforfmulti-cellfbatteryfsystemsfinfadditionftof traditionalfdcfpowerfsupplyfapplications. rev.f2.1 power management rton sc 403 b fb vout vdd vin ss bst p g n d lx pgood i l i m lxs e n / p s v enl l x b s t rilim cbst enable / psave enable ldo vext / ldo a g n d + rfb 1 rfb 2 vout cout l 1 cin vin pgood ton 1 f csoft 10 ? typical application circuit usfpatent:f2010/0033215a1
2 SC403B pin confguration ordering information marking information sc 403 b yyww xxxxxx xxxxxx agnd pad 1 vin pad 2 lx pad 3 e n l 32 t o n 31 a g n d 30 e n / p s v 29 l x s 28 i l i m 27 p g o o d 26 l x 25 24 lx lx 23 pgnd 22 pgnd 21 pgnd 20 pgnd 19 pgnd 18 pgnd 17 p g n d 16 p g n d 15 n c 14 l x b s t 13 n c 12 v i n 11 v i n 10 v i n 9 bst 8 fbl 5 ss 7 vin 6 vdd 3 agnd 4 fb 1 top view vout 2 notes: 1)f availablefinftapefandfreelfonly.ffafreelfcontainsf3000fdevices. f 2)f lead-freef packagef only.f devicef isf rohs/weeef compliantf andf halogen-free. yyww = date code xxxxxx = semtech lot number xxxxxx = semtech lot number SC403B mlpq-32; 5x5, 32 lead device package SC403Bmltrt (1)(2) mlpq-32f5x5 SC403Bevb evaluationfboard
3 SC403B absolute maximum ratings lxftofpgndf(v) .f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f -0.3f tof +30 lxftofpgndf(v)f(transientff100nsfmax.)f f. . . . . . -2ftof+30f vinftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +30 en/psv,fpgood,filim,ftofgndf(v) f. . . . . . f-0.3ftofff+(v dd f+f0.3) ss,ffvout,fffb,ffbl,ftofgndf(v) f. . . . . . . . . -0.3ftofff+(v dd f+f0.3) vddftofpgndf(v)f f. . . . . . . . . . . . . . . . . . . . . . . . . . . f -0.3f tof +6 tonftofagndf(v) .f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f -0.3ftof+(vddf-f1.5) enlf(v) f f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f -0.3f tof v inf bstftoflxf(v) f f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +6.0 bstftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +35 agndftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +0.3 esdfprotectionflevel (1)f (kv) f. . . . . . . . . . . . . . . . . . . . . . . . . . . . f2 recommended operating conditions inputfvoltagef(v)f f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . f 3.0f tof 28 vddftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.0f tof 5.5 voutftofpgndf(v)f f. . . . . . . . . . . . . . . . . . . . . . . . . . . 0.6f tof 5.5 thermal information storageftemperaturef(c) f. . . . . . . . . . . . . . . . . . . . -60ftof+150 maximumfjunctionftemperaturef(c) f. . . . . . . . . . . . . . . f 150 operatingfjunctionftemperaturef(c) f. . . . . . -40f tof +125 thermalfresistance,fjunctionftofambientf (2) f(c/w) high-sidefmosfet f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f48 low-sidefmosfet f f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . f38 pwmfcontrollerfandfldofthermalfresistancef f. . . . .f 36 peakfirfrefowftemperaturef(c) f. . . . . . . . . . . . . . . . . . . . f 260 exceedingfthefabovefspecifcationsfmayfresultfinfpermanentfdamageftofthefdeviceforfdevicefmalfunction.ffoperationfoutsidefoffthefparametersf specifedfinfthefelectricalfcharacteristicsfsectionfisfnotfrecommended. notes: (1)f testedfaccordingftofjedecfstandardfjesd22-a114.f (2)ffffcalculatedffromfpackagefinfstillfair,fmountedftof3fxf4.5f(in),f4flayerffr4fpcbfwithfthermalfviasfunderfthefexposedfpadfperfjesd51fstandards. unlessfspecifed:fv in f=12v,ft a f=f+25cfforftyp,f-40ftof+85fcfforfminfandfmax,fft j f 4 SC403B electrical characteristics (continued) parameter conditions min typ max units input supplies (continued) enlf,fen/psvf=f0v,fv dd ffromfexternalfsource 190 300 en/psvf=fv dd ,fnofload,fv fb f>f600mv (2) 0.7 ma v dd f=f5v,ff sw f=f250khz,fen/psvf=ffoatingf,fnofload 4 v dd f=f3v,ff sw f=f250khz,fen/psvf=ffoatingf,fnofload 2.5 fbfon-timefthreshold staticfv in fandfload,f0ftof+85fc 0.595 0.600 0.605 v staticfv in fandfload,f-40ftof+85fc 0.594 0.606 v frequencyfrange continuousfmodefoperation 1000 khz minimumff sw ,fen/psvf=fv dd ,fnofload 25 bootstrapfswitchfresistance 10 ? switching mosfet resistance r dson highfsideffet 30 m? lowfsideffet 10 timing on-time continuousfmodefoperation,f v in f=f15v,fv out f=f5v,fff sw =fkhz,fr ton f=f300k? 2386 2650 2915 ns 3vf 5 SC403B electrical characteristics (continued) parameter conditions min typ max units current sense zero-crossingfdetectorfthreshold lxf-fpgnd -3 0 +3 mv power good powerfgoodfthreshold upperflimit,fv fb f>finternalfreference +20 % lowerflimit,fv fb f 6 SC403B electrical characteristics (continued) parameter conditions min typ max units logic inputs/outputs (continued) en/psvfinputfforfforcedfcontinuousfoperation f maximumflevelfexpressedfinf%foffv dd 42 % minimumflevel 1 v en/psvfinputfforfdisablingfswitcher f maximumflevel 0.4 v minimumflevel 0 en/psvfinputfbiasfcurrent en/psv=fvddforfagnd -10 +10 a enlfinputfbiasfcurrent v in f=f16v 11 18 a fbl,ffbfinputfbiasfcurrent fbl,ffbf=fvddforfagnd -1 +1 a linear regulator (thefldofisfshortedftofthefvddfpinfinternally.) fblfaccuracy ldofloadf=f10ma 0.735 0.75 0.765 v ldofcurrentflimit short-circuitfprotection,fv in f=f12v,fv dd ff90%fofffnalf v dd fvalue 135 200 ldoftofvoutfswitch-overfthresholdf (4) -140 +140 mv ldoftofvoutfnon-switch-overfthresholdf (4) -450 +450 mv ldoftofvoutfswitch-overfresistance v out f=f+5v 2 ? ldofdropfoutfvoltagef (5) fromfv in ftofv dd ,fv dd f=f+5v,fi ldo f=f100ma 1.2 v notes: (1)f v in fuvlofisfprogrammablefusingfafresistorfdividerffromfvinftofenlftofagndfpins.fthefenlfvoltagefisfcomparedftofanfinternalfreference.f (2)ff forfv fb f>f0.750vffforfthefsc403fandfv fb f>f0.600v (3)f forfvddflessfthanf4.5v,fthefon-timefmayfbeflimitedfbyfthefvddfsupplyfvoltagefandfbyfthefv in .fseeftheftonflimitationfandfvddfsupplyfvoltagef sectionfinfthefapplicationsfinformation. (4)f thefswitch-overfthresholdfisfthefmaximumfvoltagefdiferentialfbetweenfthefvddfandfvoutfpinsfwhichfensuresfthatfldofwillfinternallyfswitch- overftofvout.ffthefnon-switch-overfthresholdfisfthefminimumfvoltagefdiferentialfbetweenfthefldofandfvoutfpinsfwhichfensuresfthatfldof willfnotfswitch-overftofvout. (5)ff thefldofdropfoutfvoltagefisfthefvoltagefatfwhichfthefldofoutputfdropsf2%fbelowfthefnofloadfregulationfvalue.ff
7 SC403B rton 130 k ? sc 403 b fb 1 vout 2 vdd 3 agnd 4 fbl 5 vin 6 ss 7 bst 8 v i n 9 v i n 10 v i n 11 p g n d 15 p g n d 16 17 18 19 20 21 pgnd 22 lx 23 lx 24 l x 25 p g o o d 26 i l i m 27 l x s 28 e n / p s v 29 a g n d 30 t o n 31 e n l 32 n c 14 l x b s t 13 n c 12 v i n pad 2 agnd pad 1 lx pad 3 rilim 8 . 06 k ? rldo 2 75 k ? rldo 1 425 k ? cin 2 x 10 f ( see note ) rgnd 0 cbst 1 f vin + 12 v pgnd pgnd pgnd pgnd pgnd + 10 nf l 1 1 . 5 h vout 1 . 2 v @ 6 a , 300 khz cout 330 f 9 m ? enable / psave enable ldo pgood 1 f internal ldo used as bias 3 . 3 nf component value manufacturer part number web cin 2 x 10 f / 25 v murata grm 32 dr 71 e 106 ka 12 l www . murata . com key components www . cyntec . com l 1 1 . 5 h / 6 . 7 m ? cyntec pcmb 065 t - 1 r 5 ms cout www . panasonic . com 330 f / 9 m ? panasonic eef - sx 0 e 331 er 1 f rfb 2 10 k ? rfb 1 10 k ? detailed application circuit 1
8 SC403B rton 130 k ? sc 403 b fb 1 vout 2 vdd 3 agnd 4 fbl 5 vin 6 ss 7 bst 8 v i n 9 v i n 10 v i n 11 p g n d 15 p g n d 16 17 18 19 20 21 pgnd 22 lx 23 lx 24 l x 25 p g o o d 26 i l i m 27 l x s 28 e n / p s v 29 a g n d 30 t o n 31 e n l 32 n c 14 l x b s t 13 n c 12 v i n pad 2 agnd pad 1 lx pad 3 rilim 8 . 06 k ? cin 2 x 10 f ( see note ) rgnd 0 cbst 1 f vin + 12 v pgnd pgnd pgnd pgnd pgnd + 10 nf l 1 1 . 5 h rfb 2 10 k ? vout 1 . 2 v @ 6 a , 300 khz cout 330 f 15 m ? enable / psave pgood 1 f external 5 v used as bias 3 . 3 nf 5 v component value manufacturer part number web cin 2 x 10 f / 25 v murata grm 32 dr 71 e 106 ka 12 l www . murata . com key components www . cyntec . com l 1 1 . 5 h / 6 . 7 m ? cyntec pcmb 065 t - 1 r 5 ms cout www . panasonic . com 330 f / 9 m ? panasonic eef - sx 0 e 331 er 10 ? 1 f rfb 1 10 k ? detailed application circuit 2
