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?f2010fsemtechfcorporation 1 sc418 ecospeed tm dc-dc buck controller with programmable ldo features powerfsystem inputfvoltageff3vftof28v integratedfbootstrapfswitch programmablefldofoutputff200ma 1%freferenceftolerancef f -40ftof+85fc selectablefinternal/externalfbiasfpowerfsupply ecospeed tm farchitecturefwithfpseudo-fxedffre - quencyfadaptivefon-timefcontrol logicfinput/outputfcontrolf independentfcontrolfenfforffldofandfswitcherf programmablefv in fuvlofthreshold powerfgoodfoutput selectablefp owerfsavefmode programmablef ultrasonicfpowerfsavefmode protections over-voltage/under-voltage tcfcompensatedfr ds(on)f fsensedfcurrentflimit thermalfshutdown smartfpowerfsave outputfcapacitorftypes highfesrffsp,fposcap,fosconf ceramicfcapacitors package ff3fxf3mm,f20-pinfmlpqf lead-freefandfhalogenffree rohsfandfweeefcompliant applications ofcefautomationfandfcomputing networkingfandftelecommunicationfequipment point-of-loadfpowerfsuppliesfandfmodulefreplacement ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? description thefsc418fisfafsynchronousfecospeed tmf buckfregulatorf whichfincorporatesfsemtechsfadvanced,fpatentedfadap - tivef on-timef controlf architecturef tof providef excellentf light-loadfefciencyfandffastftransientfresponse.ffitffea - turesfanfintegratedfbootstrapfswitchfandfprogrammablef ldofinfaf3fxf3mmfpackage.ffthefdevicefisfhighlyfefcientf andfusesfminimalfpcbfarea.f thefsc418fsupportsfusingfstandardfcapacitorftypesfsuchf asfelectrolyticforfspecialfpolymer,finfadditionftofceramic,fatf switchingffrequenciesfupftof1mhz.fthefprogrammablef frequency,fsynchronousfoperation,fandfprogrammablef power-savefprovidefhighfefciencyfoperationfoverfafwidef loadfrange.f additionalffeaturesfincludefcycle-by-cyclefcurrentflimit,f soft-start,funderfandfover-voltagefprotection,fprogram - mablefover-currentfprotection,fsoftfshutdown,fselectablef power-savefmodes,fandfprogrammablefultrasonicfpower- save.ffthefdevicefalsofprovidesfseparatefenablefinputsfforf thefpwmfcontrollerfandfldofasfwellfasfafpowerfgoodf outputfforfthefpwmfcontroller. thefinputfvoltagefcanfrangeffromf3vftof28v.ffthefwidef inputfvoltagefrange,fprogrammableffrequency,fandfpro - grammablefldofmakefthefdevicefextremelyffexiblefandf easyftofusefinfafbroadfrangefoffapplications.ffsupportfisf providedfforfsinglefcellforfmulti-cellfbatteryfsystemsfinf additionftoftraditionalfdcfpowerfsupplyfapplications. julyf27,f2010 power management typical application circuit vdda pgood en ton enl 1 f vext or vldo pgnd agnd vldo enable ldo rton sc 418 vddp vldo fbl 0 . 1 f 1 f enable pgood psv psv lx ilim rilim + vout cout l 1 vout fb cin dh bst dl vin vin vext or vldo
sc418 2 pin confguration ordering information marking information 418 yyww xxxx agnd pad 1 2 3 4 fb vout fbl vdda 5 6 7 8 9 10 d h pgnd psv dl vddp pgood 15 14 13 12 11 16 17 18 19 20 nc l x b s t v l d o v i n 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 sc418ultrt (1)(2) mlpq-ut20 SC418EVB evaluationfboard
sc418 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 +30 lxftofpgndf(v)f(transientff100ns)f f. . . . . . . . . . . -2ftof+30 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,ffbl,filimftofagndf(v) f. . . . . . . -0.3ftof+(vddaf+f0.3) pgood,fpsv,fftonftofagndf(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. . . . . . . . . . f 0.3f tof +6 vddaftofvddpf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3ftof+0.3 agndftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +0.3f recommended operating conditions inputfvoltagef(v)f f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . f 3.0f tof 28 vddaftofagnd,fvddpftofpgndf(v) f. . . . . . . . . . . f 4.5f tof 5.5 power-save/ultrasonicfpower-savefsupported vddaftofagnd,fvddpftofpgnd (2)f (v) f. . . . . . . . . . f 3.0f tof 4.5 power-save/ultrasonicfpower-savefnotfsupported voutftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . 