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?f2012fsemtechfcorporation 1 SC461 ecospeed ? dc-dc buck controller with integrated ldo features powerfsystem inputfvoltageff3vftof28v integratedfbootstrapfswitch fixedf5vfldofoutputff200ma 1%freferenceftolerancef f -40ftof+85fc selectablefinternal/externalfbiasfpowerfsupply ecospeed ? farchitecturefwithfpseudo-fxedffre - quencyfadaptivefon-timefcontrol logicfinputfandfoutputfcontrolf independentf enablef controlsf forf ldof andf switcherf programmablefsoft-startftime programmablefv in fuvlofthreshold powerfgoodfoutput selectablefp ower-savefmode protections automaticfrestartfonffaultfshutdownf over-voltagefandfunder-voltage tcfcompensatedfr ds(on)f fsensedfcurrentflimit thermalfshutdown smartfpower-save pre-biasfstart-up capacitorftypes:ffsp,fposcap,foscon,fandfceramic f package ff3fxf3(mm),f20-pinfmlpqf lead-freefandfhalogen-free rohsfandfweeefcompliant applications ofcefautomationfandfcomputing networkingfandftelecommunicationfequipment point-of-loadfpowerfsuppliesfandfmodulefreplacement ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? description thefSC461fisfafsynchronousfecospeed ? f buckfregulatorf whichfincorporatesfsemtechsfadvanced,fpatentedfadap - tivef on-timef controlf architecturef tof providef excellentf light-loadfefciencyfandffastftransientfresponse.ffitffea - turesfanfintegratedfbootstrapfswitchfandfaffxedf5vfldofinf af3fxf3(mm)fpackage.ffthefdevicefisfhighlyfefcientfandf usesfminimalfpcbfarea.ff thefSC461fsupportsfusingfstandardfcapacitorftypesfsuchfasf electrolyticforfspecialfpolymer,finfadditionftofceramic,fatfswitch - ingffrequenciesfupftof1mhz.fthefprogrammableffrequency,f synchronousf operation,f andf programmablef power-savef providefhighfefciencyfoperationfoverfafwidefloadfrange.f additionalffeaturesfincludefcycle-by-cyclefcurrentflimit,f programmablefsoft-start,funderfandfover-voltagefprotec - tion,fprogrammablefover-currentfprotection,fstart-upfintof pre-biasedfoutput,fautomaticffaultfrecoveryf(hiccupfrestart),f soft-shutdown,fandfafselectablefpower-savefmode.ffthef devicefalsofprovidesfseparatefenablefinputsfforfthefpwmf controllerfandfldofasfwellfasfafpowerfgoodfoutputfforfthef pwmfcontroller.ffoutputfvoltagefrangefisf0.6ftof5v,fwithf outputfvoltagesfgreaterfthanf5vfsupportedfusingfadditionalf components.ff thefinputfvoltagefcanfrangeffromf3vftof28v.ffthefwidef inputfvoltagefrange,fprogrammableffrequency,fandfinte - gratedf5vfldofmakefthefdevicefextremelyffexiblefandf easyftofusefinfafbroadfrangefoffapplications.ffsupportfisf providedfforfmulti-cellfbatteryfsystemsfinfadditionftoftra - ditionalfdcfpowerfsupplyfapplications. revisionf2.0 power management typical application circuit vdda pgood en ton enl 1 f vext or vldo pgnd agnd vldo enable ldo rton sc 461 vddp vldo ss 0 . 1 f 1 f enable pgood psv psv lx ilim rlim + vout cout l 1 vout fb cin dh bst dl vin vin vext or vldo 10 n f
SC461 2 pin confguration ordering information marking information 461 yyww xxxx agnd pad 1 2 3 4 fb vldo vout vdda 5 6 7 8 9 10 d h pgnd psv dl vddp pgood 15 14 13 12 11 16 17 18 19 20 vin l x b s t n c s s i l i m t o n e n l e n a g n d top view notes: 1)f availablefinftapefandfreelfonly.ffafreelfcontainsf3000fdevices. f 2)f lead-freef packagingf only.f devicef isf weeef andf rohsf compliantf andfhalogen-free. yyww = date code xxxx = semtech lot number mlpq-ut20 device package SC461ultrt (1)(2) mlpq-ut20 SC461evb evaluationfboard
SC461 3 absolute maximum ratings (1) 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.f -0.3f tof +28 lxftofpgndf(v)f(transientff100ns)f f. . . . . . . . . -2.0f tof +28 dh,fbstftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +35 dh,fbstftoflxf(v) f f. . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +6 dlftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +6 vinftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3ftof+30 en,ffb,filim,fpgoodftofagndf(v) f. . . -0.3ftof+(vddaf+f0.3) psv,ffss,ftonftofagndf(v) f. . . . . . . . . . -0.3ftof+(vddaf+f0.3) vldo,ffvoutftofagndf(v) f. . . . . . . . . . -0.3ftof+(vddaf+f0.3) tonftofagndf(v) .f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f.f -0.3ftof+(vddaf-1.5) enlftofagndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . f-0.3ftofvinf vddpftofpgnd,fvddaftofagndf(v)ff f. . . . . . . . . . -0.3f tof +6 vddaftofvddpf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . ff-0.3fftoff+0.3 agndftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . ff-0.3fftoff+0.3f esdfprotectionflevel (1)f (kv) f. . . . . . . . . . . . . . . . . . . . . . . . . . . . f2 recommended operating conditions inputfvoltagef(v)f f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.0f tof 28 vddaftofagnd,fvddpftofpgndf(v) f. . . . . . . . . . . f 3.0f tof 5.5 voutftofpgndf(v) (2) f. . . . . . . . . . . . . . . . . . . . . . . . . . 0.6f tof 5.5 f supportsfoutputfvoltagesfgreaterfthanf5.5vfusingf f externalfcomponents thermal information storageftemperaturef(c) f. . . . . . . . . . . . . . . . . . . . -60ftof+150 maximumfjunctionftemperaturef(c) f. . . . . . . . . . . . . . . . f150 operatingfjunctionftemperaturef(c) f. . . . . . . . -40ftof+125 thermalfresistance,fjunctionftofambient (3) f(c/w) f. . . . . .f50 peakfirfrefowftemperaturef(c) f. . . . . . . . . . . . . . . . . . . . f 260 exceedingfthefabovefspecifcationsfmayfresultfinfpermanentfdamageftofthefdeviceforfdevicefmalfunction.ffoperationfoutsidefoffthefparametersf specifedfinfthefelectricalfcharacteristicsfsectionfisfnotfrecommended. notes: (1)f testedfaccordingftofjedecfstandardfjesd22-a114.f (2)f voutfpinfmustfnotfexceedf(vddaf+f0.3v). (3)ffffcalculatedffromfpackagefinfstillfair,fmountedftof3fxf4.5f(in),f4flayerffr4fpcbfwithfthermalfviasfunderfthefexposedfpadfperfjesd51fstandards. unlessfspecifed:fv in f=12v,fvddaf=fvddpf=f5v,ft a f=f+25cfforftyp,f-40ftof+85fcfforfminfandfmax,fft j f SC461 4 electrical characteristics (continued) parameter conditions min typ max units input supplies (continued) vddaf+fvddpfsupplyfcurrent (2) enlf,fenf=f0v 3 7 a power-savefoperationf enf=f5v,fpsvf=ffopenf(foat),fv fb f>f600mv 0.4 ma forcedfcontinuousfmodefoperationff operatingff sw f=f220khz,fpsvf=fvdda,fnofload 14 fbfcomparatorfthreshold staticfv in fandfload,f0ftof+85fc 0.5952 0.600 0.6048 v staticfv in fandfload,f-40ftof+85fc 0.594 0.606 v frequencyfrange continuousfmodefoperation 1000 khz timing on-time forcedfcontinuousfmodefoperation v in f=f15v,fv out f=f3v,ffr ton f=f300k?,ffvddaf=f5v 1530 1700 1870 ns minimumfon-time 80 ns minimumfof-time 250 ns soft-start soft-startfchargefcurrent 3.0 a analog inputs/outputs voutfinputfresistance 500 k? current sense zerofcrossfdetectorfthreshold lxffwithfrespectftofpgnd -3 0 +3 mv power good powerfgoodfthreshold upperflimit,fv fb f>finternalf600mvfreference +20 % lowerflimit,fv fb f SC461 5 electrical characteristics (continued) parameter conditions min typ max units fault protection i lim fsourcefcurrent temperaturef 25c 9 10 11 a i lim fsourcefcurrentftemperaturefcoefcient 0.28 %/c i limf comparatorfofset withfrespectftofagnd -8 0 +8 mv outputfunder-voltagefthreshold v fb fwithfrespectftofinternalf600mvfreference,f f 8fconsecutivefcycles -25 % smartfpower-savefprotectionfthreshold v fb fwithfrespectftofinternalf600mvfreference +10 % over-voltagefprotectionfthreshold v fb fwithfrespectftofinternalf600mvfreference +20 % over-voltageffaultfdelay f 5 s over-temperaturefshutdown 10 cfhysteresis 165 c logic inputs/outputs logicfinputfhighfvoltageffen 1.4 v logicfinputfhighfvoltageffpsv forcedfcontinuousfmodefoperation;f psvfpinfwithfrespectftofvdda -0.4 v logicfinputflowfvoltageffen,fenl (3) withfrespectftofagnd 0.4 v enfinputfbiasfcurrent enf=fvddaforfagnd -10 +10 a enlfinputfbiasfcurrent v in f=f28v +11 a fbfinputfbiasfcurrent fbf=fvddaforfagnd -1 +1 a psvfinputfbiasfcurrent psvf=fvdda 5 16 a 0.8vf < fpsvf < f1.5v 1 a linear regulator vldofaccuracy vldofloadf=f10ma 4.875 5.0 5.125 v currentflimit vldofff4.0vf(typ) 200 vldofdropfoutfvoltage v in ftofvldo,ffldofloadf=f50ma 1.4 v
SC461 6 electrical characteristics (continued) parameter conditions min typ max units high-side driver (dh, bst, lx) peakfcurrent vddpf=f5v,fffdhfpinfsourcingforfsinking 2 a onfresistance r dh_pull-up, fflxfff0.5v,ffvddpf=f5v 1.1 2.1 ? r dh_pull-down,fff vddpf=f5v 0.65 1.20 ? riseftime c dh-lx f=f3nf,ffvddpf=f5v 22 ns fallftime c dh-lx f=f3nf,ffvddpf=f5v 12 ns propagationfdelay fromffbfinputftofdh 45 ns shoot-thrufprotectionfdelay vddpf=f5v 45 ns bootstrapfswitchfresistance vddpf=f5v 16 ? low-side driver (dl, vddp, pgnd) peakfcurrent vddpf=f5v,ffdlfsourcing 2 a vddpf=f5v,ffdlfsinking 4 a onfresistance r dl_pull-up, ffvddpf=f5v 0.85 1.60 ? r dl_pull-down,ff vddpf=f5v 0.28 0.50 ? riseftime c dl f=f3nf,fvddpf=f5v 7 ns fallftime c dl f=f3nf,fvddpf=f5v 3.5 ns notes: (1)f v in fuvlofisfprogrammablefusingfafresistorfdividerffromfvinftofenlftofagnd.ffthefenlfvoltagefisfcomparedftofanfinternalfreference.ffnotefthatf becausefthefv in fuvlofthresholdfatfthefenlfpinfisfabovefthefenablefthresholdfoffthefldo,fthefldofmustfbefusedfasfbiasfpowerfforfthefdevicef whenfusingfthefv in fuvloffeature. (2)f forffcmfoperation,fthefvddafandfvddafsupplyfcurrentfincludesfthefdh/dlfcurrentfrequiredftofdrivefthefexternalfmosfets. (3)f thefenlfpinfwillfenablefthefldofwithf0.8vftypical.ffthefv in fulvoffunctionfoffthefenlfpinfwillfdisablefthefswitcherfunlessfthefenlfpinfexceedsf thefv in fuvlofthresholdfwhichfisftypicallyf1.55v.