9 SC403B typical characteristics efciency vs. load forced continuous mode 0% 20% 40% 60% 80% 100% 0.001 0.01 0.1 1 10 i out (a dc ) eff (%) 0 0.5 1 1.5 2 2.5 p loss (w) v in = 12 v v in = 20 v v in = 6 v v in = 12 v v in = 6 v v in = 20 v p loss efficiency characteristicsfinfthisfsectionfarefbasedfonfusingfthefapplicablefdetailedfapplicationfcircuitff2. v out vs. load psave mode 1.14 1.16 1.18 1.20 1.22 1.24 1.26 0.001 0.01 0.1 1 10 i out (a dc ) v out (v dc ) 0 0.02 0.04 0.06 0.08 0.1 0.12 v peak (v rms ) v in = 12 v v in = 12 v v in = 6 v v in = 6 v v in = 20 v v in = 20 v v peak regulation efciency vs. load psave mode v out f=f1.2v,fv dd ff=ff5v,fen/psvfisffoating,fexternalfbias 0 20 40 60 80 100 0.001 0.01 0.1 1 10 i out (a dc ) eff (%) 0 0.5 1 1.5 2 2.5 p loss (w) v in = 12 v v in = 12 v v in = 6 v v in = 6 v v in = 20 v v in = 20 v p loss efficiency v out vs. load forced continuous mode 1.14 1.16 1.18 1.20 1.22 1.24 1.26 0.001 0.01 0.1 1 10 i out (a dc ) v out (v dc ) 0 0.025 0.05 0.075 0.1 0.125 0.15 v peak (v rms ) v in = 12 v v in = 12 v v in = 6 v v in = 6 v v in = 20 v v in = 20 v v peak regulation v out f=f1.2v,ffv dd f=ff5v,fen/psvfisffoating,fexternalfbias v out f=f1.2v,ffv dd f=ff5v,f=fen/psv,fexternalfbias v out f=f1.2v,ffv dd f=ff5vf=fen/psv,fexternalfbias efciency vs. load psave mode v out f=f1.2v,ffv in f=ff12v,fen/psvf=fv dd ,fexternalfbias v out vs. load psave mode v out f=f1.2v,ffv in f=ff12v,fen/psvf=fv dd ,fexternalfbias 0 20 40 60 80 100 0.001 0.01 0.1 1 10 i out (a dc ) eff (%) 0 0.5 1 1.5 2 2.5 p loss (w) v dd = 5 v v dd = 5 v v dd = 3 . 3 v v dd = 3 . 3 v p loss efficiency 1.14 1.16 1.18 1.20 1.22 1.24 1.26 0.001 0.01 0.1 1 10 i out (a dc ) v out (v dc ) 0 0.02 0.04 0.06 0.08 0.1 0.12 v peak (v rms ) v dd = 3 . 3 v v dd = 3 . 3 v v dd = 5 v v dd = 5 v regulation v peak
10 SC403B typical characteristics (continued) v in ( v dc ) v p k _ p k ( m v r m s ) v o u t ( v d c ) 1 . 425 1 . 450 1 . 475 1 . 500 1 . 525 1 . 550 1 . 575 6 . 0 8 . 8 11 . 5 14 . 3 17 . 0 19 . 8 22 . 5 25 . 3 28 . 0 0 25 50 75 100 125 150 v out v pkpk characteristicsfinfthisfsectionfarefbasedfonfusingfthefapplicablefdetailedfapplicationfcircuitff2. v out vs. line forced continuous mode frequency vs. load forced continuous mode 0 50 100 150 200 250 300 350 400 0.001 0.01 0.1 1 10 i out (a dc ) f sw (khz) v in = 12 v v in = 6 v v in = 20 v frequency vs. load psave mode 0 50 100 150 200 250 300 350 400 0.001 0.01 0.1 1 10 i out (a dc ) f sw (khz) v in = 12 v v in = 6 v v in = 20 v frequency vs. line fcm mode v in ( v dc ) f r e q u e n c y ( h z ) 0 50 100 150 200 250 300 350 400 6 . 0 8 . 8 11 . 5 14 . 3 17 . 0 19 . 8 22 . 5 25 . 3 28 . 0 v out f=f1.5v,fv ldo f=fv dd f=fenlf=f5v,fen/psvfisffoating v out f=f1.5v,fv ldo f=fv dd f=fenlf=f5v,fen/psvfisffoating v out f=f1.2v,fv dd ff=f5v,fen/psvfisffoating,fexternalfbias v out f=f1.2v,ffv dd f=fen/psvf=f5v,fexternalfbias on time vs. line 0 200 400 600 800 1000 1200 6 . 0 8 . 8 11 . 5 14 . 3 17 . 0 19 . 8 22 . 5 25 . 3 28 . 0 input voltage ( v ) o n - t i m e ( n s ) 5 v 3 . 3 v v out f=f1.5v,fv ldo f=fv dd f=fenlf=f5v,fi out f=f0a
11 SC403B powersave mode no load time (10 s/div) s/div) /div) f(20mv/div) (10v/div) v in f=f12v,fv out f=f1.2v,fi out f=f0a,fv ldo f=fv dd f=fen/psv=fenlf=f5v forced continuous mode no load time (10 s/div) s/div) /div) f(20mv/div) (10v/div) v in f=f12v,fv out f=f1.5v,fi out f=f0a,fv ldo f=fv dd f=ffenlf=f3.3v,fen/psv=ffoating typical characteristics (continued) characteristicsfinfthisfsectionfarefbasedfonfusingfthefapplicablefdetailedfapplicationfcircuitff2. time (1m s/div) s/div) /div) self-biased start-up power good true f(10v/div) (5v/div) (5v/div) v in f=f0vftof12vfstep,fv out f=f1.2v,fi out f=f0a,fv ldo f=fv dd f=fen/psv=fenlf=f5v f(500mv/div) time (1m s/div) s/div) /div) enabled loaded output f(10v/div) (5v/div) v in f=f12v,fv out f=f1.2v,fi out f=f1a,fv ldo f=fv dd f=fenlf=fen/psv=f5v (500mv/div) time (4m s/div) s/div) /div) self-biased start-up power good true f(20mv/div) (10v/div) v in f=f12vfstep,fv out f=f1.2v,fi out f=f0a,fexternalfv dd f=fen/psv=f5v time (4 s/div) s/div) /div) self-biased start-up power good true f(20mv/div) (10v/div) v in f=f12vfstep,fv out f=f1.2v,fi out f=f300ma,fexternalfv dd f=fen/psv=f5v
12 SC403B typical characteristics (continued) characteristicsfinfthisfsectionfarefbasedfonfusingfthefapplicablefdetailedfapplicationfcircuitff2. transient response time (40 s/div) s/div) /div) f(10v/div) f(50mv/div) v in f=f12v,fv out f=f1.2v,fi out f=f0aftof6a,fv ldo f=fv dd f=ffen/psv=fenlf=f5v output over-current response normal operation time (100 s/div) s/div) /div) f(500mv/div) (10v/div) f(5a/div) (5v/div) v in f=f12v,fv outf =f1.5v,fv ldo f=fv dd f=fenlf=f3.3v,fen/psv=ffoating;fi out frampedftoftripfpoint output under-voltage response normal operation time (100 s/div) s/div) /div) f(500mv/div) (10v/div) (5v/div) v in f=f12v,fv out f=f1.5v,fi out f=f0a,fv ldo f=fv dd f=fenlf=f3.3v,ffoatingfen/psv shorted output response soft-start operation time (400 s/div) s/div) /div) f(1v/div) (5v/div) f(10a/div) (5v/div) v in f=f12v,fv out f=f1.5v,fi out f=f0a,fv ldo f=fv dd f=fen/psv=fenlf=f3.3v shorted output response normal operation time (40 s/div) s/div) /div) f(1v/div) (10v/div) f(10a/div) (5v/div) v in f=f12v,fv out f=f1.5v,fv ldo f=fv dd f=fenlf=f3.3v,fen/p5vfisffoating transient response time (40 s/div) s/div) /div) f(10v/div) f(50mv/div) v in f=f12v,fv out f=f1.2v,fi out f=f6aftof0a,fv ldo f=fv dd f=ffen/psv=fenlf=f5v
13 SC403B pin descriptions pin # pin name pin function 1 fb feedbackfinputfforfswitchingfregulatorfusedftofprogramfthefoutputfvoltageffconnectftofanfexternalfresis - torfdividerffromfvoutftofagnd. 2 vout switcherfoutputfvoltagefsensefpinffalsofthefinputftofthefinternalfswitch-overfbetweenfvoutfandfvldo.f thefvoltagefatfthisfpinfmustfbeflessfthanforfequalftofthefvoltagefatfthefvddfpin. 3 vdd biasfsupplyfforftheficffwhenfusingfthefinternalfldofasfafbiasfpowerfsupply,fthefvddfisfthefldofoutput.f whenfusingfanfexternalfpowerfsupplyftofbiasfthefic,fthefldofoutputfshouldfbefdisabled. 4,f30,fpadf1 agnd analogfground 5 fbl feedbackfinputfforftheffinternalfldoffconnectftofanfexternalfresistorfdividerffromfvddftofagndftofpro - gramfthefldofoutput. 6,f9-11,f padf2 vin inputfsupplyfvoltage 7 ss thefsoftfstartftimefisfprogrammedfbyfanfinternalfcurrentfsourcefchargingfafcapacitorfonfthisfpin. 8 bst bootstrapfpinffconnectfafcapacitorfoffatfleastf100nfffromfbstftoflxftofdevelopftheffoatingfsupplyfforfthef high-sidefgatefdrive. 12 nc nofconnection 13 lxbst lxfboostffconnectftofthefbstfcapacitor. 23-25,ffpadf3 lx switchingf(phase)fnode 14 nc nofconnection 15-22 pgnd powerfground 26 pgood open-drainfpowerfgoodfindicatorffhighfimpedancefindicatesfpowerfisfgood.fanfe xternalfpull-upf resistorfisfrequired. 27 ilim currentflimitfsensefpinffusedftofprogramfthefcurrentflimitfbyfconnectingfafresistorffromfilimftoflxs. 28 lxs lxfsenseffconnectsftofr ilim . 29 en/psv enable/power-savefinputfforfthefswitchingfregulatorfffconnectftofagndftofdisablefthefswitchingfregulator.f floatftofoperatefinfforcedfcontinuousfmodef(psavefdisabled).ffconnectftofvddftofoperatefwithfpsavefmodef enabled. 31 ton on-timefprogrammingfinputffsetfthefon-timefbyfconnectingfthroughfafresistorftofagnd. 32 enl enablefinputfforfthefldoffconnectfenlftofagndftofdisablefthefldo.fdrivefwithflogicfsignalfforflogicfcon - trol,forfprogramfthefvinfuvlofwithfafresistorfdividerfbetweenfvin,fenl,fandfagndfpins.