0.5f tof 5.5 thermal information storageftemperaturef(c) f. . . . . . . . . . . . . . . . . . . . -60ftof+150 maximumfjunctionftemperaturef(c) f. . . . . . . . . . . . . . . f 150 operatingfjunctionftemperaturef(c) f. . . . . . . . -40ftof+125 thermalfresistance,fjunctionftofambient (2) f(c/w) f. . . . .f 50 peakfirfrefowftemperaturef(c) f. . . . . . . . . . . . . . . . . . . . f 260 exceedingfthefabovefspecifcationsfmayfresultfinfpermanentfdamageftofthefdeviceforfdevicefmalfunction.ffoperationfoutsidefoffthefparametersf specifedfinfthefelectricalfcharacteristicsfsectionfisfnotfrecommended. notes: (1)f thisfdevicefisfesdfsensitive.fusefoffstandardfesdfhandlingfprecautionsfisfrequired.f (2)ffffcalculatedffromfpackagefinfstillfair,fmountedftof3fxf4.5f(in),f4flayerffr4fpcbfwithfthermalfviasfunderfthefexposedfpadfperfjesd51fstandards. unlessfspecifed:fv in f=12v,fvddaf=fvddpf=f5v,ft a f=f+25cfforftyp,f-40ftof+85fcfforfminfandfmax,fft j f sc418 4 electrical characteristics (continued) parameter conditions min typ max units input supplies (continued) vddaf+fvddpfsupplyfcurrent enlf,fenf=f0v 3 7 a fenf=f5v,fr psv f=f115k?,fv fb f>f500mv (2) 1.5 ma enf=f5v,fpsvf=ffopenf(foat),fv fb f>f500mv (2) 0.4 operatingff sw f=f250khz,fpsvf=fvdda,fnofload (2) 10 fbfcomparatorfthreshold staticfvinfandfload,f0ftof+85fc 0.496 0.500 0.504 v staticfvinfandfload,f-40ftof+85fc 0.495 0.505 v frequencyfrange continuousfmodefoperation 1000 khz timing on-time forcedfcontinuousfmodefoperation,f v in f=f15v,fv out f=f5v,ffr ton f=f300k?,fvddaf=f5v 2385 2650 2915 ns vddaffinternalf500mvfreference +20 % lowerflimit,fv fb f sc418 5 electrical characteristics (continued) parameter conditions min typ max units powerfgoodfonfresistance 10 ? fault protection i lim fsourcefcurrent 9 10 11 a i lim fsourcefcurrentftemperaturefcoefcientf (2) 0.41 %/c i limf comparatorfofset withfrespectftofagnd -8 0 +8 mv outputfunder-voltagefthreshold v fb fwithfrespectftofinternalf500mvfreference,f f 8fconsecutivefcycles -25 % smartfpower-savefprotectionfthreshold (2) v fb fwithfrespectftofinternalf500mvfreference +10 % over-voltagefprotectionfthreshold v fb fwithfrespectftofinternalf500mvfreference +20 % over-voltageffaultfdelay (2) f 5 s over-temperaturefshutdown (2) 10 cfhysteresis 150 c logic inputs/outputs logicfinputfhighfvoltagef-fen,fenlf (5) 1.0 v logicfinputfhighfvoltagef-fpsv forcedfcontinuousfmodefoperation;f psvfwithfrespectftofvdda -0.4 v logicfinputflowfvoltagef-fen,fenl withfrespectftofagnd 0.4 v enfinputfbiasfcurrent enf=fvddaforfagnd -10 +10 a enlfinputfbiasfcurrent v in f=f28v -1 11 +18 a fb,ffblfinputfbiasfcurrent fb,ffblf=fvddaforfagnd -1 +1 a psvfinputfbiasfcurrent psvf=fvdda 1 a linear dropout regulator fblfaccuracy vldofloadf=f10ma 0.735 0.75 0.765 v currentflimit start-upfandffoldback;f v ldo f < f90%foffprogrammedfvalue 115 ma operatingf 135 200 vldoftofvoutfswitch-overfthreshold f(6) -140 +140 mv vldoftofvoutfnon-switch-overfthreshold f(6) -450 +450 mv vldoftofvoutfswitch-overfresistance v ldo f=fv out f=f5v v ldo f=fv out f=f3.3v f(2) 2.0 2.2 ? ldofdropfoutfvoltage (7) v in ftofv vldo ,fv vldo f=f5v,fvldofloadf=f100ma 1.2 v