SC461 7 efciency vs. load power save mode 20 % 30 % 40 % 50 % 60 % 70 % 80 % 90 % 100 % 0 1 2 3 4 5 6 7 8 9 10 i out ( adc ) e f f ( % ) 12 v 6 v 24 v characteristicsfinfthisfsectionfarefbasedfonfusingfthefdetailedfapplicationfcircuit. efciency vs. load forced continuous mode 20 % 30 % 40 % 50 % 60 % 70 % 80 % 90 % 100 % 0 1 2 3 4 5 6 7 8 9 10 ( adc ) e f f ( % ) i out 12 v 6 v 24 v load regulation forced continuous mode 1 . 75 1 . 76 1 . 77 1 . 78 1 . 79 1 . 80 1 . 81 1 . 82 1 . 83 1 . 84 1 . 85 0 1 2 3 4 5 6 7 8 9 10 12 v 24 v 6 v v o u t ( v ) i out ( adc ) efciency vs load - 5v output i out ( adc ) 20 % 30 % 40 % 50 % 60 % 70 % 80 % 90 % 100 % 0 1 2 3 4 5 6 7 8 9 10 e f f ( % ) 12 v 24 v inductor : cyntec pcmb 135 t - 3 r 3 mf high - side mosfet : irf 7811 low - side mosfet : irf 7832 frequency 360 khz efciency vs load - 12v output 20 % 30 % 40 % 50 % 60 % 70 % 80 % 90 % 100 % 0 1 2 3 4 5 6 e f f ( % ) i out ( adc ) 18 v 24 v inductor : cyntec pcmb 104 e - 4 r 7 ms high - side mosfet : irf 7811 low - side mosfet : irf 7832 frequency 400 khz load regulation power save mode 1 . 75 1 . 76 1 . 77 1 . 78 1 . 79 1 . 80 1 . 81 1 . 82 1 . 83 1 . 84 1 . 85 0 1 2 3 4 5 6 7 8 9 10 i out ( adc ) v o u t ( v ) 12 v 24 v 6 v typical characteristics internalf5vfldofbias,fpsvfenabled internalf5vfldofbias,ffcmfenabled externalf5vfbias,fpsvfenabled externalf5vfbias,ffcmfenabled externalf5vfbias,fpsvfenabled externalf5vfbias,ffcmfenabled
SC461 8 characteristicsfinfthisfsectionfarefbasedfonfusingfthefdetailedfapplicationfcircuit. startup into pre-bias output time (2ms/div) externalf5vfbias,fv in f=f12v,floadf20ma,fpsvfenabled start-up ss ramp-up time (2ms/div) start-up en input automatic restart v in transient ssf f (5v/div) vout f f(1v/div) vin (10v/div) time (40ms/div) over-current response time (100s/div) externalf5vfbias,fv in f=f12v,floadf17a pgood f (5v/div) load f (10a/div) vout f(1v/div) lx f (10v/div) automatic restart over-current time (20ms/div) ssf f (5v/div) lx f (10v/div) load f (10a/div) vout f f(1v/div) typical characteristics (continued) externalf5vfbias,fv in f=f12v,floadf1a externalf5vfbias,fv in f=f12v,fnofload,fpsvfenabled externalf5vfbias,fv in f=f12v,floadfresistancef50m?fcontinuous pgood f (5v/div) lx (10v/div) vout (1v/div) en f(5v/div) externalf5vfbias,fv in f=f12v,fnofload,fpsvfenabled enf f (5v/div) pgood f (5v/div) vout f(1v/div) lx f (10v/div) ssf f (5v/div) pgood f (5v/div) vout f(1v/div) lx f (10v/div) time (2ms/div) lx (10v/div)
SC461 9 characteristicsfinfthisfsectionfarefbasedfonfusingfthefdetailedfapplicationfcircuit. switching forced continuous mode time ( 10s/div) externalf5vfbias,fv in f=f12v,floadf10a,ffcmfenabled switching power-save mode, light load time (10s/div) switching power-save mode, no load output shutdown vout f f (1v/div) pgood f (5v/div) lxf f (10v/div) time (400s/div) transient response power-save mode time (100s/div) externalf5vfbias,fv in f=f12v,fv out f=f1.8v,fload ff 0aftof10a,fpsvfenabled voutf f (50mv/div) fb f (50mv/div) load f(10a/div) lx f (10v/div) transient response forced continuous mode time (100s/div) voutf f (50mv/div) fb f (50mv/div) lxf f (10v/div) load f f(10a/div) typical characteristics (continued) externalf5vfbias,fv in f=f12v,fv out f=f1.8v,fload ff 1a externalf5vfbias,fv in f=f12v,floadf1a,fpsvfenabled externalf5vfbias,fv in f=f12v,fv out f=f1.8v,fload ff 0aftof10a,ffcmfenabled externalf5vfbias,fv in f=f12v,fnofload,fpsvfenabled dl f (5v/div) vout f(50mv/div) lx f (10v/div) vout f(50mv/div) dl f (5v/div) lx f (10v/div) time (10ms/div) vout f(50mv/div) dl f (5v/div) lx f (10v/div) en f f (5v/div)
SC461 10 rlim 100 nf 0 w 100 n f sc 461 t o n vldo vout dl vdda pgood e n l b s t psv e n vddp a g n d fb d h n / c vin l x s s i l i m pgnd q 1 en pgood enable ldo 5 v vin vin q 2 rtop l 1 rbot 1 f 5 v vout rton 1 f vldo 1 2 3 ( 1 ) 4 5 11 12 13 14 15 6 7 8 9 10 20 19 18 17 16 cout + 1 f 154 k w 12 v to 1 . 8 v @ 10 a ctop * pad 19 . 6 k w 10 k w np cin 1 cin 2 100 n f component value manufacturer part number web cin 1 , cin 2 10 f / 25 v murata grm 32 dr 71 e 106 ka 12 l www . murata . com edc . sanyo . com cout 2 x 220 f / 15 m w / 4 v sanyo 4 tpe 220 mf www . cyntec . com l 1 1 . 5 h cyntec pcmb 1335 t - 1 r 5 mf key components www . irf . com q 1 irf 7821 i . r . irf 7821 www . irf . com q 2 irf 7832 i . r . irf 7832 6 . 81 k w css 3 . 3 nf cbst ( 1 ) ( 2 ) psv : remove 0 w resistor for power - save operation . connect 0 w resistor from psv pin to vdda for forced continuous mode operation . ( 1 ) 5 v : connect vdda and vddp to external 5 v supply for external bias . connect vdda and vddp to vldo for self - biased operation . notes : ( 2 ) detailed application circuit
SC461 11 pin descriptions pin # pin name pin function 1 fb feedbackfinputfforfswitchingfregulatorffconnectftofanfexternalfresistorfdividerffromfoutputffusedftofpro - gramfthefoutputfvoltage. 2 vout switcherfoutputfvoltagefsensefpin.ffthefvoltagefatfthisfpinfmustfnotfexceedfthefvddafpin.ffforfoutputfvoltagesf upftofvddafconnectfthisfpinfdirectlyftofthefswitcherfoutput.ffforfoutputfvoltagesfexceedingf5vfconnectfthisf pinftofthefswitcherfoutputfthroughfafresistorfdivider.ff 3 vdda supplyf inputf forf internalf analogf circuitsf f f connectf tof anf externalf 3.3vf orf 5vf supplyf orf connectf tofvldof falsofthefsensefinputfforfvddafunderfvoltageflockoutf(vddafuvlo). 4 vldo outputfoffthef5vfldoffthefvoltagefatfthisfpinfmustfnotfexceedfthefvoltagefatfthefvddafpin. 5 vin inputfsupplyfvoltageffconnectftofthefsamefsupplyfusedfforfthefhigh-sidefmosfet.ffconnectfaf100nffcapaci - torffromfthisfpinftofagndftofflterfhighffrequencyfnoise. 6 ss soft-startfffconnectfanfexternalfcapacitorftofagndftofprogramfthefsoft-startfandfautomaticfrecoveryftime. 7 nc nofconnection 8 bst bootstrapfpinffconnectfaf100nffminimumfcapacitorffromfbstftoflxftofdevelopftheffoatingfvoltagefforfthef high-sidefgatefdrive.ff 9 dh high-sidefgatefdrivefoutput 10 lx switchingf(phase)fnode 11 pgnd powerfgroundfforfthefdlfandfdhfdriversfandftheflow-sidefexternalfmosfet. 12 dl low-sidefgatefdrivefoutput 13 vddp supplyfinputfforfthefdhfandfdlfgatefdrivesffconnectftofthefsamef3.3vforf5vfsupplyfusedfforfvdda. 14 psv power-savefprogrammingfinputfffoatfpinftofselectfpower-savefwithfnofminimumffrequencyffpullfupftof vddaftofdisablefpower-savefandfselectfforcedfcontinuousfmode. 15 pgood open-drainfpowerfgoodfindicatorffhighfimpedancefindicatesfthefswitchingfregulatorfoutputfisfgood.ffanf e xternalfpull-upfresistorfisfrequired. 16 ilim currentflimitfsensefpinffusedftofprogramfthefcurrentflimitfbyfconnectingfafresistorffromfilimftoflx. 17 en enablefinputfforfswitchingfregulatorffflogicflowfdisablesfthefswitchingfregulatorffflogicfhighfenablesfthef switchingfregulator. 18 agnd analogfground 19 ton onftimefprogrammingfinputffsetfthefon-timefbyfconnectingfthroughfafresistorftofagnd. 20 enl enablefinputfforfthefldofandfvinfuvlofinputfforfthefswitchingfregulatorffconnectfenlftofagndftofdisablef thefldofffdriveftoflogicfhighf(>1.7v)ftofenablefthefldofandfinhibitfv in fuvloffconnectftofresistorfdividerf fromfvinftofagndftofprogramfthefv in fuvlofthreshold. pad agnd analogfground