14 SC403B block diagram reference soft start fb agnd on - time generator control & status pgood gate drive control vin pgnd ton vout zero cross detector valley current limit ilim enl fbl vldo switchover mux a y b ldo vdd bst fb comparator - lx en / psv bypass comparator bypass comparator nc nc a 12 8 27 14 32 5 3 2 31 1 26 29 a = connected to pins 6 , 9 - 11 , pad 2 b = connected to pins 23 - 25 , pad 3 c = connected to pins 15 - 22 d = connect to pins 4 , 30 , pad 1 b c d v in vdd vdd v in bootstrap switch lo - side mosfet hi - side mosfet lxbst 13 lxs 28 dl dl vdd vdd vdd ss 7
15 SC403B synchronous buck converter thefSC403Bfisfafstepfdownfsynchronousfdc-dcfbuckf converterfwithfintegratedfpowerfmosfetsfandfaf200maf programmablefldo.ffthefdevicefoperatesfatfafcurrentfupf tof6afatfveryfhighfefciency.fafspacefsavingf5x5f(mm)f32- pinfpackagefisfused.ffthefprogrammablefoperatingffre - quencyf off upf tof 1mhzf enablesf optimizingf thef confgurationfforfpcbfareafandfefciency. thefbuckfcontrollerfusesfafpseudo-fxedffrequencyfadap - tivefon-timefcontrol.fthisfcontrolfmethodfallowsffastftran - sientfresponsefwhichfpermitsfthefusefoffsmallerfoutputf capacitors. infadditionftoftheffollowingfinformation,fthefuserfcanf clickfonfthefapplicableflinkftofgoftofthefSC403Bfonline f c-simfdesignfandfsimulationftoo l ,fwhichfwillfleadfthef userfthroughfthefdesignfprocess. input voltage requirements thefSC403Bfrequiresftwofinputfsuppliesfforfnormalfopera - tion:fv in fandfv dd .ffv in foperatesfoverfafwidefrangeffromf3vftof 28v.ffvddfrequiresfafsupplyfvoltagefbetweenf3vftof5vfthatf canfbefanfexternalfsourceforfthefinternalfldoffromfv in . power up sequence thefSC403Bfinitiatesfafstartfupfwhenfvin,fvdd,fandfen/ psvfpinsfarefabovefthefapplicablefthresholds.ffwhenfusingf anfexternalfbiasfsupplyfforfthefv dd fvoltage,fitfisfrecom - mendedfthatfthefv dd fisfappliedftofthefdevicefonlyfafterf theffv in fvoltagefisfpresentfbecausefv dd fcannotfexceedfv in f atfanyftime.ffaf10?fresistorfmustfbefplacedfbetweenfthef externalfv dd fsupplyfandfthefvddfpinftofavoidfdamageftof thefdevicefduringfpower-upfandforfshutdownfsituationsf wherefv dd fcouldfexceedffv in funexpectedly. shutdown thefSC403Bfcanfbefshutdownfbyfpullingfeitherfvddforf en/psvfpinfbelowfitsfthreshold.ffwhenfusingfanfexternalf supplyfvoltagefforfv dd ,fthefvddfpinfmustfbefdeactivatedf whilefthefv in fvoltagefisfstillfpresent.ffaf10?fresistorfmustf befplacedfbetweenfthefexternalfv dd fsupplyfandfthefvddf pinftofavoidfdamageftofthefdevice. whenfthefvddfpinfisfactivefandfen/psvfisfatflowflogicflevel,f thefoutputfvoltagefdischargesfthroughfanfinternalffet.ff psuedo-fxed frequency adaptive on-time control thefpwmfcontrolfmethodfusedfbyfthefSC403Bfisfpseudo- fxedffrequency,fadaptivefon-time,fasfshownfinffiguref1.f thefripplefvoltagefgeneratedfatfthefoutputfcapacitorfesrf isfusedfasfafpwmframpfsignal.fthisfripplefisfusedftoftriggerf thefcontrollerfon-time.f q 1 q 2 l c out esr + c in v out fb threshold v fb v lx v lx ton fb v in figure 1 pwm control method, v out ripple thefadaptivefon-timefisfdeterminedfbyfanfinternalfone- shotftimer.ffwhenfthefone-shotfisftriggeredfbyfthefoutputf ripple,fthefdevicefsendsfafsinglefon-timefpulseftofthefhigh- sidefmosfet.fthefpulsefperiodfisfdeterminedfbyfv outff andf v in .fthef periodf isf proportionalf tof outputf voltagef andf inverselyfproportionalftofinputfvoltage.ffwithfthisfadaptivef on-timefarrangement,fthefdevicefautomaticallyfantici - patesfthefon-timefneededftofregulatefv out fforfthefpresentf v in fconditionfandfatfthefselectedffrequency.ff thefadvantagesfoffadaptivefon-timefcontrolfare: predictablefoperatingffrequencyfcomparedftof otherfvariableffrequencyfmethods.f reducedfcomponentfcountfbyfeliminatingfthef errorfampliferfandfcompensationfcomponents. reducedf componentf countf byf removingf thef needftofsensefandfcontrolfinductorfcurrent. fastftransientfresponseffthefresponseftimefisf controlledfbyfaffastfcomparatorfinsteadfoffaftypi - callyfslowferrorfamplifer. reducedfoutputfcapacitancefdueftoffastftran - sientfresponse ? ? ? ? ? applications information
16 SC403B one-shot timer and operating frequency one-shotftimerfoperationfisfshownfinffiguref2.ftheffbf comparatorfoutputfgoesfhighfwhenfv fb fisflessfthanfthef internalfreference.fthisffeedsfintofthefgatefdrivefandfturnsf onfthefhigh-sidefmosfet,fandfstartsfthefone-shotftimer.ff thefone-shotftimerfusesfanfinternalfcomparatorfandfaf capacitor.ffonefcomparatorfinputfisfconnectedftofv out ,fthef otherfinputfisfconnectedftofthefcapacitor.ffwhenfthefon- timefbegins,fthefinternalfcapacitorfchargesffromfzerofvoltsf throughfafcurrentfwhichfisfproportionalftofv in .fwhenfthef capacitorfvoltagefreachesfv out ,fthefon-timefisfcompletedf andfthefhigh-sidefmosfetfturnsfof.fff gate drives fb comparator one - shot timer on - time = k x r ton x ( v out / v in ) v out v in fb q 1 q 2 l c out v in esr + v out v lx fb dh dl r ton + - figure 2 on-time generation thisfmethodfautomaticallyfproducesfanfon-timefthatfisf proportionalf tofv out f andf inverselyf proportionalf tofv in .ff underfsteady-statefconditions,fthefswitchingffrequencyf canfbefdeterminedffromfthefon-timefbyftheffollowingf equation. in on out sw v t v f u thefSC403Bfusesfanfexternalfresistorftofsetfthefon-timef whichfindirectlyfsetsftheffrequency.ffthefon-timefcanfbef programmedftofprovidefanfoperatingffrequencyffromf 200khzftof1mhzfusingfafresistorfbetweenftheftonfpinfandf ground.fffthefresistorfvaluefis selected by the following equation. out in on ton v pf 25 v ) ns 10 t ( r u u thefmaximumfr ton fvaluefallowedfisfshownfbyftheffollow - ingfequation. a 5 . 1 10 v r min _ in max _ ton p u immediatelyfafterfthefon-time,fthefdlf(drivefsignalfforfthef lowfsideffet)foutputfdrivesfhighftofturnfonftheflow-sidef mosfet.ffdlfhasfafminimumfhighftimefoff~320ns,fafterf whichfdlfcontinuesftofstayfhighfuntilfonefofftheffollowingf occurs:f vfbffallsfbelowfthefreference thefzerofcrossfdetectorfsensesfthatfthefvoltagef onftheflxfnodefisfbelowfground.fpowerfsavefisf activatedf eightf switchingf cyclesf afterf af zerof crossingfisfdetected. ton limitations and v dd supply voltage forfv dd fbelowf4.5v,ftheftonfaccuracyfmayfbeflimitedfbyf thefinputfvoltage.ff theforiginalfr ton fequationfisfaccuratefiffv in fsatisfesfthef relationshipfoverfthefentirefv in frange,fasffollows. v in < (v dd - 1.6v) x 10 iffv in fexceedsf( v dd f-f1.6v)fxf10,fforfallforfpartfoffthefv inf range,fthefr ton fequationfisfnotfaccurate.ffinfallfcasesfwheref v in f>ff( v dd f-f1.6v)fxf10,fthefr ton fequationfmustfbefmodifed,f asffollows.f out dd on ton v 25pf 10 1.6v) (v 10ns) (t r u u u notefthatfwhenfv in f>ff( v dd f-f1.6v)fxf10f,fthefactualfon-timef isffxedfandfdoesfnotfvaryfwithfv in .ffwhenfoperatingfinfthisf condition,fthefswitchingffrequencyfwillfvaryfinverselyfwithf v in fratherfthanfapproximatingfaffxedffrequency.ffff v out voltage selection thefswitcherfoutputfvoltagefisfregulatedfbyfcomparingf v out fasfseenfthroughfafresistorfdividerfatftheffbfpinftofthef internalfreferencefvoltage,fseeffiguref3.f r 1 to fb pin r 2 v out figure 3 output voltage selection ? ? applications information (continued)