sc418 6 electrical characteristics (continued) parameter conditions min typ max units high-side driver (dh, bst, lx) peakfcurrent (2) vddpf=f5v 2.0 a onfresistance r dh_pull-up, fflxfff0.5v,fvddpf=f5v 1.0 2.0 ? r dh_pull-down,ff vddpf=f5v 0.6 1.2 ? riseftime (2) c dh-lx f=f3nf,fvddpf=f5v 22 ns fallftime (2) c dh-lx f=f3nf,fvddpf=f5v 12 ns propagationfdelay (2) fromffbfcomparatorfinputftofdh 30 45 60 ns shoot-thrufprotectionfdelay (2) 10 20 30 ns bootstrapfswitchfresistance 10 ? low-side driver (dl, vddp, pgnd) peakfcurrent (2) vddpf=f5v 4.0 a onfresistance r dl_pull-up, ffvddpf=f5v 1.3 2.1 ? r dl_pull-down,ff vddpf=f5v 0.50 0.86 ? riseftime (2) c dl f=f3nf,fvddpf=f5v 7 ns fallftime (2) c dl f=f3nf,fvddpf=f5v 3.5 ns notes: (1)f vinfuvlofisfprogrammablefusingfafresistorfdividerffromfvinftofenlftofagnd.fthefenlfvoltagefisfcomparedftofanfinternalfreference. (2)f guaranteedfbyfdesign. (3)f forfvddaflessfthanf4.5v,fthefon-timefmayfbeflimitedfbyfthefvddafsupplyfvoltagefandfbyfv in .ffseeftheftonflimitationsfandfvddafsupplyf voltagefsectionfinfthefapplicationsfinformation. (4)f thefzerofcrossfdetectorfandfthefpower-savefandfultrasonicfpower-saveffeaturesfarefnotfsupportedfforfvddaflessfthanf4.5v. f (5)f thefenlfpinfwillfenablefthefldofwithf1.0vftypical.ffthefenlfpinsfvinfulvoffunctionfwillfdisablefthefswitcherfunlessfthefenlfpinfexceedsfthef v in fuvlofthreshold. (6)f thefswitch-overfthresholdfisfthefmaximumfvoltagefdiferentialfbetweenfthefvldofandfvoutfpinsfwhichfensuresfthatfvldofwillfinternallyf switch-overftofvout.fthefnon-switch-overfthresholdfisfthefminimumfvoltagefdiferentialfbetweenfthefvldofandfvoutfpinsfwhichfensuresf thatfvldofwillfnotfswitch-overftofvout. (7)f thefldofdropfoutfvoltagefisfthefvoltagefatfwhichfthefldofoutputfdropsf2%fbelowfthefnominalfregulationfpoint.
sc418 7 rilim 100 nf rpsv ( 2 ) 100 n f sc 418 t o n vout fbl dl vdda pgood e n l b s t psv e n vddp a g n d fb d h v l d o n / c l x v i n i l i m pgnd q 1 en pgood enable ldo 5 v vin vin q 2 rfb 1 l 1 rfb 2 10 nf 5 v vout + rton rldo 2 rldo 1 1 f vldo 1 2 3 4 5 11 12 13 14 15 6 7 8 9 10 20 19 18 17 16 cout 1 + cout 2 1 f 154 k ? 12 v to 1 . 05 v @ 10 a , 250 khz cff pad 11 k ? 10 k ? 100 pf 56 . 2 k ? cin 1 cin 2 100 n f 10 k ? 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 www . panasonic . com cout 1 , cout 2 220 f / 15 m ? / 6 . 3 v panasonic eefue 0 j 221 www . vishay . com l 1 ( option 1 ) 0 . 88 h / 2 . 3 m ? nec - tokin mpc 1040 lr 88 c key components www . irf . com q 1 irf 7821 i . r . irf 7821 www . irf . com q 2 irf 7832 i . r . irf 7832 10 k ? note 2 - rpsv : use 115 k ? for ultrasonic operation remove rpsv for power - save operation connect psv pin to vdda for forced continuous mode operation note 1 - 5 v : connect vdda / vddp to external 3 . 3 v or 5 v supply for external bias connect vdda / vddp to vldo for self - biased operation l 1 ( option 2 ) 1 . 0 h / 2 . 3 m ? vishay ihlp 4040 dzer 1 rom 11 www . nec - tokin . com detailed application circuit
sc418 8 efciency vs. load forced continuous mode 50 60 70 80 90 100 0 1 2 3 4 5 6 7 8 9 10 e f f i c i e n c y ( % ) i out ( a ) 85 % internally biased at v ldo = 5 v , v in = 12 v , v out = 1 . 050 v characteristicsfinfthisfsectionfarefbasedfonfusingfthefdetailedfapplicationfcircuit. efciency vs. load ultrasonic power-save mode 20 40 60 80 100 0 . 01 0 . 1 1 10 i out ( a ) e f f i c i e n c y ( % ) internally biased at v ldo = 5 v , v in = 12 v , v out = 1 . 050 v , r psv = 115 k ? v out vs. load ultrasonic power-save mode 0 . 950 0 . 975 1 . 000 1 . 025 1 . 050 1 . 075 1 . 100 0 1 2 3 4 5 6 7 8 9 10 i out ( a ) v o u t ( v ) internally biased at v ldo = 5 v , v in = 12 v , v out = 1 . 