SC461 12 block diagram 8 vdda uvlo soft start / automatic restart fb agnd on - time generator control & status pgood gate drive control vin ton vout zero cross detector current limit ilim enl 5 v ldo vldo bst fb comparator vdda lx en dl 5 16 20 4 2 19 1 3 15 17 10 a vddp vddp vin vdda psv 14 pgnd 11 dl 12 dh 9 vddp 13 bootstrap switch reference vin ulvo detect a = connected to pins 18 and pad dl vin to control & status vin ulvo vdda ss 6
SC461 13 synchronous buck converter thefSC461fisfafstepfdownfsynchronousfdc-dcfbuckfcon - trollerfwithfanfinternalf5vfldo.ffitfprovidesfefcientfopera - tionfinfafspacefsavingf3x3f(mm)f20-pinfpackage.ffthef programmablefoperatingffrequencyfrangefupftof1mhzf enablesfoptimizingfthefconfgurationfforfpcbfareafandf efciency.ffff thefcontrollerfusesfafpseudo-fixedffrequencyfadaptivef on-timefcontrol.ffthisfallowsffastftransientfresponsefwhichf permitsfthefusefoffsmallerfoutputfcapacitors. input voltage requirements thefSC461frequiresftwofinputfsuppliesfforfnormalfopera - tion:ffv in fandfvdda/vddp.ffv in foperatesfoverfthefwidef rangefoff3vftof28v.ffvddafandfvddpfrequirefaf3.3vforf5vf supplyfwhichfcanfbeffromfanfexternalfsourceforffromfthef internalfldo.ffvddafandfvddpfmustfbefderivedffromfthef samefsourcefvoltage.ff psuedo-fxed frequency adaptive on-time control thefpwmfcontrolfmethodfusedfbyfthefSC461fisfpseudo- f fxedffrequency,fadaptivefon-time,fasfshownfinffiguref1.f thefripplefvoltagefgeneratedfatfthefoutputfcapacitorfesrf isfusedfasfafpwmframpfsignal.ffthisfripplefisfusedftoftriggerf thefon-timefoffthefcontroller.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.fthefpulsefdurationfisfdeterminedfbyfv outff andfv in .fthefdurationfisfproportionalftofoutputfvoltagef andfinverselyfproportionalftofinputfvoltage.ffwithfthisf adaptivefon-timefconfguration,fthefdevicefautomaticallyf anticipatesfthefon-timefneededftofregulatefv out fforfthef presentfv in fconditionfandfatfthefselectedffrequency.ff thefadvantagesfoffadaptivefon-timefcontrolfare: predictablefoperatingffrequencyfcomparedftof otherfvariableffrequencyfmethods.f reducedfcomponentfcountfbyfeliminatingfthef errorfampliferfandfcompensationfcomponents. reducedf componentf countf byf removingf thef needftofsensefandfcontrolfinductorfcurrent. fastftransientfresponseffthefresponseftimefisf controlledfbyfaffastfcomparatorfinsteadfoffaftypi - callyfslowferrorfamplifer. reducedfoutputfcapacitancefdueftoffastftran - sientfresponse. one-shot timer and operating frequency one-shotftimerfoperationfisfshownfinffiguref2.ftheffbf comparatorfoutputfgoesfhighfwhenfv fb fisflessfthanfthef internalf600mvfreference.fthisffeedsfintofthefdhfgatef drivefandfturnsfonfthefhigh-sidefmosfet,fandfalsofstartsf thefone-shotftimer.ffthefone-shotftimerfusesfanfinternalf comparatorfandfafcapacitor.ffonefcomparatorfinputfisfcon - nectedftofv out ,fthefotherfinputfisfconnectedftofthefcapaci - tor.ffwhenfthefon-timefbegins,fthefcapacitorfchargesffromf zerofvoltsfthroughfafcurrentfwhichfisfproportionalftofv in .f whenfthefcapacitorfvoltagefreachesfv out ,fthefon-timefisf completedfandfthefhigh-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 ref q 1 q 2 l c out v in esr + v out v lx fb dh dl r ton + - figure 2 on-time generation ? ? ? ? ? applications information
SC461 14 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 thefSC461fusesfanfexternalfresistorftofsetfthefon-timefwhichf indirectlyfsetsftheffrequency.ffthefon-timefcanfbefpro - grammedftofprovidefanfoperatingffrequencyfoffupftof1mhzf usingfafresistorfbetweenftheftonfpinfandfground.fthefresis - torfvaluefis selected by the following equation. 28 7 ,1 6: ,1 28 7 28 7 ,1 21 721 9 s) 9 qv i 9 9 9 s) 9 qv 7 5 u u ? ? 1 ? u u u thefmaximumfrecommendedfr ton fvaluefisfshownfbyfthef followingfequation. $ 9 5 ,1b0,1 721b0$; u immediatelyfafterfthefon-time,fthefdlfoutputfdrivesfhighf tofenergizeftheflow-sidefmosfet.ffdlfhasfafminimumfhighf timefoff~250ns,fafterfwhichfdlfcontinuesftofstayfhighfuntilf onefofftheffollowingfoccurs:f theffbfinputffallsfbelowfthef600mvfreference thefzerofcrossfdetectorftripsfiffpower-savefisf active ton limitations and vdda supply voltage forfvddafbelowf4.5v,ftheftonfaccuracyfmayfbeflimitedfbyf v in .ffthefpreviousfrtonfequationfisfaccuratefiffv in fsatisfesf thefbelowfrelationfoverfthefentirefv in frange: v in < (vdda - 1.6v) x 10 iff v in fexceedsf((vddaf-f1.6v)fxf10)fforfallforfpartfoffthef v in f range,fthefpreviousfrtonfequationfisfnotfaccurate.finfallf casesfwherefvinf>f((vddaf-f1.6v)fxf10),fthefrtonfequationf mustfbefmodifedfasffollows.f ? ? 28 7 21 721 9 s) 9 9''$ qv 7 5 u u u notefthatfwhenf v in fisfgreaterfthanff((vddaf-f1.6v)fxf10),fthef actualfon-timefisffxedfandfdoesfnotfvaryfwithfv in .ffwhenf operatingfinfthisfcondition,fthefswitchingffrequencyfwillf varyfinverselyfwithfv in fratherfthanfapproximatingffxedf frequency. v out voltage selection thefswitcherfoutputfvoltagefisfregulatedfbyfcomparingf v out fasfseenfthroughfafresistorfdividerfatftheffbfpinftofthef internalf600mvfreferencefvoltagef(seeffiguref3).f r 1 to fb pin r 2 v out figure 3 output voltage selection notefthatfthisfcontrolfmethodfregulatesfthefvalleyfoffthef outputfripplefvoltage,fnotfthefdcfvalue.ffthefdcfvaluefoff v out fisfofsetfbyfthefoutputfripplefaccordingftoftheffollow - ingfequation. ? 1 ? ? ? 1 ? u 2 v r r 1 0.6 v ripple 2 1 out infsomefapplicationsfafsmallfcapacitorfc top fisfplacedfinf parallelfwithfr1ftofprovidefaflargerfripplefsignalffromfv out f toftheffbfpin.ffinfthesefapplications,fthefoutputfvoltagef v out fisfcalculatedfaccordingftoftheffollowingfequationfinf whichf z frepresentsfthefswitchingffrequency. 723 723 5,33/ ( 28 7 & & 5 5 5 5 & & 5 9 5 5 9 ? ? 1 ? u u ? 1 ? ? ? 1 ? u confguring v out greater than 5v thefswitcherfoutputfvoltagefcanfbefprogrammedfhigherf thanf5vfwithfcarefulfattentionftofthefvoutfandfrtonfpins.ff infthesefapplicationsfthefvoutfpinfcannotfconnectfdirectlyf applications information (continued)