17 SC403B notefthatfthisfcontrolfmethodfregulatesfthefvalleyfoffthef outputfripplefvoltage,fnotfthefdcfvalue.ffthefdcfoutputf voltagefv out fisfofsetfbyfthefoutputfripplefaccordingftofthef followingfequation. ? 1 ? ? ? 1 ? u 2 v r r 1 v v ripple 2 1 fb out whenfaflargefcapacitorfisfplacedfinfparallelfwithfr1f(c top )f v out fisfshownfbyftheffollowingfequation. 2 top 1 2 1 2 2 top 1 ripple 2 1 fb out c r r r r 1 ) c r ( 1 2 v r r 1 v v ? ? 1 ? z u z u ? 1 ? ? ? 1 ? u enable and power save input thefen/psvfinputfisfusedftofenableforfdisablefthefswitch - ingfregulator.fwhenfen/psvfisflowf(grounded),fthefswitch - ingfregulatorfisfoffandfinfitsflowestfpowerfstate.ffwhenfof,f thefoutputfoffthefswitchingfregulatorfsoft-dischargesfthef outputfintofaf15?finternalfresistorfviafthefv out fpin.ffwhenf en/psvfisfallowedftoffoat,fthefpinfvoltagefwillffoatftof33%f offthefvoltagefatfvdd.fthefswitchingfregulatorfturnsfonf withfpsavef(powerfsave)fdisabledfandfallfswitchingfisfinf forcedfcontinuousfmode.ff whenfen/psvfisfhighf(abovef45%foffthefvoltagefatfvdd),f thef switchingf regulatorf turnsf onf withf power-savef enabled.ff forced continuous mode operation thef SC403Bf operatesf thef switcherf inf fcmf (forcedf continuousfmode)fbyffoatingfthefen/psvfpinf(seeffiguref 4).ffinfthisfmodefonefoffthefpowerfmosfetsfisfalwaysfon,f withfnofintentionalfdeadftimefotherfthanftofavoidfcross- conduction.ffthisffeaturefresultsfinfuniformffrequencyf acrossftheffullfloadfrangefwithftheftrade-offbeingfpoorf efciencyfatflightfloadsfdueftofthefhigh-frequencyfswitch - ingfoffthefmosfets.ffdhfisfthefgatefsignalfdrivingfthef upperfmosfet.fdlfisftheflowerfgatefsignalfdrivingfthef lowerfmosfet. fb ripple voltage ( v fb ) fb threshold dl dh inductor current dc load current dh on - time is triggered when v fb reaches the fb threshold . on - time ( t on ) dl drives high when on - time is completed . dl remains high until v fb falls to the fb threshold . figure 4 forced continuous mode operation power-save operation thefSC403Bfprovidesfpower-savefoperationfatflightfloadsf withfnofminimumfoperatingffrequency.fwithfpower-savef enabled,fthefinternalfzerofcrossingfcomparatorfmonitorsf thefinductorfcurrentfviafthefvoltagefacrossftheflow-sidef mosfetfduringfthefof-time.fiffthefinductorfcurrentffallsftof zerofforf8fconsecutivefswitchingfcycles,fthefcontrollerf entersfpower-savefoperation.fitfwillfturnfofftheflow-sidef mosfetf onf eachf subsequentf cyclef providedf thatf thef currentfcrossesfzero.fatfthisftimefbothfmosfetsfremainfoff untilf v fb f dropsf tof thef 600mvf threshold.f becausef thef mosfetsf aref off,f thef loadf isf suppliedf byf thef outputf capacitor. applications information (continued)
18 SC403B iffthefinductorfcurrentfdoesfnotfreachfzerofonfanyfswitch - ingfcycle,fthefcontrollerfimmediatelyfexitsfpower-savefandf returnsftofforcedfcontinuousfmode.f figuref5fshowsfpower-savefoperationfatflightfloads. fb ripple voltage ( v fb ) fb threshold dl dh inductor current zero ( 0 a ) dh on - time is triggered when v fb reaches the fb threshold . on - time ( t on ) dl drives high when on - time is completed . dl remains high until inductor current reaches zero . dead time varies according to load figure 5 psave operation smart psave protection activefloadsfmayfleakfcurrentffromfafhigherfvoltagefintof thefswitcherfoutput.ffunderflightfloadfconditionsfwithf psavefenabled,fthisfcanfforcefv out ftofslowlyfrisefandfreachf thefover-voltagefthreshold,fresultingfinfafhardfshutdown.f smartfpsavefpreventsfthisfcondition.fwhenftheffbfvoltagef exceedsf10%fabovefnominal,fthefdevicefimmediatelyfdis - ablesfpsave,fandfdlfdrivesfhighftofturnfonftheflow-sidef mosfet.ffthisfdrawsfcurrentffromffv out fthroughfthefinduc - torfandfcausesfv out ftoffall.ffwhenfv fb fdropsfbackftofthef internalfreferenceftripfpoint,fafnormalft on fswitchingfcyclef begins.fthisfmethodfpreventsfafhardfovpfshutdownfandf alsofcyclesfenergyffromfv out fbackftofv in .ffitfalsofminimizesf operatingfpowerfbyfavoidingfforcedfconductionfmodef operation.ffiguref6fshowsftypicalfwaveformsfforfthefsmartf psaveffeature. fb threshold high - side drive ( dh ) low - side drive ( dl ) v out drifts up to due to leakage current flowing into c out dh and dl off dl turns on when smart psave threshold is reached smart power save threshold dl turns off when fb threshold is reached single dh on - time pulse after dl turn - off v out discharges via inductor and low - side mosfet normal dl pulse after dh on - time pulse normal v out ripple figure 6 smart psave smartdrive tm forfeachfdhfpulsefthefdhfdriverfinitiallyfturnsfonfthefhigh- sidefmosfetfatfaflowerfspeed,fallowingfafsofter,fsmoothf turn-offofftheflow-sidefdiode.foncefthefdiodefisfoffandf theflxfvoltagefhasfrisenf1vfabovefpgnd,fthefsmartdrivef circuitfautomaticallyfdrivesfthefhigh-sidefmosfetfonfatfaf rapidfrate.ffthisftechniquefreducesfswitchingfpowerflossf whilefmaintainingfhighfefficiencyfandfalsofavoidsfthef needfforfsnubbersforfseriesfresistorsfinfthefgatefdrive. current limit protection programmablefcurrentflimitingfisfaccomplishedfbyfusingf thefrds on fofftheflowerfmosfetfforfcurrentfsensing.ffthef currentflimitfisfsetfbyfthefr ilim fresistor.fthefr ilim fresistorfcon - nectsffromfthefilimfpinftoftheflxsfpinfwhichfisfalsofthefdrainf offtheflow-sidefmosfet.ffwhenftheflow-sidefmosfetfisfon,f anfinternalf~10 afcurrentfflowsffromfthefilimfpinfandf throughfthefr ilim fresistor,fcreatingfafvoltagefdropfacrossfthef resistor.ffwhileftheflow-sidefmosfetfisfon,fthefinductorf currentffowsfthroughfitfandfcreatesfafvoltagefacrossfthef rds on .fthefvoltagefacrossfthefmosfetfisfnegativefwithf respectftofground.ffiffthisfmosfetfvoltagefdropfexceedsfthef voltagefacrossfr ilim ,fthefvoltagefatfthefilimfpinfwillfbefnega - tivefandfcurrentflimitfwillfactivate.fthefcurrentflimitfthenf keepsftheflow-sidefmosfetfonfandfwillfnotfallowfanotherf high-sidefon-time,funtilfthefcurrentfinftheflow-sidefmosfetf reducesfenoughftofbringfthefilimfvoltagefbackfupftofzero.ff thisfmethodfregulatesfthefinductorfvalleyfcurrentfatfthef levelfshownfbyfilimfinffiguref7.fff applications information (continued)
19 SC403B time i peak i load i lim i n d u c t o r c u r r e n t figure 7 valley current limit settingfthefvalleyfcurrentflimitftof6afresultsfinfafpeakfinduc - torfcurrentfoff6afplusfthefpeak-to-peakfripplefcurrent.finf thisfsituation,fthefaveragef(load)fcurrentfthroughfthefinduc - torfisf6afplusfone-halffthefpeak-to-peakfripplefcurrent. thefinternalf10 afcurrentfsourcefisftemperaturefcompen - satedfatf4100ppmfinforderftofprovideftrackingfwithfthef rds on . thef r ilim fvaluefisfcalculatedfbyftheffollowingfequation. r ilim = 1176 x i lim x [0.088 x (5v - v dd ) + 1] ( ? ) where i lim is in amps. whenfselectingfafvaluefforfr ilim fdofnotfexceedfthefabsolutef maximumfvoltagefvaluefforfthefilimfpin.fnotefthatfbecausef theflow-sidefmosfetfwithflowfrds on fisfusedfforfcurrentf sensing,fthefpcbflayout,fsolderfconnections,fandfpcbfconnec - tionftoftheflxfnodefmustfbefdonefcarefullyftofoptimizefopera - tion.fr ilim fshouldfbefconnectedfdirectlyftoflxsf(pinf28). soft-start of pwm regulator SC403Bfhasfafprogrammablefsoft-startftimefthatfisfcon - trolledfbyfanfexternalfcapacitorfatfthefssfpin.fafterfthef controllerfmeetsfbothfuvlofandfen/psvfthresholds,fthef controllerfhasfanfinternalfcurrentfsourcefoff3affowingf throughfthefssfpinftofchargefthefcapacitor.ffduringfthef startfupfprocessf(figuref8),fafpercentagefoffthefvoltagefatf thefssfpinfisfusedfasfthefreferencefforftheffbfcomparator.f thefpercentagefisf50%fforfthefsc403fandf40%fforfthef SC403B.ffthefpwmfcomparatorfissuesfanfon-timefpulsef whenfthefvoltagefatftheffbfpinfisflessfthanf50%f(orf40%)foff thefssfpin.ffasfresult,fthefoutputfvoltageffollowsfthefssf startfvoltage.fthefoutputfvoltagefreachesfandfmaintainsf regulationfwhenfthefsoftfstartfvoltagefisf > f1.5v.fftheftimef betweenftheffrstflxfpulsefandfwhenfv out fmeetsfregulationf isfthefsoftfstartftimef(t ss ).ffthefcalculationfforfthefsoft-startf timefisfshownfbyftheffollowingfequation. a 3 v 5 . 