050 v , r psv = 115 k ? efciency vs. load power-save mode 60 70 80 90 100 0 . 01 0 . 1 1 10 i out ( a ) e f f i c i e n c y ( % ) externally biased at vdda = vddp = 5 v , v in = 12 v , v out = 1 . 050 v v out vs. load power-save mode 0 . 950 0 . 975 1 . 000 1 . 025 1 . 050 1 . 075 1 . 100 0 1 2 3 4 5 6 7 8 9 10 i out ( a ) v o u t ( v ) externally biased at vdda = vddp = 5 v , v in = 12 v , v out = 1 . 050 v v out vs. load forced continuous mode 0 . 950 0 . 975 1 . 000 1 . 025 1 . 050 1 . 075 1 . 100 0 1 2 3 4 5 6 7 8 9 10 v o u t ( v ) i out ( a ) internally biased at v ldo = 5 v , v in = 12 v , v out = 1 . 050 v typical characteristics
sc418 9 characteristicsfinfthisfsectionfarefbasedfonfusingfthefdetailedfapplicationfcircuit. power-save mode no load time (2s/div) v out f f (50mv/div) lx f (10v/div) iout f(5a/div) fpgood f (5v/div) externallyfbiasedfatfvdda/vddpf=f5v,fv in f=f12v,ffv out f=f1.050v ultrasonic powersave mode no load time (5s/div) v outf f (50mv/div) dh f (10v/div) lxf f (10v/div) dlf f (5v/div) forced continuous mode no load time (2s/div) v out f (50mv/div) dh (10v/div) lxf (10v/div) dl f f(5v/div) internallyfbiasedfatfv ldo f=f5v,fv in f=f12v,ffv out f=f1.050v,fi out f=f0a enabled loaded output power good true v outf f (500mv/div) lx f (10v/div) pgoodf f (5v/div) time (400s/div) transient response load rising time (40s/div) externallyfbiasedfatfvdda/vddpf=f5v,fv in f=f12v,fv out f=f1.050v,fi outf =f0aftof10a v out f f (50mv/div) lx f (10v/div) iout f(5a/div) fpgood f (5v/div) transient response load falling time (40s/div) v out f f (500mv/div) lx f (5a/div) pgoodf f (5v/div) iout f f(5a/div) typical characteristics (continued) internallyfbiasedfatfv ldo f=f5v,fv in f=f12v,ffv out f=f1.050v,fi out f=f0a internallyfbiasedfatfv ldo f=f5v,fv in f=f12v,ffv out f=f1.050v,fr psv f=f115kf?f externallyfbiasedfatfvdda/vddpf=f5v,fv in f=f12v,fv out f=f1.050v,fi outf =f10aftof0a
sc418 10 typical characteristics (continued) characteristicsfinfthisfsectionfarefbasedfonfusingfthefdetailedfapplicationfcircuit. frequency vs. load power-save mode 200 220 240 260 280 300 320 0 1 2 3 4 5 6 7 8 9 10 i out ( a ) f r e q u e n c y ( k h z ) internally biased at v ldo = 5 v , v in = 12 v , v out = 1 . 050 v v ripple vs. load forced continuous mode 0 . 00 0 . 05 0 . 10 0 . 15 0 . 20 0 1 2 3 4 5 6 7 8 9 10 i out ( a ) vout p - p internally biased at v ldo = 5 v , v in = 12 v , v out = 1 . 050 v v r i p p l e ( v p - p ) output over-current response normal operation time (100s/div) voutf f (500mv/div) lx f (5a/div) pgoodf f (5v/div) v in f=f12v,fv outf =f1.05v,ffv ldo f=f5vf,fi out frampedftoftripfpoint iout f f(5a/div) frequency vs. load ultrasonic power-save mode 0 50 100 150 200 250 300 0 1 2 3 4 5 6 7 8 9 10 i out ( a ) f r e q u e n c y ( k h z ) internally biased at v ldo = 5 v , v in = 12 v , v out = 1 . 050 v , r psv = 115 k ? externally biased at vdda / vddp = 5 v , v in = 12 v , v out = 1 . 050 v 0 50 100 150 200 250 300 0 1 2 3 4 5 6 7 8 9 10 i out ( a ) f r e q u e n c y ( k h z ) frequency vs. load forced continuous mode
sc418 11 pin descriptions pin # pin name pin function 1 fb feedbackfinputfforfswitchingfregulatorffconnectftofanfexternalfresistorfdividerffromfoutputffusedftof programfthefoutputfvoltage. 2 fbl feedbackfinputfforfthefldoffconnectftofanfexternalfresistorfdividerffromfvldoftofagndffusedftofpro - gramfthefldofoutput. 3 vdda supplyfinputfforfinternalfanalogfcircuitsfffconnectftofexternalf3.3vforf5vfsupply,forfconfgurefthefldofforf 3.3vforf5vfandfconnectftofvldoffalsofthefsensefinputfforfvddafuvlo. 