SC461 15 tofthefswitcherfoutputfdueftofitsfmaximumfvoltagefrating.ff anf additionalf resistorf dividerf networkf isf requiredf tof connectffromfthefswitcherfoutputftofthefvoutfpinfasf shownfinffiguref4. to vout pin vout > 5 v r v 1 cout l lx r v 2 figure 4 resistor divider for v out exceeding 5v thefresistorsfmustfbefchosenfsofthatfthefvoutfdoesfnotf exceedfthefvddafsupply.ffnotefthatfthefvoutfpinfhasfanf internalf500k w fresistorfconnectedftofagnd.fftofminimizef thefefectfoffthisfresistorfonfthefresistorfdividerfratio,fthef maximumfrecommendfvaluefforfresistorfr v2 finffiguref4fisf 10k w .ff infadditionftofthefresistorfdivider,fthefrtonfresistorfvaluef mustfbefadjusted.ffthefon-timefisfcalculatedfaccordingftof thefvoltagefatfthefvoutfpin.ffinforderftofselectfthefdesiredf on-timef andf operatingf frequency,f thef rtonf resistorf shouldfbefadjustedftofafhigherfvalueftofcompensatefforf thefreducedfvoltagefatfthefvoutfpin.ffforfoutputfvoltagesf exceedingf5v,fthefrequiredfrtonfvaluefcanfbefdeterminedf byftheffollowingfequation.f ? ? 1 ? u u u ? ? 1 ? u 9 9 28 7 ,1 6: ,1 28 7 72 1 5 5 9 s) 9 qv i 9 9 5 forfapplicationsfwherefv out fexceedsf5v,ffcmfoperationfisf recommended.ff forced continuous mode operation thefSC461foperatesfthefswitcherfinfforcedfcontinuousf modef(fcm)fbyfconnectingfthefpsvfpinftofvdda.ffthefpsvf pinfshouldfneverfexceedfthefvddafsupply.ffseeffiguref5f forf fcmf waveforms.f f inf thisf modef onef off thef powerf mosfetsfisfalwaysfon,fwithfnofintentionalfdeadftimefotherf thanf tof avoidf cross-conduction.f fthisf resultsf inf moref uniformffrequencyfacrossftheffullfloadfrange,fwithfthef trade-offbeingfreducedfefciencyfatflightfloadsfdueftof thefhigh-frequencyfswitchingfoffthefmosfets. thefpsvfpinfcontainsfaf5afcurrentfsinkftofpreventfstrayf leakagefcurrentffromfpullingfthefpsvfpinfupftofthefvddaf supplyfwhenfthefpsvfpinfisffoatedftofselectfpower-savef operation.fftofselectfforcedfcontinuousfmodefoperation,f thef maximumf recommendedf resistancef betweenf thef vddafsupplyfandfthefpsvfpinfisf40k w . ff ff 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 5 forced continuous mode operation power-save mode operation thefSC461fprovidesfpower-savefoperationfatflightfloadsf withfnofminimumfoperatingffrequency,fselectedfbyffoat - ingfthefpsvfpinf(nofconnection).ffinfthisfmodefoffopera - tion,fthefzerofcrossfcomparatorfmonitorsfinductorfcurrentf viafthefvoltagefacrossftheflow-sidefmosfetfduringfthef of-time.fiffthefinductorfcurrentffallsftofzerofforf8fconsecu - tivefswitchingfcycles,fthefcontrollerfentersfpower-savef operation.fitfwillfthenfturnfofftheflow-sidefmosfetfonf eachfsubsequentfcycle,fprovidedfthatfthefcurrentffallsftof zero.ffafterftheflow-sidefmosfetfisfof,fbothfhigh-sidefandf low-sidesfmosfetsfremainfoffuntilfv fb fdropsftofthef600mvf threshold.ffwhilefthefmosfetsfarefoffthefloadfisfsuppliedf applications information (continued)
SC461 16 byfthefoutputfcapacitor.ffiffthefinductorfcurrentfdoesfnotf reachfzerofonfanyfswitchingfcycle,fthefcontrollerfimmedi - atelyfexitsfpower-savefandfreturnsftofforcedfcontinuousf mode.f figuref 6f showsf power-savef operationf atf lightf loads.f 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 6 power-save operation smart power-save protection activefloadsfmayfleakfcurrentffromfafhigherfvoltagefintof thefswitcherfoutput.ffunderflightfloadfconditionsfwithf power-savefenabled,fthisfcanfforcefv out ftofslowlyfrisefandf reachfthefover-voltagefthreshold,fresultingfinfanfover- voltagefshutdown.fsmartfpower-savefpreventsfthisfcondi - tion.fwhenftheffbfvoltagefexceedsf10%fabovefnominalf (exceedsf660mv),fthefdevicefimmediatelyfdisablesfpower- savefandfdlfdrivesfhighftofturnfonftheflow-sidefmosfet.ff thisfdrawsfcurrentffromfv out fthroughfthefinductorfandf causesfv out ftoffall.ffwhenfv fb fdropsfbackftofthef600mvftripf point,fafnormalft on fswitchingfcyclefbegins.fthisfmethodf preventsfover-voltagefshutdownfbyfcyclingfenergyffromf v out fbackftofv in .ffitfalsofminimizesfoperatingfpowerfunderf lightfloadfconditionsfbyfavoidingfforcedfcontinuousffmodef operation.f figuref7fshowsftypicalfwaveformsfforfthefsmartfpower- saveffeature. 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 7 smart power-save smartdrive tm forfeachfdhfpulse,fthefdhfdriverfinitiallyfturnsfonfthef high-sidefmosfetfatfafslowerfspeed,fallowingfafsofter,f smoothfturn-offofftheflow-sidefdiode.foncefthefdiodefisf offfandftheflxfvoltagefhasfrisenf0.8vfabovefpgnd,fthef smartdrivef circuitf automaticallyf drivesf thef high-sidef mosfetfonfatfafrapidfrate.ffthisftechniquefreducesfswitch - ingfnoisefwhilefmaintainingfhighfefciency,freducingfthef needfforfsnubbers. enablefinputfforfswitchingfregulator thef enf inputf isf af logicf levelf input.f f whenf enf isf lowf (grounded),fthefswitchingfregulatorfisfoffandfinfitsflowestf powerfstate.ffwhenfenfisflowfandfvddafisfabovefthefvddaf uvlof threshold,f thef outputf off thef switchingf regulatorf soft-dischargesf intof thef voutf pinf throughf anf internalf 2k?fresistor.ffwhenfenfisfaflogicfhighf( > 1v)fthefswitchingf regulatorfisfenabled.f thef enf inputf hasf internalf resistorsf f 2m ? f pullupf tof vdda,fandfaf1m ? fpulldownftofagnd.ffthesefresistorsfwillf normallyfcausefthefenfvoltageftofbefnearftheflogicfhighf tripf pointf asf vddaf reachesf thef vddaf uvlof threshold.ff tofpreventfundesiredftogglingfoffenfandferraticfstart-upf performance,fthefenfpinfshouldfnotfbefallowedftoffoatfasf open-circuit.ff applications information (continued)
SC461 17 notefthatfthefldofenablefpinf(enl)fcanfalsofdisablefthef switchingfregulatorfthroughfthefv in fuvloffunction.ffreferf tofthefenlfpinfandfv in fuvlofsection.ff current limit protection thefSC461ffeaturesfprogrammablefcurrentflimiting,fwhichf isfaccomplishedfusingfthefrds (on) fofftheflowerfmosfetfforf currentfsensing.ffthefcurrentflimitfisfsetfbyfr lim fresistorf whichfconnectsffromfthefilimfpinftofthefdrainfofftheflow- sidefmosfet.ffwhenftheflow-sidefmosfetfisfon,fanfinternalf 10 afcurrentffowsffromfthefilimfpinfandfthroughfthefr lim f resistor,fcreatingfafvoltagefdropfacrossfthefresistor.ffwhilef theflow-sidefmosfetfisfon,fthefinductorfcurrentfflowsf throughfitfandfcreatesfafvoltagefacrossfthefrds (on) .fthef voltagefacrossfthefmosfetfisfnegativefwithfrespectftof pgnd.ffiffthisfmosfetfvoltagefdropfexceedsfthefvoltagef acrossfr lim ,fthefvoltagefatfthefilimfpinfwillfbefnegativefandf currentflimitfwillfactivate.fthefcurrentflimitfthenfkeepsfthef low-sidefmosfetfonfandfpreventsfanotherfhigh-sidefon- time,funtilfthefcurrentfinftheflow-sidefmosfetfreducesf enoughftofbringfthefilimfpinfvoltagefupftofzero.ffthisf methodfregulatesfthefinductorfvalleyfcurrentfatftheflevelf shownfbyfi lim finffiguref8. time i peak i load i lim i n d u c t o r c u r r e n t figure 8 valley current limit thefcurrentflimitfschematicfwithfthefr lim fresistorfisfshownf inffiguref9. v out v in + cin q 2 + d 2 r lim c bst q 1 l pgnd dl ilim lx dh bst c out figure 9 valley current limit settingfthefvalleyfcurrentflimitftof10afresultsfinfafpeakf inductorfcurrentfoff10afplusfpeakfripplefcurrent.finfthisf situationfthefaveragefcurrentfthroughfthefinductorfisf10af plusfone-halffthefpeak-to-peakfripplefcurrent. thefr lim fvaluefisfcalculatedfbyfthefnextfequation. $ , 5'6 5 /, 0 21 /, 0 u fff thefinternalf10afcurrentfsourcefisftemperaturefcompen - satedfatf2800fppmfinforderftofprovideftrackingfwithfthef rds on . soft-start of pwm regulator thefSC461fhasfafprogrammablefsoft-startftimefthatfisfcon - trolledfbyfanfexternalfcapacitorfatfthefssfpin.fduringfthef soft-startftime,fthefcontrollerfsourcesf3affromfthefssfpinf tof chargef thef capacitor.f duringf thef start-upf processf (figuref10),f40%ffoffthefvoltageframpfatfthefssfpinfisfusedf asfthefreferencefforftheffbfcomparator.fthefpwmfcompara - torfissuesfanfon-timefpulsefwhenftheffbfvoltagefisflessf thanf 40%f off thef ssf voltage,f whichf forcesf thef outputf voltageftoffollowfthefssframp.fthefoutputfvoltagefreachesf regulationfwhenfthefssfpinfvoltagefexceedsf1.5vfandfthef fbfreachesfthef600mvfthreshold.fftheftimefbetweenfthef frstflxfpulsefandfvoutfreachingfthefregulationfpointfisf thefsoft-startftimef(t ss ).fthefcalculationfforfthefsoft-startf timefisfshownfbyftheffollowingfequation. applications information (continued)