1 c t ss ss p u thefvoltagefatfthefssfpinfcontinuesftoframpfupfandfeventu - allyfisfequalftof64%foffv dd .ffafterfsoftfstartfcompletes,fftheffbf pinfvoltagefisfcomparedftofthefinternalfreference.fthefdelayf timefbetweenfthefv out fregulationfpointfandfpgoodfgoingf highfisfshownfbyftheffollowingfequation. f a 3 ) v 5 . 1 v 64 . 0 ( c t dd ss delay - pgood p u u time (400 s/div)s/div) /div) f(10mv/div) (2v/div) f (5v/div) (500mv/div) figure 8 soft-start timing diagram pre-bias startup SC403Bfcanfstartfupfasfiffinfafsoft-startfconditionfwithfanf existingfoutputfvoltageflevel.ffthefsoftfstartftimefisfstillfthef samefasfnormalfstartfupf(whenfthefoutputfvoltagefstartsf fromfzero).fthefoutputfvoltagefstartsftoframpfupfwhenf 40%foffthefvoltagefatfssfpinfmeetsfthefpre-chargeffbf voltageflevel.ffpre-biasfstartupfisfachievedfbyfturningfoff theflowerfgatefwhenfthefinductorfcurrentffallsfbelowfzero.f thisf methodf preventsf thef outputf voltagef fromf decreasing.f power good output thefpgoodf(powerfgood)foutputfisfanfopen-drainfoutputf whichfrequiresfafpull-upfresistor.ffwhenfthefvoltagefatfthef fbfpinfisf10%fbelowfthefnominalfvoltage,fpgoodfisfpulledf low.fitfisfheldflowfuntilfthefoutputfvoltagefreturnsfabovef 92%foffnominal.ff applications information (continued)
20 SC403B pgoodfwillftransitionflowfiffthefv fb fpinfexceedsf+20%foff nominal,fwhichfisfalsofthefover-voltagefshutdownfthresh - old.fpgoodfalsofpullsflowfiffthefen/psvfpinfisflowfwhenf v dd fvoltagefisfpresent. output over-voltage protection over-voltagefprotectionfbecomesfactivefasfsoonfasfthef devicefisfenabled.fthefthresholdfisfsetfatf20%fabovefthef feedbackfreferencefvoltage.ffwhenfv fb fexceedsfthefovpf threshold,fdlflatchesfhighfandftheflow-sidefmosfetfisf turnedfon.fdlfremainsfhighfandfthefcontrollerfremainsfof,f untilfthefen/psvfinputfisftoggledforfvddfisfcycled.ftherefisf af5sfdelayfbuiltfintofthefovpfdetectorftofpreventffalsef transitions.fpgoodfisfalsoflowfafterfanfovpfevent. output under-voltage protection whenfv fb ffallsf25%fbelowfitsfnominalfvoltagefforfeightf consecutivefswitchingfcycles,fthefswitcherfisfshutfoffandf thef dhf andf dlf drivesf aref pulledf lowf tof tri-statef thef mosfets.fthefcontrollerfstaysfoffuntilfen/psvfisftoggledf orfvddfisfcycled. vdd uvlo, and por uvlof(under-voltageflock-out)fcircuitryfinhibitsfswitch - ingfandftri-statesfthefdh/dlfdriversfuntilfv dd frisesfabovef 3.0v.fanfinternalfporf(power-onfreset)foccursfwhenfv dd f exceedsf3.0v,fwhichfresetsftheffaultflatchfandfsoft-startf counterftofpreparefforfsoft-start.fthefSC403Bfthenffbeginsf afsoft-startfcycle.fthefpwmfwillfshutfoffiffvddffallsfbelowf 2.4v. ldo regulator SC403Bfhasfanfoptionftofbiasfthefswitcherfbyfusingfanfinter - nalfldoffromfv in .ffthefldofoutputfisfconnectedftofvddf internally.ffthefoutputfoffthefldofisfprogrammablefbyfusingf externalfresistorsffromfthefvddfpinftofagnd.fftheffeedbackf pinf(fbl)fforfthefldofisfregulatedftof750mvf(seeffiguref9).ff r ldo 1 to fbl pin vdd r ldo 2 figure 9 ldo voltage divider thefldofoutputfvoltagefisfsetfbyftheffollowingfequation. ? ? 1 ? u 2 ldo 1 ldo r r 1 mv 750 vldo afminimumf1ffcapacitorfreferencedftofagndfisfrequiredf alongf withf af minimumf 1.0ff capacitorf referencedf tof pgndftofflterfthefgatefdrivefpulses.ffthefcapacitanceftof pgndfwillfneedftofincreaseftoff10uffiffvldofisfsetftofaf voltageflowerfthanf4.5v.freferftoftheflayoutfguidelinesf sectionfforfcomponentfplacementfsuggestions.fff ldo enl functions thefenlfinputfisfusedftofcontrolfthefinternalfldo.fwhenf enlfisflowf(grounded),fthefldofisfof.fffwhenfenlfisffabovef thef v in f uvlof threshold,f thef ldof isf enabledf andf thef switcherf isf alsof enabledf iff en/psvf andfvddf meetf thef thresholds.ff thefenlfpinfalsofactsfasfthefswitcherfuvlof(under-voltagef lockout)fforfthefv inf supply.ffthefv in fuvlofvoltagefisfpro - grammablefviafafresistorfdividerfatfthefvin,fenlfandfagndf pins. iffthefenlfpinftransitionsffromfhighftoflowfwithinf2fswitch - ingfcyclesfandfisflessfthanf1v,fthenfthefldofwillfturnfoff butfthefswitcherfremainsfon.fiffthefenlfgoesfbelowfthefv in f uvlofthresholdfandfstaysfabovef1v,fthenfthefswitcherfwillf turnfoffbutfthefldofremainsfon.fthefv in fuvloffunctionf hasfaftypicalfthresholdfoff2.6vfonfthefv in frisingfedge.fthef fallingfedgefthresholdfisf2.4v. notefthatfitfisfpossibleftofoperatefthefswitcherfwithfthef ldofdisabled,fbutfthefenlfpinfmustfbefbelowftheflogicf lowfthresholdf(0.4vfmaximum).finfthisfcase,fthefuvlof functionfforfthefinputfvoltagefcannotfbefused.ftheftablef belowfsummarizesftheffunctionfoffthefenlfandfenfpins,f withfrespectftofthefrisingfedgefoffenl. en enl ldo status switcher status low low,ff2.6v on of high high,f>f2.6v on on applications information (continued)
21 SC403B figuref10fshowsfthefenlfvoltagefthresholdsfandftheirf efectfonfldofandfswitcherfoperation. agnd enl low threshold ( min 0 . 4 v ) 2 . 6 v 2 . 4 v ldo on ldo on ldo off v in uvlo hysteresis enl voltage switcher on if en = high switcher on if en = high switcher off by v in uvlo figure 10 enl threshold beforefstart-up,fthefldofchecksfthefstatusfofftheffollowingf signalsftofensurefproperfoperationfcanfbefmaintained. enlfpin v in finputfvoltage whenfthefenlfpinfisfhighfandfv in fisfabovefthefuvlofpoint,ff thefldofwillfbeginfstart-up.ffduringfthefinitialfphase,fwhenf thefv dd fvoltagef(whichfisfthefldofoutputfvoltage)fisflessf thanf0.75v,fthefldofinitiatesfafcurrent-limitedfstart-upf (typicallyf65ma)ftofchargefthefoutputfcapacitorsfwhilef protectingffromfafshortfcircuitfevent.ffwhenfv dd fisfgreaterf thanf0.75vfbutfstillflessfthanf90%foffitsffinalfvaluef(asf sensedfatftheffblfpin),fthefldofcurrentflimitfisfincreasedf tof~115ma.fwhenfv dd fhasfreachedf90%fofftheffnalfvaluef (asfsensedfatftheffblfpin),fthefldofcurrentflimitfisfincreasedf tof~200mafandfthefldofoutputfisfquicklyfdrivenftofthef nominalfvaluefbyfthefinternalfldofregulator.ffitfisfrecom - mendedf thatf duringf ldof start-upf tof holdf thef pwmf switchingfoffuntilfthefldofhasfreachedf90%fofftheffnalf value.ffthisffpreventsfoverloadingfthefcurrent-limitedfldof outputfduringfthefldofstart-up.fff dueftofthefinitialfcurrentflimitationsfonfthefldofduringf powerfupf(figuref11),fanyfexternalfloadfattachedftofthef vddfpinfmustfbeflimitedftof20mafbeforefthefldofhasf reachedf90%foffitffnalfregulationfvalue. 1. 2. f v vldo final 90 % of v vldo final constant current startup @ ~ 115 ma voltage regulating with ~ 200 ma current limit short - circuit protection @ ~ 65 ma 0 . 7 v figure 11 ldo start-up ldo switch-over operation thefswitch-overffunctionfisfprovidedftofincreasefefciencyf byfusingfthefmorefefcientfdc-dcfconverterftofpowerfthef ldofoutput,favoidingftheflessfefficientfldofregulatorf whenfpossible.fthefswitch-overffunctionfconnectsfthef vddfpinfdirectlyftofthefvoutfpinfusingfanfinternalfswitch.ff whenfthefswitch-overfisfcompletefthefldofisfturnedfof,f whichfresultsfinfpowerfsavingsfandfmaximizesfefciency.f iffthefldofoutputfisfusedftofbiasfthefSC403B,fthenfafterf switch-overfthefdevicefisfself-poweredffromfthefswitchingf regulatorfwithftheffldofturnedfof. thefswitch-overflogicfwaitsfforf32fswitchingfcyclesfbeforef itfstartsfthefswitch-over.fwhenfthefldofisfalreadyfinfregu - lationfandfthefdc-dcfconverterfisflaterfenabled,fasfsoonfasf thefpgoodfoutputfgoesfhigh,fthef32fcyclesfarefstarted.f thefvoltagesfatfthefvddfandfvoutfpinsfarefthenfcom - pared;fifftheftwofvoltagesfarefwithinf300mvfoffeachf other,fthefvddfpinfconnectsftofthefvoutfpinfusingfanf internalfswitch,fandfthefldofisfturnedfof. switch-over limitations on vout, enl, and vdd becausefthefinternalfswitch-overfcircuitfalwaysfcomparesf thefvoutfandfvddfpinsfatfstart-up,ftherefareflimitationsf onfpermissiblefcombinationsfoffthesefpins.ffconsiderfthef casefwherefv out fisfprogrammedftof3.0vfandfv ldo fisfpro - grammedftof3.3v.ffafterfstart-up,fthefdevicefwouldfconnectf voutftofvddfandfdisablefthefldo,fsinceftheftwofvoltagesf arefwithinfthef300mvfswitch-overfwindow.fftofavoidf unwantedfswitch-over,fthefminimumfdiferencefbetweenf thefvoltagesfforfv out fandfv ldo fshouldfbef500mv.f applications information (continued)