4 vout switcherfoutputfvoltagefsensefpinffalsofthefinputftofthefinternalfswitch-overfmosfetfbetweenfvoutfandf vldo.ffthefvoltagefatfthisfpinfmustfbeflessfthanforfequalftofthefvoltagefatfthefvddafpin. 5 nc nofconnection 6 vin inputfsupplyfvoltageffconnectftofthefsamefsupplyfusedfforfthefhigh-sidefmosfet.ffconnectfaf100nffca - pacitorffromfthisfpinftofagnd. 7 vldo ldofoutputffthefvoltagefatfthisfpinfmustfbeflessfthanforfequalftofthefvoltagefatfthefvddafpin. 8 bst bootstrapfpinffconnectfaf100nffminimumfcapacitorffromfbstftoflxftofdevelopftheffoatingfvoltagefforfthef high-sidefgatefdrive. 9 dh high-sidefgatefdrivefoutput 10 lx switchingf(phase)fnode 11 pgnd powerfground 12 dl low-sidefgatefdrivefoutput 13 vddp supplyfinputfforfthefdhfandfdlfgatefdrivesffconnectftofthefsamef3.3vforf5vfsupplyfusedfforfvdda. 14 psv power-savefprogrammingfinputffconnectfafresistorftofagndftofsetfafminimumf(ultrasonic)fpower-savef frequency f ffoatfpinftofselectfpower-savefwithfnofminimumffrequencyffpullfupftofvddaftofdisablef power-savefandfselectfforcedfcontinuousfmode. 15 pgood open-drainfpowerfgoodfindicatorffhighfimpedancefindicatesfthefswitchingfregulatorfoutputfisfgood.fanf e xternalfpull-upfresistorfisfrequired. 16 ilim currentflimitfsensefpinffusedftofprogramfthefcurrentflimitfbyfconnectingfafresistorffromfilimftoflx. 17 en enablefinputfforfswitchingfregulatorffflogicflowfdisablesfthefswitchingfregulatorffflogicfhighfenablesfthef switchingfregulator. 18 agnd analogfground 19 ton onftimefprogrammingfinputffsetfthefon-timefbyfconnectingfthroughfafresistorftofagnd. 20 enl enablefinputfforfthefldofandfvinfuvlofinputfforfthefswitchingfregulatorffconnectfenlftofagndftofdisablef thefldofffdriveftoflogicfhighf(>3v)ftofenablefthefldofandfinhibitfvinfuvloffconnectftofresistorfdividerf fromfvinftofagndftofprogramfthefv in fuvlofthreshold. pad agnd analogfground
sc418 12 block diagram 8 vdda uvlo soft start fb agnd on - time generator control & status pgood gate drive control vin ton vout zero cross detector current limit ilim enl fbl vldo switchover mux a y b ldo vldo bst fb comparator vdda lx en vldo switchover comparator dl 6 16 20 2 7 4 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 vin ulvo vin ulvo
sc418 13 synchronous buck converter thefsc418fisfafstepfdownfsynchronousfdc-dcfbuckfcon - trollerfwithfafprogrammablefldo.ffitfprovidesfhighfef - ciencyf operationf inf af spacef savingf 3x3f (mm)f 20-pinf package.ffthefprogrammablefoperatingffrequencyfrangef off200khzftof1mhzfenablesfoptimizingfthefconfgurationf forfpcbfareafandfefciency. thefcontrollerfusesfafpseudo-fixedffrequencyfadaptivef on-timefcontrol.ffthisfallowsffastftransientfresponsefwhichf permitsfthefusefoffsmallerfoutputfcapacitors. infadditionftoftheffollowingfinformation,fthefuserfcanf clickfonfthefapplicableflinkftofgoftofthefsc418fonline f c-simfdesignfandfsimulationftoo l , fwhichfwillfleadfthef userfthroughfthefdesignfprocess. input voltage requirements thefsc418frequiresfthreefinputfsuppliesfforfnormalfopera - tion:ffv in ,fvdda,fandfvddp.ffv in foperatesfoverfthefwidef rangefoff3vftof28v.ffvddafandfvddpfrequirefaf3.3vforf5vf supplyfwhichfcanfbeffromfanfexternalfsourceforffromfthef internalfldofconfguredftof3.3vforf5v.ffvddafandfvddpf shouldfderiveffromfthefsamefsourcefvoltage.ffpower-savef operationfandfultrasonicfpower-savefarefnotfsupportedf forfvdda/vddpfbelowf4.5v. psuedo-fxed frequency adaptive on-time control thefpwmfcontrolfmethodfusedfbyfthefsc418fisfpseudo- f fxedffrequency,fadaptivefon-time,fasfshownfinffiguref1.f thefripplefvoltagefgeneratedfatfthefoutputfcapacitorfesrf isfusedfasfafpwmframpfsignal.fthisfripplefisfusedftoftriggerf 