SC461 18 a 3 v 5 . 1 c t ss ss p u afterfthefssfcapacitorfvoltagefreachesf1.5v,fthefssfcapaci - torfcontinuesftofchargefuntilfthefssfvoltagefisfequalftof 67%foffvdda.fatfthisftimefthefpowerfgoodfmonitorfcom - paresftheffbfpinfandfsetsfthefpgoodfoutputfhighf(openf drain)fiffvoutfisfinfregulation.fftheftimefbetweenfvoutf reachingfthefregulationfpointfandfthefpgoodfoutputf goingfhighfisfshownfbyftheffollowingfequation. ? 1 ? u u 9 9''$ $ & w 66 3*22' theftimeffromfthefrisingfedgefoffthefenfpinfftofthefpgoodf outputfgoingfhighfisfshownfbyftheffollowingfequation. ? 1 ? u u 9''$ $ & w 66 (1b3*22' afterfthefpowerfgoodfstart-upfdelayftimefisfcompleted,f thefssfpinfisfinternallyfpulledfupftofthefvddafsupply. thefsoft-startfcyclefandfpowerfgoodftimingfcanfbefseenfinf theffiguref10.ff en ss fb pgood css charging current 3 ua v ss = 1 . 5 v vout in regulation v ss = 67 % vdda t ss t pgood figure 10 soft-start cycle and power good timing pre-bias start-up SC461fcanfsupportfsoft-startfwithfanfoutputfpre-bias.fthef ssfcapacitorframpftimefisfthefsamefasfafnormalfstart-upf whenfthefoutputfvoltagefstartsffromfzero.funderfafpre- biasfstart-up,fthefdhfandfdlfdriversfinhibitfswitchingfuntilf 40%f off thef rampf atf thef ssf pinf equalsf thef pre-biasf fbf voltageflevel.fpre-biasfstart-upfisfachievedfbyfturningfoff theflowerfmosfetfwhenfthefinductorfcurrentfreachesfzerof duringfthefsoft-startfcycle.fthisfmethodfhelpsfpreventfthef outputfvoltageffromfdecreasing. power good output thefpgoodf(powerfgood)foutputfisfanfopen-drainfoutputf whichfrequiresfafpull-upfresistor.ffduringfstart-up,fpgoodf isfheldflowfandfisfnotfallowedftoftransitionfhighfuntilfthef outputfvoltagefisfinfregulationfandfthefssfpinfhasfreachedf 67%foffvdda.fftheftimeffromfenfgoingfhighftofpgoodf goingfhighfisftypicallyf12.5msfforfcssf=f10nffandfvddaf=f 5v.ffffforfcssf=f10nffandfvddaf=f3vftheftypicalfpgoodf timefisf7.5ms. whenfthefvoltagefatftheffbfpinfisf10%fbelowfthefnominalf voltage,fpgoodfisfpulledflow.foncefpgoodfpullsflowf therefisftypicallyf2%fhysteresisftofpreventfchatterfonfthef pgoodfoutput.fff pgoodfwillftransitionflowfifftheffbfvoltagefexceedsf+20%f offnominalf(720mv),fwhichfisfalsofthefover-voltagefshut - downfthreshold.fpgoodfalsofpullsflowfiffthefenfpinfisflowf andfvddafisfpresent.ff output over-voltage protection over-voltagefprotectionf(ovp)fbecomesfactivefasfsoonfasf thefdevicefisfenabled.fthefovpfthresholdfisfsetfatf600mvf +f20%f(720mv).fftherefisfaf5sfdelayfbuiltfintofthefovpf detectorftofpreventffalseftransitions.fwhenfv fb fexceedsfthef ovpfthreshold,fdlfisfdrivenfhighfandftheflow-sidefmosfetf isfturnedfon.fdlfremainsfhighfandfthefcontrollerfremainsf of.ffifftheffbfpinfremainsfabovefthefovpfthreshold,fdlf remainsfhighfandftheficfwillfmaintainfthisfmaintainfthisf statefwithfnofautomaticfrecovery.ffifffbffallsfbelowfthefovpf threshold,fthefdevicefgoesfthroughfthefautomaticffaultf recoveryfcycle.ffwhenfthefautomaticfrecoveryfcyclefisf completed,fthefdevicefwillfattemptfafnewfsoft-startfcycle.ff atfthefstartfoffthefsoft-startfcycle,fthefdlfoutputfwillfgof lowfforftypicallyf30usfwhilefthefcontrollerfinitializesfthef applications information (continued)
SC461 19 soft-startf sequence.f f pgoodf isf alsof lowf afterf anf ovpf event. output under-voltage protection whenfv fb ffallsf25%fbelowfitsfnominalfvoltagef(fallsftof 450mv)fforfeightfconsecutivefclockfcycles,fthefswitcherfisf shutfoffandfthefdhfandfdlfdrivesfarefpulledflowftoftri- statef thef mosfets.fthef controllerf staysf offf whilef thef devicefgoesfthroughfthefautomaticffaultfrecoveryfcycle. automatic fault recovery thefSC461fincludesfanfautomaticfrecoveryffeaturef(hiccupf modefuponffault).ffiffthefswitcherfoutputfisfshutfdownfduef tofaffaultfcondition,fthefdevicefusesfthefssfcapacitorfasfaf timer.ffuponffaultfdetectionfthefssfpinfisfpulledflowfandf thenfbeginsfchargingfthroughfthefinternalf3 afcurrentf source.ffwhenfthefssfcapacitorfreachesf67%foffvdda,fthef ssfpinfisfagainfpulledflow,fafterfwhichfthefssfcapacitorf beginsfanotherfchargingfcycle.ffthefssfcapacitorfwillfbef usedfforf15fcyclesfoffchargingffromf0ftof67%foffvdda.ff(forf over-voltagefandfover-temperatureffaults,fthefcountfwillf bef 16f cyclesf insteadf off 15).f f duringf thesef cyclesf thef switcherfisfoffandftherefisfnofmosfetfswitching.ff duringf thef nextf chargingf cycle,f thef normalf soft-startf routinefisfimplementedfandfthefmosfetsfbeginfswitch - ing.ffswitchingfcontinuesfuntilfthefpowerfgoodfstart-upf delayftimefisfreached.ffiffthefswitcherfoutputfisfstillfinfaf faultfcondition,fthefswitcherfwillfagainfshutfdownfandf forcef15fcyclesfoffssfchargingf(16fcyclesfinfthefcasefoffanf over-voltageforfover-temperatureffault)fbeforefattempt - ingfanotherfsoft-start.ffftheflongfdelayfbetweenfsoft-startf cyclesf reducesf thef averagef powerf lossf inf thef powerf components.fff thefautomaticfrecoveryftimingfisfshownfinffiguref11. 15 cycles ss t hiccup = 15 x t en _ pgood t en _ pgood 67 % x vdda 1 soft - start cycle fault applied 15 cycles 1 soft - start cycle t en _ pgood t en _ pgood t hiccup = 15 x t en _ pgood figure 11 automatic recovery timing thef controlf off thef low-sidef mosfetf duringf anf over- voltageffaultfisfhandledfdiferentlyffromfotherffaults.ffiffthef faultfwasfdueftofanfover-voltagefcondition,fthefdlfoutputf willfremainfhighfduringf16fssfchargingfcycles.ffforfallfotherf faults,fthefdlfoutputfwillfremainflow.ffhowever,fifftheffbf pinfexceedsfthefover-voltagefthreshold,fthefchargingfoff thefssfcapacitorfwillfnotfoccur,fandfthefdlfoutputfwillf remainfhigh.ffifftheffbfpinffallsfbelowfthefovpfthreshold,f 16fssfchargingfcyclesfwillfoccurfwhilefdlfremainsfhigh.ff whenfthefnextfstart-upfcyclefcommences,fdlfwillfdrivef lowfforftypicallyf30usfasfthefcontrollerfre-initializesfthef internalfsoft-startfroutine.f vdda uvlo and por thefvddafunder-voltageflock-outf(uvlo)fcircuitryfinhib - itsfswitchingfandftri-statesfthefdh/dlfdriversfuntilfvddaf risesfabovef2.84v.ffwhenfvddafexceedsf2.84v,fanfinternalf