22 SC403B applications information (continued) infmanyfapplications,fthefen/psvfpinfwillfbefpulledfhighftof thefvddfnodeftofallowfcontrolfoffthefpwmfandfldofenlf pin.fiffthefswitchfoverffeaturefisfused,ffthisfcircuitfmustfbef implementedfwithfcautionforfthefcircuitfmayfbefdamaged.f infthefcasefwherefthefenlfpinfisfbeingfcontrolledfbyfafgpiof signalforfisftiedfdirectlyftofthefinputfvoltage,fthefenlfpinfcanf befpulledflowfwhilefthefpwmfisfstillfgeneratingfanfoutputf voltagefthatfisfseenfacrossfonefoffthefswitch-overfdiodes.f thisfmayfresultfinfthefvddfnodefbeingfheldfabovefitsfuvlof thresholdffwhilefthefldofisfdeactivated.fthisftypefoffopera - tionfcanfpotentiallyfdamageftheffpart. infthefcasefwherefthefenlfpinfisfusedftofcontrolfthefinputf uvlo,fitfisfacceptableftofconnectfen/psvfdirectlyftofthef vddfnode. itfisfnotfrecommendedftofusefthefswitch-overffeaturefforf anfoutputfvoltageflessfthanf3vfsincefthisfdoesfnotfprovidef sufcientfvoltagefforfthefgate-sourcefdriveftofthefinternalf p-channelfswitch-overfmosfet.ff switch-over mosfet parasitic diode thefswitch-overfmosfetfcontainsfafparasiticfdiodefthatfisf inherentftofitsfconstruction,fasfshownfinffiguref12. switchover mosfet parasitic diode vdd ldo v out switchover control figure 12 switch-over mosfet parasitic diodes iffv out fisfhigherfthanfvdd,fthenfthefdiodefwillfturnfonfandf thefSC403Bfoperatingfcurrentfwillffowfthroughfthisfdiode.ff thisfhasfthefpotentialfoffdamagingfthefdevice. therefarefsomefimportantfdesignfrulesfthatfmustfbeffol - lowedftofpreventfforwardfbiasfoffthisfdiode.fftheffollowingf condition,fv dd ffv out fneedsftofbefsatisfedfinforderfforfthef parasiticf diodef tof stayf offf andf preventf damagingf thef device.fmanyfapplicationsfconnectfthefenfpinftofv5vfandf controlfthefon/offoffthefldofandfpwmfsimultaneouslyf withfthefenlfpin.fthisfallowsfonefsignalftofcontrolfbothf thefbiasfandfpowerfoutputfoffthefSC403B.fwhenfv out f>f 3.0vfthisfconfigurationfcanfcausefproblemsfdueftofthef parasiticfdiodesfinfthefldofswitchoverfcircuitry.fafterfthef v out f>f3.0vfpwmfoutputfisfupfandfrunningfthefswitchoverf diodesfcanfholdfupfv5vf>fuvlofevenfiffthefenlfpinfisf grounded,fturningfoffthefldo.foperatingfinfthisfwayfcanf potentiallyfdamagefthefpart. design procedure whenfdesigningfafswitchfmodefsupplyfthefinputfvoltagef range,f loadf current,f switchingf frequency,f andf inductorf ripplefcurrentfmustfbefspecifed. thefmaximumfinputfvoltagef(v inmax )fisfthefhighestfspeci - fedfinputfvoltage.fthefminimumfinputfvoltagef(fv inmin )fisf determinedfbyftheflowestfinputfvoltagefafterfevaluatingf thefvoltagefdropsfdueftofconnectors,ffuses,fswitches,fandf pcbftraces. theffollowingfparametersfdefnefthefdesign. nominalfoutputfvoltagef(v out ) staticforfdcfoutputftolerance transientfresponse maximumfloadfcurrentf(i out ) therefareftwofvaluesfoffloadfcurrentftofevaluateffcon - tinuousfloadfcurrentfandfpeakfloadfcurrent.fcontinuousf loadf currentf relatesf tof thermalf stressesf whichf drivef thef selectionfoffthefinductorfandfinputfcapacitors.fpeakfloadf currentfdeterminesfinstantaneousfcomponentfstressesfandf flteringfrequirementsfsuchfasfinductorfsaturation,foutputf capacitors,fandfdesignfoffthefcurrentflimitfcircuit. theffollowingfvaluesfarefusedfinfthisfdesign. v in f=f12vf + f10% v out f=f1.5vf + f4% f sw f=f300khz loadf=f6afmaximum v fb f=f600mvf ffffff f ? ? ? ? ? ? ? ? ?
23 SC403B applications information (continued) frequency selection selectionfoffthefswitchingffrequencyfrequiresfmakingfaf trade-offbetweenfthefsizefandfcostfoffthefexternalfflterf componentsf (inductorf andf outputf capacitor)f andf thef powerfconversionfefciency.ff thefdesiredfswitchingffrequencyfisf300khzfwhichfresultsf fromfusingffcomponentsfselectedfforfoptimumfsizefandf cost.ff afresistorf(r ton )fisfusedftofprogramfthefon-timef(indirectlyf settingftheffrequency)fusingftheffollowingfequation. out in on ton v pf 25 v ) ns 10 t ( r u u tofselectfr ton ,fusefthefmaximumfvaluefforfv in ,fandfforft on f usefthefvaluefassociatedfwithfmaximumfv in . sw inmax out on f v v t u t on = 379 ns at 13.2v in , 1.5v out , 300khz substitutingfforfr ton fresultsfinftheffollowingfsolution. r ton = 129.9k ? , use r ton = 130k ? inductor selection inforderftofdeterminefthefinductance,fthefripplefcurrentf mustffrstfbefdefned.fflowfinductorfvaluesfresultfinfsmallerf sizefbutfcreatefhigherfripplefcurrentfwhichfcanfreducefef - ciency.ffhigherfinductorfvaluesfwillfreducefthefripplefcurrentf andfripplefvoltagefandfforfafgivenfdcfresistancefarefmoref efcient.fhowever,flargerfinductanceftranslatesfdirectlyfintof largerfpackagesfandfhigherfcost.ffcost,fsize,foutputfripple,f andfefciencyfarefallfusedfinfthefselectionfprocess.f thefripplefcurrentfwillfalsofsetfthefboundaryfforfpsavef operation.fffthefswitchingfwillftypicallyfenterfpsavefmodef whenf thef loadf currentf decreasesf tof 1/2f off thef ripplef current.fforfexample,fiffripplefcurrentfisf4afthenfpsavef operationfwillftypicallyfstartfforfloadsflessfthanf2a.ffiffripplef currentfisfsetfatf40%foffmaximumfloadfcurrent,fthenfpsavef willfstartfforfloadsflessfthanf20%foffmaximumfcurrent. thefinductorfvaluefisftypicallyfselectedftofprovidefafripplef currentfthatfisfbetweenf25%ftof50%foffthefmaximumfloadf current.ffthisfprovidesfanfoptimalftrade-offbetweenfcost,f efciency,fandftransientfperformance. duringfthefdhfon-time,fvoltagefacrossfthefinductorfisf f (v in f-fv out ).ffthefequationfforfdeterminingfinductancefisf shownfnext.f ripple on out in i t ) v v ( l u example infthisfexample,fthefinductorfripplefcurrentfisfsetfequalftof 50%f off thef maximumf loadf current.f fthereforef ripplef currentfwillfbef50%fxf6aforf3a.fftoffindfthefminimumf inductancefneeded,fusefthefv in fandft on fvaluesfthatfcorre - spondftofv inmax .f h 48 . 1 a 3 ns 379 ) 5 . 1 2 . 13 ( l p u af slightlyf largerf valuef off 1.5hf isf selected.f thisf willf decreaseftheftypicalfi ripple ftof2.7a. notefthatfthefinductorfmustfbefratedfforfthefmaximumfdcf loadfcurrentfplusf1/2foffthefripplefcurrent. thefripplefcurrentfunderfminimumfv in fconditionsfisfalsof checkedfusingftheffollowingfequations. ns 461 ns 10 v v r pf 25 t inmin out ton vinmin _ on u u l t ) v v ( i on out in ripple u a 38 . 2 ) 2 . 0 1 ( h 5 . 1 ns 461 ) 5 . 1 8 . 10 ( i min _ ripple u p u a 7 . 3 ) 2 . 0 1 ( h 5 . 1 ns 379 ) 5 . 1 8 . 10 ( i max _ ripple u p u thefvaluefofflfhasfbeenfadjustedfbyf + 20%fforfthefequa - tionsfabovefassumingfanfinductorftolerancefoff + 20%.