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.fthefpulsefperiodfisfdeterminedfbyfv outff andf v in .fthef periodf isf proportionalf tof outputf voltagef andf inverselyfproportionalftofinputfvoltage.ffwithfthisfadaptivef on-timefconfguration,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. one-shot timer and operating frequency one-shotftimerfoperationfisfshownfinffiguref2.ftheffbf comparatorfoutputfgoesfhighfwhenfv fb fisflessfthanfthef internalf500mvfreference.fthisffeedsfintofthefdhfgatef drivefandfturnsfonfthefhigh-sidefmosfet,fandfalsofstartsf thefone-shotftimer.ffthefone-shotftimerfusesfanfinternalf ? ? ? ? ? applications information
sc418 14 comparatorfandfafcapacitor.ffonefcomparatorfinputfisfcon - nectedf tof v out ,f thef otherf inputf isf connectedf tof thef capacitor.ffwhenfthefon-timefbegins,fthefinternalfcapaci - torfchargesffromfzerofvoltsfthroughfafcurrentfwhichfisf proportionalftofv in .fwhenfthefcapacitorfvoltagefreachesf v out ,fthefon-timefisfcompletedfandfthefhigh-sidefmosfetf turnsfof.fff gate drives fb comparator one - shot timer on - time = k x r ton x ( v out / v in ) v out v in fb 500 mv 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 thefsc418fusesfanfexternalfresistorftofsetfthefon-timefwhichf indirectlyfsetsftheffrequency.ffthefon-timefcanfbefpro - grammedftofprovidefanfoperatingffrequencyffromf200khzf tof1mhzfusingfafresistorfbetweenftheftonfpinfandfground.f thefresistorfvaluefis selected by the following equation. out in on ton v pf 25 v ) ns 10 t ( r u u thefmaximumfr ton fvaluefallowedfisfshownfbyftheffollow - ingfequation. a 15 v r min _ in max _ ton p immediatelyfafterfthefon-time,fthefdlfoutputfdrivesfhighf tofenergizeftheflow-sidefmosfet.ffdlfhasfafminimumfhighf timefoff~250ns,fafterfwhichfdlfcontinuesftofstayfhighfuntilf onefofftheffollowingfoccurs:f vfbffallsfbelowfthef500mvfreference ? thefzerofcrossfdetectorftripsfiffpower-savefisf active ton limitations and vdda supply voltage forfvddafbelowf4.5v,ftheftonfaccuracyfmayfbeflimitedf byfv in .ffthefpreviousfrtonfequationfisfaccuratefiffv in fsatis - fesfthefbelowfrelationfoverfthefentirefv 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 mustfbefmodiffedfasffollows.f out on ton v 25pf 10 1.6v) (v5v 10ns) (t r u u u notefthatfwhenf v in fisfgreaterfthanff((vddaf-f1.6v)fxf10),fthef actualfon-timefisffxedfandfdoesfnotfvaryfwithfv in .ffwhenf operatingfinfthisfcondition,fthefswitchingffrequencyfwillf varyfinverselyfwithfv in fratherfthanfapproximatingffxedf frequency. whenfaflargefcapacitorfisfplacedfinfparallelfwithfr1f(ctop)f voutfisfshownfbyftheffollowingfequation. 2 top 1 2 1 2 2 top 1 ripple 2 1 out c r r r r 1 ) c r ( 1 2 v r r 1 5 . 0 v ? ? 1 ? z u z u ? 1 ? ? ? 1 ? u f thefswitcherfoutputfvoltagefcanfbefprogrammedfhigherf thanf5vfwithfcarefulfdesign.ffinfthisfcasefthefvoutfpinf cannotfconnectfdirectlyftofthefswitcherfoutputfdueftofitsf thef maximumf voltagef rating.f f anf additionalf resistorf dividerfnetworkfisfrequiredftofconnectffromfthefswitcherf outputftofthefvoutfpin.ffthefvoltagefatfthefvoutfpinf shouldfbefatfleastf500mvflowerfthanfthefvddafsupply,ftof preventfthefvldofswitch-overffunction.fffforfexample,fthef voltagefatfthefvoutfpinfcanfbef4vfiffvddafisfsetfforf5v.f whenfthefsc418foperatesffromfanfexternalfpowerfsourcef andfthefldofisfdisabledfbyfgroundingfthefenlfpin,fthef voltagef atf thefvoutf pinf canf bef asf highf asf shownf inf recommendedfoperatingfconditions.ffnotefthatfrtonf mustf bef adjustedf higherf byf thef samef dividerf ratiof tof ? applications information (continued)