porf(power-onfreset)fresetsftheffaultflatchfandfthefsoft- startfcircuitryfandfthenfthefSC461fisfreadyftofbeginfafsoft- startfcycle.fthefswitcherfwillfshutfoffiffvddaffallsfbelowf 2.62v.fffvddpfdoesfnotfhavefuvlofprotection. ldo regulator whenfthefldofisfprovidingfbiasfpowerftofthefdevice,faf minimumf0.1ffcapacitorfreferencedftofagndfisfrequired,f alongfwithfafminimumf1ffcapacitorfreferencedftofpgndf tofflterfthefgatefdrivefpulses.ffreferftofthefpcbflayoutf guidelinesfsection.fff enl pin and v in uvlo thefenlfpinfisfalsofusedfforfthefv in funder-voltageflockoutf (v in fuvlo)fforfthefswitcher.ffthefv in fuvlofvoltagefisfpro - grammablefviafafresistorfdividerfatfthefv in ,fenlfandfagndf pins.ffthefv in fuvloffunctionfhasfaftypicalfthresholdfoff1.55vf onfthefv inf risingfedge.fftheffallingfedgefthresholdfisf1.24v. notefthatfwhenfthefv in fuvloffeaturefisfused,fthefldofisf enabledfbecausefthefenlfpinfisfabovefthefldofenablef thresholdf(0.8vftypical).ffinfthesefcasesfthefSC461fmustfusef thefinternalfldofforfbiasfpower.f timingfisfimportantfwhenfdrivingfenlfwithflogicfandfnotf usingfthefv in fuvlofcapability.ffthefenlfpinfmustftransitionf fromfhighftoflowfwithinf2fswitchingfcyclesftofavoidfthef pwmfoutputfturningfof.ffiffenlfgoesfbelowfthefv in fuvlof applications information (continued)
SC461 20 applications information (continued) thresholdfandfstaysfabovef1v,fthenfthefswitcherfwillfturnf offbutfthefldofwillfremainfon. notefthatfitfisfpossibleftofoperatefthefswitcherfwithfthef ldofdisabled,fbutfthefenlfpinfmustfbefbelowftheflogicf lowfthresholdf(0.4vfmaximum),fotherwisefthefv in fuvlof functionfwillfdisablefthefswitcher. thefnextftablefsummarizesftheffunctionfoffthefenlfandfenf pins.ff en enl ldo status switcher status low low,ff1.55v on of high high,f>f1.55v on on figuref12fshowsfthefenlfvoltagefthresholdsfandftheirf efectfonfldofandfswitcherfoperation. agnd enl low threshold ( min 0 . 4 v ) 1 . 55 v 1 . 24 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 12 enl thresholds enl logic control of pwm operation whenfthefenlfinputfexceedsfthefv in fuvlofthresholdfoff 1.55v,finternalflogicfchecksfthefpgoodfsignal.ffiffpgoodf isfhigh,fthefswitcherfisfalreadyfrunningfandfthefldofwillf startfwithoutfafectingfthefswitcher.ffiffpgoodfisflow,fthef devicefdisablesfpwmfswitchingfuntilfthefldofoutputfhasf reachedf80%foffitsffnalfvalue.ffthisfdelayfpreventsfthef additionalfcurrentfneededfbyfthefdhfandfdlfgatefdrivesf fromfoverloadingfthefldofatfstart-up. ldofstart-up beforefldofstart-up,fthefdevicefchecksfthefstatusfoffthef followingf signalsf tof ensuref properf operationf canf bef maintained. enlfpin vldofoutput v in finputfvoltage whenfthefenlfpinfisfhighfandfv in fvoltagefisfavailable,fthef ldofwillfbeginfstart-up.ffduringfthefinitialfphasefwhenf v ldo fisfbelowf1v,fthefldofinitiatesfafcurrent-limitedfstart- upf(typicallyf15ma).ffthisfprotectsfthefldoffromfthermalf damagefiffthefvldofpinfisfshortedftofground.ffasfv ldo f exceedsf1v,fthefstart-upfcurrentfgraduallyfincreasesftof 96ma.f f whenf v ldo f reachesf 4v,f thef ldof currentf limitf increasesftof200mafandfthefldofoutputfrisesfquicklyftof 5v.fffthefldofstart-upfproflefisfshownfinffiguref13. f 5 v 4 v 15 ma constant current voltage regulating with 200 ma current limit 1 v vldo increasing current f figure 13 ldo start-up using the internal ldo to bias the SC461 theffollowingfstepsfmustfbeffollowedfwhenfusingfthef internalfldoftofbiasfthefdevice. connectf vddaf andf vddpf tof vldof beforef enablingfthefldo. duringfthefinitialfstart-upfthefldo,fwhenfthef ldofoutputfisflessfthanf1v,fthefexternalfloadf shouldfnotfexceedf10ma.ffabovef1v,fanyfexter - nalfloadfonfvldofshouldfnotfexceedf40mafuntilf thefldofvoltagefhasfreachedf4v.ff 1. 2. 3. ? ?
SC461 21 applications information (continued) whenfthefldofisfusedfasfbiasfpowerfforfthefdevice,fthefenf andfenlfinputsfmustfbefusedfcarefully.ffdofnotfconnectf thefenfpinfdirectlyftofvddaforfanotherfsupplyfvoltage.ffiff thisfisfdone,fdrivingfthefenlfpinflowf(tofagnd)fwillfturnfoff thef ldof andf thef ldof switch-overf mosfet,f butf thef switcherfcanfcontinuefoperating.ffiffv out fexceedsf2.5v,fthef outputfvoltagefcanffeedfintofthefvddafsuppliesfthroughf internalfparasiticfdiodesfviafthefvoutfpin.ffthisfcanfpoten - tiallyfdamagefthefdevice,fandfalsofcanfpreventfthefswitcherf fromfshuttingfoffuntilfthefvddafsupplyfdropsfbelowfthef vddafuvlofthreshold.ffforfthesefapplicationsfafdedicatedf logicfsignalfisfrequiredftofdrivefenflowfandfdisablefthef switcher.ffthisfsignalfcanfbefcombinedfwithfthefenlfsignalf iffneeded,fasflongfasfthefenfpinfdoesfnotfexceedfabsolutef maximumfratings. ldo usage at low input voltage applicationsfrequiringfsteady-stateforftransientfoperationf atflowfinputfvoltagesf(v in fbelowf6.5v)fmayfusefthefinternalf ldoftofbiasfthefvdda/vddpfpinsfwithinflimitations.fftheref areflimitationsftofbothfstartupfandfnormalfoperationfasf explainedfbelow. whenfstartingfupfusingfthefinternalfldo,fswitcherfopera - tionfisfinhibitedfuntilfthefldofoutputfreachesf4v.ffduringf thisftime,fthefldofstart-upfisfimplementedfusingfafcurrentf source.ffatflowfv in fitfisfimportantftofnotfapplyfanfexternalf loadftofthefldo,finforderftofallowfthefldofoutputftofreachf thef4vfthresholdfandfallowfswitchingftofbegin.f oncefswitchingfbegins,fldofoperationftransitionsffromf current-sourcef operationf tof voltagef regulation.f fthef minimumfoperatingfv in fisfthenflimitedfbyfthefrds on foffthef internalfldofmosfet.ffthefcurrentfrequiredftofpowerfthef SC461fandfexternalfmosfetfgatesfcausesfafvoltagefdropf fromfthefv in fpinftofthefvldofpin.ffthefvldofpinfmustfstayf abovef4v,fotherwisefthefldofcontrolfwillfrevertfbackftof current-sourcefoperation,fcausingfmorefvoltagefdropfatf thefldofoutput.ffthefrds on foffthefldofmosfetfatflowfv in f isftypicallyf28fohmsfatf25c. design procedure whenfdesigningfafswitchfmodefsupplyfthefinputfvoltagef range,f loadf current,f switchingf frequency,f andf inductorf ripplefcurrentfmustfbefspecifed. thefmaximumfinputfvoltagef(v inmax )fisfthefhighestfspecifedf inputfvoltage.fthefminimumfinputfvoltagef(fv inmin )fisfdeter - minedfbyftheflowestfinputfvoltagefincludingfthefvoltagef dropsfdueftofconnectors,ffuses,fswitches,fandfpcbftraces. 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=f24vf + f10% v out f=f1.8vf + f4% f sw f=f220khz loadf=f10afmaximum frequency selection selectionfoffthefswitchingffrequencyfrequiresfmakingfaf trade-offbetweenfthefsizefandfcostfoffthefexternalfflterf componentsf (inductorf andf outputf capacitor)f andf thef powerfconversionfefciency.ff thefdesiredfswitchingffrequencyfisf220khz.ff afresistor,fr ton fisfusedftofprogramfthefon-timef(indirectlyf settingftheffrequency)fusingftheffollowingfequation. 28 7 ,1 6: ,1 28 7 28 7 ,1 21 721 9 s) 9 qv i 9 9 9 s) 9 qv 7 5 u u ? ? 1 ? u u u tofselectfr ton ,fusefthefmaximumfvaluefforfv in ,fandfforft on f usefthefvaluefassociatedfwithfmaximumfv in . ? ? ? ? ? ? ? ?