24 SC403B applications information (continued) output capacitor selection thefoutputfcapacitorsfarefchosenfbasedfuponfrequiredf esrfandfcapacitance.ffthefmaximumfesrfrequirementfisf controlledfbyfthefoutputfripplefrequirementfandfthefdcf tolerance.fthefoutputfvoltagefhasfafdcfvaluefthatfisfequalf tofthefvalleyfoffthefoutputfripplefplusf1/2foffthefpeak-to- peakfripple.ffafchangefinfthefoutputfripplefvoltagefwillf leadftofafchangefinfdcfvoltagefatfthefoutput.ff thefdesignfgoalfisfthatfthefoutputfvoltagefregulationfbef 4%funderfstaticfconditions.ffthefinternalfreferenceftoler - ancefisf1%.fallowingf1%ftoleranceffromftheffbfresistorf divider,fthisfallowsf2%ftolerancefdueftofv out fripple.ffsincef thisf2%ferrorfcomesffromf1/2foffthefripplefvoltage,fthef allowablefripplefisf4%,forf60mvfforfaf1.5vfoutput. thef maximumf ripplef currentf off 3.7af createsf af ripplef voltagefacrossfthefesr.ffthefmaximumfesrfvaluefallowedf isfshownfbyftheffollowingfequations. a 7 . 3 mv 60 i v esr ripplemax ripple max esr max = 16.2 m ? thefoutputfcapacitancefisfusuallyfchosenftofmeetftran - sientf requirements.f af worst-casef loadf release,f fromf maximumfloadftofnofloadfatfthefexactfmomentfwhenfthef inductorfcurrentfisfatfthefpeak,fdeterminesfthefrequiredf capacitance.ffiffthefloadfreleasefisfinstantaneousf(loadf changesf fromf maximumf tof zerof inf 25 SC403B applications information (continued) andfmaximumf16m ?fesr ,fselectfonefcapacitorfratedfatf 330ffandf9m ?fesr . itfisfrecommendedfthatfanfadditionalfsmallfcapacitorfbef placedfinfparallelfwithfc out finforderftofflterfhighffrequencyf switchingfnoise. soft start capacitor selection forfafsoft-startftimef(t ss )foffapproximatelyf3ms,fsolvefthef followingfequationfforfc ss . nf 6 c v 5 . 1 a 3 t c ss ss ss p iffc ss fisfselectedfasf4.7fnf,fthenft ss fwillfbef2.4fms. thenfthefpgoodfdelay,ftheftimeffromfv out fregulationftof pgoodfsignalfhighfisfshownfbyftheffollowingfequation. a 3 ) v 5 . 1 v 64 . 0 ( nf 7 . 4 t dd delay - pgood p u atfv dd f=f5v,ffthefpgoodfdelayfwillfbef2.7ms. stability considerations unstablefoperationfisfpossiblefwithfadaptivefon-timefcon - trollers,fandfusuallyftakesfthefformfoffdouble-pulsingforf esrfloopfinstability.fff double-pulsingfoccursfdueftofswitchingfnoisefseenfatfthef fbfinputforfbecauseftheffbfripplefvoltagefisftooflow.fthisf causesftheffbfcomparatorftoftriggerfprematurelyfafterfthef 250nsfminimumfof-timefhasfexpired.ffinfextremefcasesf thefnoisefcanfcausefthreeforfmorefsuccessivefon-times.ff double-pulsingfwillfresultfinfhigherfripplefvoltagefatfthef output,fbutfinfmostfapplicationsfitfwillfnotfafectfopera - tion.fthisfformfoffinstabilityfcanfusuallyfbefavoidedfbyf providingftheffbfpinfwithfafsmooth,fcleanfripplefsignalf thatfisfatfleastf10mvp-p,fwhichfmayfdictatefthefneedftof increasefthefesrfoffthefoutputfcapacitors.ffitfisfalsofimpera - tiveftofprovidefafproperfpcbflayoutfasfdiscussedfinfthef layoutfguidelinesfsection.f anotherfwayftofeliminatefdoubling-pulsingfisftofaddfaf smallf(~f10pf)fcapacitorfacrossfthefupperffeedbackfresis - tor,fasfshownfinffiguref13.ffthisfcapacitorfshouldfbefleftf unpopulatedfuntilfitfcanfbefconfrmedfthatfdouble-pulsingf exists.ffaddingfthefc top fcapacitorfwillfcouplefmorefripplef intoffbftofhelpfeliminatefthefproblem.fanfoptionalfcon - nectionfonfthefpcbfshouldfbefavailablefforfthisfcapacitor. v out to fb pin r 2 r 1 c top figure 13 capacitor coupling to fb pin esrffloopfinstabilityfisfcausedfbyfinsufficientfesr.ffthef detailsf off thisf stabilityf issuef aref discussedf inf thef esrf requirementsfsection.ffthefbestfmethodfforfcheckingfsta - bilityfisftofapplyfafzero-to-fullfloadftransientfandfobservef thefoutputfvoltagefripplefenvelopefforfovershootfandf ringing.fringingfforfmorefthanfonefswitchingfcyclefafterf thefinitialfstepfisfanfindicationfthatfthefesrfshouldfbef increased. onefsimplefwayftofsolvefthisfproblemfisftofaddftracefresis - tancefinfthefhighfcurrentfoutputfpath.ffafsidefefectfoff addingftracefresistancefisfafdecreasefinfloadfregulationf performance.ff esr requirements afminimumfesrfisfrequiredfforftwofreasons.ffonefreasonfisf tof generatef enoughf outputf ripplef voltagef tof providef 10mvp-pfatftheffbfpinf(afterfthefresistorfdivider)ftofavoidf double-pulsing.fff thefsecondfreasonfisftofpreventfinstabilityfdueftofinsuf - cientfesr.ffthefon-timefcontrolfregulatesfthefvalleyfoffthef outputfripplefvoltage.ffthisfripplefvoltagefisfthefsumfoffthef twofvoltages.ffonefisfthefripplefgeneratedfbyfthefesr,fthef otherfisfthefripplefdueftofcapacitivefchargingfandfdis - chargingfduringfthefswitchingfcycle.ffforfmostfapplica - tionsfthefminimumfesrfripplefvoltagefisfdominatedfbyfthef
26 SC403B applications information (continued) outputfcapacitors,ftypicallyfspforfposcapfdevices.ffforf stabilityfthefesrfzerofoffthefoutputfcapacitorfshouldfbef lowerfthanfapproximatelyfone-thirdfthefswitchingffre - quency.ffthefformulafforfminimumfesrfisfshownfbyfthef followingfequation.f sw out min f c 2 3 sr e u u s u using ceramic output capacitors forfapplicationsfusingfceramicfoutputfcapacitors,fthefesrf isfnormallyftoofsmallftofmeetfthefabovefesrfcriteria.finf thesefapplicationsfitfisfnecessaryftofaddfafsmallfvirtualfesrf networkfcomposedfofftwofcapacitorsfandfonefresistor,fasf shownfinffiguref14.ffthisfnetworkfcreatesfaframpfvoltagef acrossfc l ,fanalogousftoftheframpfvoltagefgeneratedfacrossf thefesrfoffafstandardfcapacitor.ffthisframpfisfthenfcapaci - tivelyfcoupledfintoftheffbfpinfviafcapacitorfc c . r 1 r 2 fb pin c c c out l low - side high - side c l r l virtual esr network figure 14 virtual esr ramp circuit dropout performance thefoutputfvoltagefadjustfrangefforfcontinuous-conduc - tionf operationf isf limitedf byf thef fixedf 80nsf (typical)f minimumfof-timefoffthefone-shot.ffwhenfworkingfwithf lowfinputfvoltages,fthefduty-factorflimitfmustfbefcalcu - latedfusingfworst-casefvaluesfforfonfandfofftimes.ff thefduty-factorflimitationfisfshownfbyfthefnextfequation. ) max ( off ) min ( on ) min ( on t t t duty thefinductorfresistancefandfmosfetfon-statefvoltagef dropsf mustf bef includedf whenf performingf worst-casef dropoutfduty-factorfcalculations. system dc accuracy (v out controller) threeffactorsfafectfv out faccuracy:ftheftripfpointfofftheffbf errorfcomparator,fthefripplefvoltagefvariationfwithflinef andfload,fandfthefexternalfresistorftolerance.fftheferrorf comparatorfofsetfisftrimmedfsofthatfunderfstaticfcondi - tionsfitftripsfwhenftheffeedbackfpinfisfwithinf + 1%foffthef referencefvoltage. thefon-timefpulseffromfthefSC403Bfinfthefdesignfexamplef isfcalculatedftofgivefafpseudo-fxedffrequencyfoff300khz.f someffrequencyfvariationfwithflinefandfloadfisfexpected.ff thisfvariationfchangesfthefoutputfripplefvoltage.ffbecausef constantfon-timefconvertersfregulateftofthefvalleyfoffthef outputfripple,f?foffthefoutputfripplefappearsfasfafdcfregu - lationferror.ffforfexample,fiffthefoutputfripplefisf50mvfwithf v in f=f6fvolts,fthenfthefmeasuredfdcfoutputfwillfbef25mvf abovefthefcomparatorftripfpoint.ffiffthefripplefincreasesftof 80mvfwithfv in f=f25v,fthenfthefmeasuredfdcfoutputfwillfbef 40mvfabovefthefcomparatorftrip.ffthefbestfwayftofmini - mizefthisfefectfisftofminimizefthefoutputfripple.f tofcompensatefforfvalleyfregulation,fitfmayfbefdesirableftof usefpassivefdroop.ftakeftheffeedbackfdirectlyffromfthef outputfsidefoffthefinductorfandfplacefafsmallfamountfoff tracefresistancefbetweenfthefinductorfandfoutputfcapaci - tor.fthisftracefresistancefshouldfbefoptimizedfsofthatfatf fullfloadfthefoutputfdroopsftofnearftheflowerfregulationf limit.fpassivefdroopfminimizesfthefrequiredfoutputfcapaci - tancefbecausefthefvoltagefexcursionsfdueftofloadfstepsf arefreducedfasfseenfatfthefload. thefusefoff1%ffeedbackfresistorsfmayfresultfinfupftof1%f error.fifftighterfdcfaccuracyfisfrequired,f0.1%fresistorsf shouldfbefused.f
27 SC403B applications information (continued) thefoutputfinductorfvaluefmayfchangefwithfcurrent.ffthisf willfchangefthefoutputfripplefandfthereforefwillfhavefaf minorfefectfonfthefdcfoutputfvoltage.ffthefoutputfesrf alsofafectsfthefoutputfripplefandfthusfhasfafminorfefectf onfthefdcfoutputfvoltage.ff switching frequency variation thefswitchingffrequencyfwillfvaryfdependingfuponflinef andfloadfconditions.fftheflinefvariationfisfafresultfofffxedf propagationfdelaysfinfthefon-timefone-shot,fasfwellfasf unavoidablefdelaysfinfthefexternalfmosfetfswitching.ffasf v in fincreases,ftheseffactorsfmakefthefactualfdhfon-timef slightlyflongerfthanfthefidealfon-time.ffthefnetfefectfisf thatffrequencyftendsftoffallsfslightlyfwithfincreasingfinputf voltage.f thefswitchingffrequencyfalsofvariesfwithfloadfcurrentfasfaf resultfoffthefpowerflossesfinfthefmosfetsfandfthefinduc - tor.ffforfafconventionalfpwmfconstant-frequencyfcon - verter,f asf loadf increasesf thef dutyf cyclef alsof increasesf slightlyftofcompensatefforfirfandfswitchingflossesfinfthef mosfetsfandfinductor.ffafconstantfon-timefconverterf mustfalsofcompensatefforfthefsameflossesfbyfincreasingf thef effectivef dutyf cyclef (moref timef isf spentf drawingf energyffromfv in fasflossesfincrease).ffthefon-timefisfessen - tiallyfconstantfforfafgivenfv out /v in fcombination,ftofofsetf theflossesfthefof-timefwillftendftofreducefslightlyfasfloadf increases.fthef netf effectf isf thatf switchingf frequencyf increasesfslightlyfwithfincreasingfload.fff
28 SC403B applications information (continued) allfotherfdecouplingfcapacitorsfmustfbeflocatedf asfclosefasfpossibleftofthefic. pgnd plane pgndfrequiresfitsfownfcopperfplanefwithfnof otherfsignalftracesfroutedfonfit. copperfplanes,fmultiplefviasfandfwideftracesfaref neededftofconnectfpgndftofinputfcapacitors,f outputfcapacitors,fandfthefpgndfpinsfonfthef device. thef pgndf copperf areaf betweenf thef inputf capacitors,foutputfcapacitors,fandfpgndfpinsf mustfbefasftightfandfcompactfasfpossibleftof reducefthefareafoffthefpcbfthatfisfexposedftof noisefdueftofcurrentffowfonfthisfnode. connectfpgndftofagndfwithfafshortftraceforf f 0 ? f resistor.fthisfconnectionfshouldfbefasfclosef tofthefdevicefasfpossible. ? ? ? ? ? vout plane on top and bottom layer l c in c s s rfb 2 rfb 1 r i l i m c b s t lx plane on top and bottom layer all components shown top side agnd plane on inner layer v in plane on top and / or bottom layer rgnd agnd connects to pgnd close to ic pin 1 marking ic with vias for lx , agnd , vin c out pgnd on inner or bottom layer vout sense trace on inner layer vdd decoupling capacitor v d d cer . c in sp or poscap r t o n ctop pgnd on top layer figure 15 pcb layout pcb layout guidelines thefoptimumflayoutfforfthefSC403Bfisfshownfinffiguref15.ff thisflayoutfshowsfanfintegratedffetfbuckfregulatorfwithfaf maximumfcurrentfoff6a.ftheftotalfpcbfareafisfapproxi - matelyf25fxf29fmm. critical layout guidelines theffollowingfcriticalflayoutfguidelinesfmustfbeffollowedf tofensurefproperfperformancefoffthefdevice. icfdecouplingfcapacitors pgndfplane agndfisland fb,fvout,fandfotherfanalogfcontrolfsignals bst,filim,fandflx cinfandfcoutfplacementfandfcurrentfloops ic decoupling capacitors af0.1fffcapacitorfmustfbeflocatedfasfclosefasf possibleftoftheficfandfdirectlyfconnectedftofpinsf 3f(vdd)fandf4f(agnd).f ? ? ? ? ? ? ?