sc418 15 maintainf thef desiredf on-time;f on-timef isf calculatedf accordingftofthefvoltagefatfthefvoutfpin. v out voltage selection thefswitcherfoutputfvoltagefisfregulatedfbyfcomparingf v out fasfseenfthroughfafresistorfdividerfatftheffbfpinftofthef internalf500mvfreferencefvoltagef(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 5 . 0 v ripple 2 1 out forced continuous mode operation thefsc418foperatesfthefswitcherfinfforcedfcontinuousf modef(fcm)fbyfconnectingfthefpsvfpinftofvdda.ff(thefpsvf pinfshouldfneverfexceedfthefvddafsupply.)ffseeffiguref4f forf fcmf waveforms.f f inf thisf modef onef off thef powerf mosfetsfisfalwaysfon,fwithfnofintentionalfdeadftimefotherf thanf tof avoidf cross-conduction.f fthisf resultsf inf moref uniformffrequencyfacrossftheffullfloadfrangefwithfthef trade-offbeingfreducedfefciencyfatflightfloadsfdueftof thefhigh-frequencyfswitchingfoffthefmosfets.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 . ( 500 mv ) 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 programmable ultrasonic power-save operation thefdevicefprovidesfprogrammablefultrasonicfpower-savef operationf atf lightf loads;f thef minimumf operatingf fre - quencyfisfprogrammedfbyfconnectingfafresistorffromfpsvf tofagnd.ffthefsc418fusesfthefpsvfresistorftofsetfanfinter - nalftimerfthatfmonitorsftheftimefbetweenfconsecutivef high-sidefgatefpulses.ffifftheftimefexceedsfthefprogrammedf timer,fdlfdrivesfhighftofturnftheflow-sidefmosfetfon.ffthisf drawsfcurrentffromfv out fthroughfthefinductor,fforcingfbothf v out fandfv fb ftoffall.ffwhenfv fb fdropsftofthef500mvfthresh - old,fthefnextfdhfon-timefisftriggered.ffafterfthefon-timefisf completedfthefhigh-sidefmosfetfisfturnedfoffandfthef low-sidef mosfetf turnsf on,f andf thef internalf timerf isf restarted.f fthef low-sidef mosfetf remainsf onf untilf thef inductorfcurrentframpsfdownftofzero,fatfwhichfpointfthef low-sidefmosfetfisfturnedfof.ffthisfendsfthefcyclefuntilfv fb f fallsfbelowfthef500mvfthreshold,forfthefinternalftimerff forcesfanotherfdlfturn-on.f becausef thef periodf betweenf on-timesf isf limitedf tof af maximumfvalue,fafminimumfoperatingffrequencyfisfmain - tained.fffiguref5fshowsfultrasonicfpower-savefoperation. applications information (continued)
sc418 16 fb ripple voltage ( v fb ) fb threshold ( 500 mv ) inductor current dh dl ( 0 a ) minimum frequency after the programmable time - out , dl drives high if v fb has not reached the fb threshold . dh on - time is triggered when v fb reaches the fb threshold on - time ( t on ) programmable time - out figure 5 ultrasonic power-save operation thefequationfforfdeterminingfthefr psv fresistorfvaluefisf shownfnext.ffthefdesiredfminimumffrequencyfisff swmin . swmin psv f pf 350 1 r u note:fthefpower-savefandfultrasonicfpower-saveffeaturesf arefnotfsupportedfforfvddaflessfthanf4.5v. power-savefmodefoperation thefdevicefprovidesfpower-savefoperationfatflightfloadsf withfnofminimumfoperatingffrequency,fselectedfbyffoat - ingfthefpsvfpinf(nofconnection).ffinfthisfmodefoffopera - tion,fthefzerofcrossfcomparatorfmonitorsfinductorfcurrentf viafthefvoltagefacrossftheflow-sidefmosfetfduringfthef of-time.fiffthefinductorfcurrentffallsftofzerofforf8fconsecu - tivefswitchingfcycles,fthefcontrollerfentersfpower-savef operation.fitfwillfthenfturnfofftheflow-sidefmosfetfonf eachfsubsequentfcycle,fprovidedfthatfthefcurrentfcrossesf zero.ffafterftheflow-sidefmosfetfisfof,fbothfhigh-sidefandf low-sidesfmosfetsfremainfoffuntilfv fb fdropsftofthef500mvf threshold.ffwhilefthefmosfetsfarefoffthefloadfisfsuppliedf byfthefoutputfcapacitor.ffiffthefinductorfcurrentfdoesfnotf reachfzerofonfanyfswitchingfcycle,fthefcontrollerfimmedi - atelyfexitsfpower-savefandfreturnsftofforcedfcontinuousf mode.ffiguref6fshowsfpower-savefoperationfatflightfloads.f note:fthefpower-savefandfultrasonicfpower-saveffeaturesf arefnotfsupportedfforfvddaflessfthanf4.5v. 