SC461 22 applications information (continued) sw inmax out on f v v t u t on = 310 nsec at 26.4v in , 1.8v out , 220khz substitutingfforfr ton fresultsfinftheffollowingfsolution. r ton = 156k?, use r ton = 154k? inductor selection inforderftofdeterminefthefinductance,fthefripplefcurrentf mustffrstfbefdefned.fflowfinductorfvaluesfresultfinfsmallerf sizefbutfcreatefhigherfripplefcurrentfwhichfcanfreducef efciency.ffhigherfinductorfvaluesfwillfreducefthefripplef current/voltagefandfforfafgivenfdcfresistancefarefmoref efcient.ffhowever,flargerfinductanceftranslatesfdirectlyf intoflargerfpackagesfandfhigherfcost.ffcost,fsize,foutputf ripple,fandfefciencyfarefallfusedfinfthefselectionfprocess.f thefripplefcurrentfwillfalsofsetfthefboundaryfforfpower- savefoperation.fffthefswitchingfwillftypicallyfenterfpower- savefmodefwhenfthefloadfcurrentfdecreasesftof1/2foffthef ripplefcurrent.fforfexample,fiffripplefcurrentfisf4afthenf power-savefoperationfwillftypicallyfstartfforfloadsflessfthanf 2a.ffiffripplefcurrentfisfsetfatf40%foffmaximumfloadfcurrent,f thenf power-savef willf startf forf loadsf lessf thanf 20%f off maximumfcurrent. thefinductorfvaluefisftypicallyfselectedftofprovidefafripplef currentfthatfisfbetweenf25%ftof60%foffthefmaximumfloadf current.ffthisfprovidesfanfoptimalftrade-offbetweenfcost,f efciency,fandftransientfperformance. duringfthefdhfon-time,fvoltagefacrossfthefinductorfisf f (v in f-fv out ).fftheffollowingfequationfforfdeterminingfinduc - tancefisfshown.f ripple on out in i t ) v v ( l u infthisfexamplefthefinductorfripplefcurrentfisfsetfapproxi - matelyfequalftof50%foffthefmaximumfloadfcurrent.ffthusf ripplefcurrentftargetfwillfbef50%fxf10aforf5a.f ftoffndfthefminimumfinductancefneeded,fusefthefv in fandf t on fvaluesfthatfcorrespondftofv inmax .f + $ qv / u afslightlyfsmallerfvaluefoff1.5hfisfselected.f notefthatfthefinductorfmustfbefratedfforfthefmaximumfdcf loadfcurrentfplusf1/2foffthefripplefcurrent. thefripplefcurrentfunderfminimumfv in fconditionsfisfalsof checkedfusingftheffollowingfequations. qv qv 9 9 5 s) 7 ,10, 1 28 7 721 21b9,10, 1 u u l t ) v v ( i on out in ripple u $ + qv , 0, 1 5,33/(b9, 1 u capacitor selection thefoutputfcapacitorsfarefchosenfbasedfonfrequiredfesrf andfcapacitance.ffthefmaximumfesrfrequirementfisfcon - trolledfbyfthefoutputfripplefrequirementfandfthefdcftoler - ance.fthefoutputfvoltagefhasfafdcfvaluefthatfisfequalftof thefvalleyfoffthefoutputfripplefplusf1/2foffthefpeak-to-peakf ripple.ffchangefinfthefoutputfripplefvoltagefwillfleadftofaf changefinfdcfvoltagefatfthefoutput.ff thefdesignfgoalfisfforfthefoutputfvoltagefregulationftofbef 4%funderfstaticfconditions.ffthefinternalf600mvfrefer - enceftolerancefisf1%.fallowingf1%ftoleranceffromftheffbf resistorfdivider,fthisfallowsf2%ftolerancefdueftofv out fripple.ff sincefthisf2%ferrorfcomesffromf1/2foffthefripplefvoltage,f thefallowablefripplefisf4%,forf72mvfforfaf1.8vfoutput. thefmaximumfripplefcurrentfoff5afcreatesfafripplefvoltagef acrossfthefesr.ffthefmaximumfesrfvaluefallowedfisfshownf byftheffollowingfequations. $ p 9 , 9 (65 5,33/(0$; 5,33/ ( 0$; esr max = 14.4 m? thef outputf capacitancef isf chosenf tof meetf transientf requirements.fafworst-casefloadfrelease,ffromfmaximumf
SC461 23 applications information (continued) loadf tof nof loadf atf thef exactf momentf whenf inductorf currentfisfatfthefpeak,fdeterminesfthefrequiredfcapaci - tance.ffiffthefloadfreleasefisfinstantaneousf(loadfchangesf fromfmaximumftofzerofinf SC461 24 applications information (continued) anotherfwayftofeliminatefdoubling-pulsingfisftofaddfaf smallf capacitorf acrossf thef upperf feedbackf resistor,f asf shownfinffiguref16.ffthisfcapacitorfshouldfbefleftfunpopu - latedf unlessf itf canf bef confirmedf thatf double-pulsingf exists.ffaddingfthefc top fcapacitorfwillfcouplefmorefripplef intoffbftofhelpfeliminatefthefproblem.fanfoptionalfcon - nectionfonfthefpcbfshouldfbefavailablefforfthisfcapacitor.fff v out to fb pin r 2 r 1 c top figure 16 capacitor coupling to fb pin note:ffthefc top fcapacitorfcanfmoderatelyfafectfthefdcfoutputfvolt - age,freferftofthefsectionfonfv out fvoltagefselection. esrfloopfinstabilityfisfcausedfbyfinsufficientfesr.ffthef detailsf off thisf stabilityf issuef aref discussedf inf thef esrf requirementsfsection.ffthefbestfmethodfforfcheckingfsta - bilityfisftofapplyfafzero-to-fullfloadftransientfandfobservef thefoutputfvoltagefripplefenvelopefforfovershootfandf ringing.fringingfforfmorefthanfonefcyclefafterfthefinitialf stepfisfanfindicationfthatfthefesrfshouldfbefincreased. onefsimplefwayftofsolvefthisfproblemfisftofaddftracefresis - tancefinfthefhighfcurrentfoutputfpath.ffafsidefefectfoff addingftracefresistancefisfdecreasedfloadfregulation.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 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 whenf usingf highf esrf valuef capacitors,f thef feedbackf voltagefrippleflagsfthefphasefnodefvoltagefbyf90fdegreesf andfthefconverterfisfeasilyfstabilized.ffwhenfusingfceramicf outputfcapacitors,fthefesrfvaluefisfnormallyftoofsmallftof meetfthefabovefesrfcriteria.ffasfafresult,ftheffeedbackf voltagefripplefisf180fdegreesffromfthefphasefnodefleadingf tofunstablefoperation.ffinfthisfapplicationfitfisfnecessaryftof addfafsmallfvirtualfesrfnetworkfthatfisfcomposedfofftwof capacitorsfandfonefresistor,fasfshownfbyfr l ,fc l ,fandfc c finf figuref17.fff r 1 r 2 fb pin c c c out l c l r l dcr d x v in + - v l figure 17 virtual esr ramp circuit thefripplefvoltagefatffbfisfafsuperpositionfofftwofvoltagef sources:f thef voltagef acrossf c l f andf thef outputf ripplef voltage.fftheyfarefdefnedfinftheffollowingfequations. & 5 6 '&5 / v '&5 , 9f / / / / u u u 6: / 28 7 i & , 9 u ' ' figuref18fshowsfthefequivalentfcircuitfforfcalculatingfthef magnitudefoffthefripplefcontributionfatftheffbfpinfdueftofc l .f
SC461 25 applications information (continued) r 1 r 2 fb pin c c l c l r l dcr d x v in + - v l figure 18 fb voltage by cl voltage thefmagnitudefofftheffbfripplefcontributionfdueftofc l fisf shownfbyftheffollowingfequation. 1 c s r // r c s r // r vc vfbc c 2 1 c 2 1 l l u u u u u figuref19fshowsfthefequivalentfcircuitfforfcalculatingfthef magnitudefoffthefripplefcontributionfdueftofthefoutputf voltagefripple. r 1 r 2 fb pin c c c out l c l r l dcr v l v out r 1 r 2 v out c c fb pin c out figure 19 fb voltage by output voltage thefmagnitudefofftheffbfripplefcontributionfdueftofoutputf voltagefripplefisfshownfbyftheffollowingfequation. & 28 7 28 7 5 & 6 5 5 9 9 9) % u u ' ' thefpurposefoffthisfnetworkfisftofcouplefthefinductorf currentfripplefinformationfintoftheffeedbackfvoltagefsuchf thatftheffeedbackfvoltagefhasf90fdegreesfphaseflagftofthef switchingfnodefsimilarftofthefcasefoffusingfstandardfhighf esrfcapacitors.ffthisfisfillustratedfinffiguref20. v out lx i l fb contribution by c l fb contribution by output voltage ripple combined fb figure 20 fb voltage in phaser diagram thefmagnitudefofftheffeedbackfripplefvoltage,fwhichfisf dominatedfbyfthefcontributionffromfc lf ,ffisfcontrolledfbyf thefvaluefoffr 1 ,fr 2 fandfc cf .ffiffthefcornerffrequencyfoff(r 1 //f r 2 )fxfc cfff isftoofhigh,fthefripplefmagnitudefatftheffbfpinfwillf befsmaller,fwhichfcanfleadftofdouble-pulsing.ffconversely,f iffthefcornerffrequencyfoff(r 1 //fr 2 )fxfc cfff isftooflow,fthefripplef magnitudefatffbfpinfwillfbefhigher.ffsincefthefSC461ffregu - latesftofthefvalleyfoffthefripplefvoltagefatftheffbfpin,fafhighf ripplefmagnitudefisfundesirablefasfitfsignifcantlyfimpactsf thefoutputfvoltagefregulation.ffasfafresult,fitfisfdesirableftof selectf af cornerf frequencyf forf (r 1 //f r 2 )f xf c cf f tof achievef enough,fbutfnotfexcessive,fripplefmagnitudefandfphasef margin.ffthefcomponentfvaluesfforfr 1 ,fr 2 ,fandfc cf fshouldf befcalculatedfusingftheffollowingfprocedure. selectfc l f(typicalf10nf)fandfr l ftofmatchfwithflfandfdcrf timefconstantfusingftheffollowingfequation. / / & '&5 / 5 u selectfc cff byfusingftheffollowingfequation. vz & i 5 5 & u s u u | thefresistorfvaluesf(r 1 fandfr 2 )finfthefvoltagefdividerfcircuitf setfthefv out fforfthefswitcher.fftheftypicalfvaluefforfc cff isff fromf10pfftof1nf.f dropout performance thefoutputfvoltagefadjustfrangefforfcontinuous-conduc - tionf operationf isf limitedf byf thef fixedf 250nsf (typical)f
SC461 26 applications information (continued) minimumfof-timefoffthefone-shot.ffwhenfworkingfwithf lowfinputfvoltages,fthefduty-factorflimitfmustfbefcalcu - latedfusingfworst-casefvaluesfforfonfandfofftimes.ff thef duty-factorf limitationf isf shownf byf thef followingf equation. ) 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 - tionsfitftripsfwhenftheffeedbackfpinfisf600mv,f + f 1%. thefon-timefpulseffromfthefSC461finfthefdesignfexamplef isfcalculatedftofgivefafpseudo-fxedffrequencyfoff220khz.f someffrequencyfvariationfwithflinefandfloadfisfexpected.ff thisfvariationfchangesfthefoutputfripplefvoltage.ffbecausef adaptivefon-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%ffeedbackfresistorsfcontributesfupftof1%f error.fifftighterfdcfaccuracyfisfrequired,f0.1%fresistorsf shouldfbefused.f thefoutputfinductorfvaluefmayfchangefwithfcurrent.ffthisf willfchangefthefoutputfripplefandfthereforefwillfhavefaf minorfefectfonfthefdcfoutputfvoltage.ffthefoutputfesrf alsofafectsfthefoutputfripplefandfthusfhasfafminorfefectf onfthefdcfoutputfvoltage.ff switching frequency variations thefswitchingffrequencyfwillfvaryfdependingfonflinefandf loadfconditions.fftheflinefvariationsfarefafresultfofffxedf propagationfdelaysfinfthefon-timefone-shot,fasfwellfasf unavoidablefdelaysfinfthefexternalfmosfetfswitching.ffasf v in fincreases,ftheseffactorsfmakefthefactualfdhfon-timef slightlyflongerfthanfthefidealfon-time.ffthefnetfefectfisf thatffrequencyftendsftoffallsfslightlyfwithfincreasingfinputf voltage.ff thefswitchingffrequencyfalsofvariesfwithfloadfcurrentfasfaf resultfoffthefpowerflossesfinfthefmosfetsfandfthefinduc - tor.ffforfafconventionalfpwmfconstant-frequencyfcon - verter,f asf loadf increasesf thef dutyf cyclef alsof increasesf slightlyftofcompensatefforfirfandfswitchingflossesfinfthef mosfetsfandfinductor.ffanfadaptivefon-timefconverterf mustfalsofcompensatefforfthefsameflossesfbyfincreasingf thef effectivef dutyf cyclef (moref timef isf spentf drawingf energyffromfv in fasflossesfincrease).ffbecausefthefon-timefisf essentiallyfconstantfforfafgivenfv out /v in fcombination,ftof ofsetftheflossesfthefof-timefwillfreducefslightlyfasfloadf increases.fthef netf effectf isf thatf switchingf frequencyf increasesfslightlyfwithfincreasingfload.fff pcb layout guidelines afswitch-modefconverterfrequiresfgoodfpcbflayoutfwhichf isfessentialftofachievingfhighfperformance.fftheffollowingf guidelinesfwillfprovidefanfoptimumfpcbflayout. thefdeviceflayoutfrecommendationsfconsistfofffourfparts. groundingfforfpgndfandfagnd powerfcomponentsf low-noisefanalogfcircuits bypassfcapacitors grounding for pgnd and agnd afgroundfplaneflayerfforfpgndfisfrecommendedf tofminimizefthefefectsfoffswitchingfnoise,fresis - tiveflosses,fandftofmaximizefheatfremovalffromf thefpowerfcomponents.f ? ? ? ? ?