29 SC403B applications information (continued) agnd island agndfshouldfhavefitsfownfislandfoffcopperfwithf nofotherfsignalftracesfroutedfonfthisflayerfthatf connectsfthefagndfpinsfandfpadfoffthefdevicef tofthefanalogfcontrolfcomponents. allfoffthefcomponentsfforfthefanalogfcontrolfcir - cuitryfshouldfbeflocatedfsofthatftheffconnectionsf tofagndfarefdonefbyfwidefcopperftracesforfviasf downftofagnd. connectfpgndftofagndfwithfafshortftraceforf0 ? f resistor.fthisfconnectionfshouldfbefasfcloseftof thefdevicefasfpossible. fb, vout, and other analog control signals thef connectionf fromf thef v out f powerf tof thef analogfcontrolfcircuitryfmustfbefroutedffromfthef outputfcapacitorsfandflocatedfonfafquietflayer. theftracesfbetweenfvoutfandfthefanalogfcontrolf circuitryf(vout,fandffbfpins)fmustfbefshortfandf routedfawayffromfnoisefsources,fsuchfasfbst,fflx,f vin,fandfpgndfbetweenfthefinputfcapacitors,f outputfcapacitors,fandfthefdevice. ilimfandftonfnodesfmustfbefasfshortfasfpossiblef tofensurefthefbestfaccuracyfinfcurrentflimitfandf onftime. thefr ilim fresistorfshouldfbefcloseftofthefdevicef andfconnectedftoflxfwithfafkelvinftraceftofpinf 28fonfthefdevice.fallfofftheflxfpinsfarefconnectedf toftheflxfpadfonfthefdevice,fwhichfshouldfbefaf sufcientfconnectionfandfwillfpreventfthefneedf tof connectf thef resistorf furtherf intof thef lxf plane. theffeedbackfcomponentsfforfthefswitcherffandf thefldofneedftofbeffasfcloseftoftheffbfandffblf pinsfoffthefdevicefasfpossibleftofreducefthefpos - sibilityfoffnoisefcorruptingfthesefanalogfsignals. ? ? ? ? ? ? ? ? bst, ilim, and lx lxfandfbstfarefveryfnoisyfnodesfandfmustfbef routedftofminimizedfthefpcbfareafthatfisfexposedf tofthesefsignals. thef connectionsf forf f thef boostf capacitorf betweenfthefdevicefandflxfmustfbefshortfandf directlyfconnectedftoftheflxbstf(pinf13). thefconnectionsfforfthefcurrentflimitfresistorf betweenfthefilimfpinfandflxfmustfbefasfshortfasf possiblefandfdirectlyfconnectedftofpinf28f(lxs). theflxfnodefbetweenfthefficfandfthefinductorf shouldfbefwidefenoughftofhandlefthefinductorf currentfandfshortfenoughftofeliminatefthefpos - sibilityfofflxfnoisefcorruptingfotherfsignals.f multiplefviasfshouldfbefusedftofprovidefafgoodf connectionftoflxfbetweenfthefdevicefandfthef inductor. capacitors and current loops thefcurrentfloopsfbetweenfthefinputfcapacitors,f thefdevice,fthefinductor,ffandfthefoutputfcapaci - torsfmustfbefasfclosefasfpossibleftofeachfotherftof reducefirfdropfacrossfthefcopper. allfbypassfandfoutputfcapacitorsfmustfbefcon - nectedfasfclosefasfpossibleftofthefpinfonfthef device. soft-start capacitor thef capacitorf usedf forf soft-startf shouldf bef locatedfawayffromfthefbstfpinfandfitsfcapacitor. iff possiblef locatef thef boostf capacitorf onf thef oppositefsidefoffthefboardfformftheficfandfsoft- startfcapacitor.f ? ? ? ? ? ? ? ? ?
30 SC403B outline drawing mlpq-5x5-32 b aaa c c seating plane 1 2 n bbb c a b coplanarity applies to the exposed pad as well as the terminals . controlling dimensions are in millimeters ( angles in degrees ). notes : 2 . 1 . a pin 1 indicator ( laser mark ) d e a 2 bxn a a 1 e lxn e 1 0 . 76 0 . 76 3 . 48 1 . 05 d 1 1 . 66 1 . 49 pin 1 identification r 0 . 20 3 . 61 millimeters 0 . 50 bsc . 002 - 0 . 00 . 000 a 1 . 193 . 193 . 135 . 076 . 012 . 007 e 1 aaa bbb n e l a 2 d 1 d e b . 020 bsc . 137 . 016 . 003 . 004 32 . 197 (. 008 ) . 078 . 197 . 010 - 3 . 43 . 139 . 020 0 . 30 . 201 . 201 . 080 - . 012 4 . 90 4 . 90 1 . 92 - 0 . 18 . 031 min dim a max dimensions inches - nom . 039 0 . 80 min - 0 . 05 5 . 10 5 . 10 3 . 53 2 . 02 0 . 50 0 . 30 3 . 48 0 . 40 0 . 10 0 . 08 32 5 . 00 ( 0 . 20 ) 1 . 97 5 . 00 0 . 25 - 1 . 00 max - nom
31 SC403B land pattern mlpq-5x5-32 1 . 74 3 . 48 h 2 k 3 . 61 h 1 k 1 y h ( c ) g this land pattern is for reference purposes only . consult your manufacturing group to ensure your notes : 2 . dim x y h k p c g millimeters inches ( 4 . 95 ) . 012 . 030 . 165 . 020 . 078 . 137 (. 195 ) 0 . 30 0 . 75 3 . 48 0 . 50 1 . 97 4 . 20 dimensions company ' s manufacturing guidelines are met . 5 . 70 . 224 z failure to do so may compromise the thermal and / or functional performance of the device . shall be connected to a system ground plane . thermal vias in the land pattern of the exposed pad 3 . 4 . square package - dimensions apply in both x and y directions . controlling dimensions are in millimeters ( angles in degrees ). 1 . x p z h 1 . 059 1 . 49 h 2 . 065 1 . 66 k 1 . 041 1 . 05 1 . 74
32 SC403B contact information semtechfcorporation powerfmanagementfproductsfdivision 200fflynnfroad,fcamarillo,fcaff93012 phone:ff(805)f498-2111ffffax:ff(805)f498-3804 www.semtech.com ?fsemtechf2011 allfrightsfreserved.freproductionfinfwholeforfinfpartfisfprohibitedfwithoutfthefpriorfwrittenfconsentfoffthefcopyrightf owner.fthefinformationfpresentedfinfthisfdocumentfdoesfnotfformfpartfoffanyfquotationforfcontract,fisfbelievedftofbef accuratefandfreliablefandfmayfbefchangedfwithoutfnotice.fnofliabilityfwillfbefacceptedfbyfthefpublisherfforfanyfconse - quencefoffitsfuse.fpublicationfthereoffdoesfnotfconveyfnorfimplyfanyflicensefunderfpatentforfotherfindustrialforfintellec - tualfpropertyfrights.fsemtechfassumesfnofresponsibilityforfliabilityfwhatsoeverfforfanyffailureforfunexpectedfoperationf resultingffromfmisuse,fneglectfimproperfinstallation,frepairforfimproperfhandlingforfunusualfphysicalforfelectricalfstressf including,fbutfnotflimitedfto,fexposureftofparametersfbeyondfthefspecifedfmaximumfratingsforfoperationfoutsidefthef specifedfrange. semtechfproductsfarefnotfdesigned,fintended,fauthorizedforfwarrantedftofbefsuitablefforfusefinflife- supportfapplications,fdevicesforfsystemsforfotherfcriticalfapplications.finclusionfoffsemtechfproductsf infsuchfapplicationsfisfunderstoodftofbefundertakenfsolelyfatfthefcustomersfownfrisk.fshouldfafcustomerf purchaseforfusefsemtechfproductsfforfanyfsuchfunauthorizedfapplication,fthefcustomerfshallfindemnifyfandfholdf semtechfandfitsfofcers,femployees,fsubsidiaries,fafliates,fandfdistributorsfharmlessfagainstfallfclaims,fcostsfdamagesf andfattorneyffeesfwhichfcouldfarise. notice:fallfreferencedfbrands,fproductfnames,fservicefnamesfandftrademarksfarefthefpropertyfofftheirfrespectivef owners.

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