SC461 27 applications information (continued) afseparatefgroundfplaneforfislandfshouldfbefusedf forfagndfandfallfassociatedfcomponents.fthef agndfislandfshouldfavoidfoverlappingfswitchingf signalsfonfotherflayersf(dh/dl/bst/lx). connectfpgndfandfagndftogetherfwithfafzerof ohmfresistorforfcopperftrace.f makefthefconnec - tionfnearfthefagndfandfpgndfpinsfoffthefic.f f f power components usefshort,fwideftracesfbetweenftheffollowingf powerfcomponents. inputfcapacitorsfandfhigh-sidefmosfets high-sidef andf low-sidef mosfetsf andf in - ductorf(lxfconnection).ffusefwidefcopperf tracesftofprovidefhighfcurrentfcarryingfca - pacityfandfforfheatfdissipation.ff inductorfandfoutputfcapacitors. allfpgndfconnectionsffthefinputfcapaci - tors,flow-sidefmosfets,foutputfcapacitors,f andfthefpgndfpinfoffthefSC461.ff anfinnerflayerfgroundfplanefisfrecommended.ff eachfpowerfcomponentfrequiresfafshort,flowf impedancefconnectionftofthefpgndffplane. placefviasftofthefpgndfplanefdirectlyfnearfthef componentfpins. usefshortfwideftracesfforfthefpinfconnectionsf fromfthefSC461f(lx,fdh,fdlfandfbst).fdofnotf routeftheseftracesfnearfthefsensitiveflow-noisef analogfsignalsf(fb,fss,fton,fv out ). avoidfoverlappingfoffthefdlftracefwithflx/dh/ bst.ffthisfhelpsfreduceftransientfpeaksfonfthef gatefofftheflow-sidefmosfetfduringfthefturn-onf offthefhigh-sidefmosfet. low-noise analog circuits low-noisefanalogfcircuitsfarefsensitivefcircuitsfthatfaref referencedftofagnd.ffdueftoftheirfhighfimpedancefandf sensitivityftofnoise,fitfisfimportantfthatfthesefcircuitsfbef locatedfasffarfasfpossibleffromfthefswitchingfsignals.f usefafplaneforfsolidfareafforfagnd.ffplacefallf componentsf connectedf tof agndf abovef thisf area.ff usefshortfdirectftracesfforfthefagndfcon - nectionsftofallfcomponents.f placefviasftofthefagndfplanefdirectlyfnearf thefcomponentfpins. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? properfroutingfoffthefv out fsenseftracefisfessentialf sincefitffeedsfintoftheffbfresistorfdivider.ffnoisef onftheffbfwaveformfwillfcausefinstabilityfandf multiplefpulsing.f connectf thef v out f sensef tracef directlyf tof thef outputf capacitorf orf af ceramicf bypassf capacitor.f routefthisftracefoverftofthefvoutfpin,fcare - fullyf avoidingf allf switchingf signalsf andf powerfcomponents.ff routefthisftracefinfafquietflayerfiffpossible.ff routef thisf tracef awayf fromf thef switch - ingf tracesf andf components,f evenf iff thef tracefisflonger.favoidfshorterftracefroutingf throughfthefpowerfswitchingfarea. iffafbypassfcapacitorfisfusedfatftheficfsidef offthef v out fsensef trace,fitfshouldfbefplacedf nearftheffbfresistorfdivider. allfcomponentsfconnectedftoftheffbfpinfmustf beflocatedfnearfthefpin.fftheffbftracesfshouldfbef keptfsmallfandfnotfroutedfnearfanyfnoisyfswitch - ingfconnectionsforfpowerfcomponents. placefthefssfcapacitorfnearfthefssfpinfwithfaf shortfdirectfconnectionftofthefagndfplane. placefthefr lim fresistorfnearfthefic.ffforfanfaccuratef i lim fcurrentfsensefconnection,froutefthefr lim ftracef directlyftofthefdrainfofftheflow-sidefmosfetf(lx).ff usefanfinnerfroutingflayerfiffneeded.fff placefthefr ton fresistorfnearftheftonfpin.ffroutef r tonf toftheftonfpinfandftofagndfusingfshortf tracesfandfavoidfallfswitchingfsignals. ff bypass capacitors thefdevicefrequiresfbypassfcapacitorsfforftheffollowingf pins.ff vddaf pinf withf respectf tof agnd.f fthisf 0.1ff minimumfcapacitorfmustfbefplacedfandfroutedf closeftoftheficfpins,fonfthefsameflayerfasfthefic.ff thisfcapacitorfalsoffunctionsfasfbypassfforfthef ldofoutput,fsincefthefvddafandfvldofpinsfaref adjacent. vddpfwithfrespectftofpgnd.ffthisf1ffminimumf capacitorfmustfbefplacedfandfroutedfcloseftof theficfpinsfandfonfthefsameflayerfasfthefic. ? ? ? ? ? ? ? ? ? ? ? ?
SC461 28 applications information (continued) bstfpinfwithfrespectftoflx.fthisf0.1ffminimumf capacitorfmustfbefplacedfnearfthefic,fonfeitherf sidefoffthefpcb.ffusefshortftracesfforfthefroutingf betweenfthefcapacitorfandfthefic. vinf pinf withf respectf tof agnd.f thisf 0.1ff minimumfcapacitorfmustfbefplacedfandfroutedf closef tof thef icf pins.f fthisf capacitorf providesf noisefflteringfforfthefinputftofthefinternalfldo. ? ?
SC461 29 outline drawing mlpq-ut20 3x3 e 1 2 n pin 1 indicator ( laser mark ) a 1 a a 2 c seating plane e / 2 d / 2 b . 006 . 008 . 010 0 . 15 0 . 20 0 . 25 lxn bxn d 1 e 1 coplanarity applies to the exposed pad as well as the terminals . 2 . controlling dimensions are in millimeters ( angles in degrees ). 1 . inches dimensions nom e bbb aaa a 1 a 2 d 1 e 1 dim n l e min d a millimeters max min max nom e b d . 114 . 118 3 . 00 . 122 2 . 90 3 . 10 notes : bbb c a b aaa c . 003 . 061 20 . 067 . 000 . 020 - - (. 006 ) 0 . 08 20 . 071 1 . 55 . 024 . 002 0 . 00 0 . 50 1 . 80 1 . 70 0 . 05 0 . 60 ( 0 . 1524 ) - - . 004 0 . 10 1 . 55 2 . 90 1 . 70 1 . 80 3 . 00 3 . 10 0 . 40 bsc . 016 bsc 0 . 30 . 012 . 020 . 016 0 . 40 0 . 50 . 122 . 118 . 114 . 071 . 067 . 061 a dap is 1 . 90 x 190 mm . 3 .
SC461 30 land pattern mlpq-ut20 3x3 this land pattern is for reference purposes only . consult your manufacturing group to ensure your company ' s manufacturing guidelines are met . notes : 1 . c z r y x g p h . 146 . 004 . 008 . 031 . 083 . 067 . 016 3 . 70 0 . 20 0 . 80 0 . 10 1 . 70 0 . 40 2 . 10 dim ( 2 . 90 ) millimeters dimensions (. 114 ) inches controlling dimensions are in millimeters ( angles in degrees ). 2 . k . 067 1 . 70 thermal vias in the land pattern of the exposed pad shall be connected to a system ground plane . functional performance of the device . failure to do so may compromise the thermal and / or 3 . h k r ( c ) x p y g z
SC461 31 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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