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 . ( 500 mv ) 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,fresultingfinfafhardfshut - down.fsmartfpower-savefpreventsfthisfcondition.fwhenf thef fbf voltagef exceedsf 10%f abovef nominalf (exceedsf 550mv),fthefdevicefimmediatelyfdisablesfpower-savefandf dlfdrivesfhighftofturnfonftheflow-sidefmosfet.ffthisfdrawsf currentffromfv out fthroughfthefinductorfandfcausesfv out ftof fall.ffwhenfv fb fdropsfbackftofthef500mvftripfpoint,fafnormalf t on fswitchingfcyclefbegins.fthisfmethodfpreventsfafhardf ovpfshutdownfandfcyclesfenergyffromfv out fbackftofv in .ffitf alsofminimizesfoperatingfpowerfbyfavoidingfforcedfcon - ductionfmodefoperation.ffiguref7fshowsftypicalfwave - formsfforfthefsmartfpower-saveffeature. applications information (continued)
sc418 17 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 ( 550 mv ) 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 offfandftheflxfvoltagefhasfrisenf0.5vfabovefpgnd,fthef smartdrivef circuitf automaticallyf drivesf thef high-sidef mosfetfonfatfafrapidfrate.ffthisftechniquefreducesfswitch - ingfnoisefwhilefmaintainingfhighfefciency,freducingfthef needfforfsnubbersforfseriesfresistorsfinfthefgatefdrive. 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 15?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 tof preventf undesiredf togglingf off enf andf erraticf startupf performance,fthefenfpinfshouldfnotfbefallowedftoffoatfasf open-circuit.ff notefthatfthefldofenablefpinf(enl)fcanfalsofdisablefthef switchingfregulatorfthroughfthefv in fuvloffunction.ffreferf tofthefenlfpinfandfv in fuvlofsection.ff currentflimitfprotection thefsc418ffeaturesfprogrammablefcurrentflimiting,fwhichf isfaccomplishedfusingfthefrds (on) fofftheflowerfmosfetfforf currentfsensing.ffthefcurrentflimitfisfsetfbyfr ilim fresistorf whichfconnectsffromfthefilimfpinftofthefdrainfofftheflow- sidefmosfet.ffwhenftheflow-sidefmosfetfisfon,fanfinternalf 10 afcurrentffowsffromfthefilimfpinfandfthroughfthefr ilim f resistor,fcreatingfafvoltagefdropfacrossfthefresistor.ffwhilef theflow-sidefmosfetfisfon,fthefinductorfcurrentfflowsf throughfitfandfcreatesfafvoltagefacrossfthefrds (on) .fthef voltagefacrossfthefmosfetfisfnegativefwithfrespectftof pgnd.ffiffthisfmosfetfvoltagefdropfexceedsfthefvoltagef acrossfr ilim ,fthefvoltagefatfthefilimfpinfwillfbefnegativefandf currentflimitfwillfactivate.fthefcurrentflimitfthenfkeepsfthef low-sidefmosfetfonfandfpreventsfanotherfhigh-sidefon- time,funtilfthefcurrentfinftheflow-sidefmosfetfreducesf enoughftofbringfthefi lim fvoltagefupftofzero.ffthisfmethodf regulatesfthefinductorfvalleyfcurrentfatftheflevelfshownfbyf i 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 ilim fresistorfisfshownf inffiguref9. applications information (continued)
sc418 18 v out v in + cin q 2 + d 2 r ilim 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 ilim fvaluefisfcalculatedfbyfthefnextfequation. a 10 i r r lim dson ilim p u fff thefinternalf10afcurrentfsourcefisftemperaturefcompen - satedfatf4100ppmfinforderftofprovideftrackingfwithfthef rds on . notefthatfmosfetfrds (on) fincreasesfsignificantlyfiffthef vddpfvoltagefisf3.3vfcomparedftof5.0v.fffwhenfselectingf thefr ilim fvalue,fusefthefrds (on)f fvaluefthatfcorrespondsftof thefvddpfvoltagefusedfinfthefapplication. soft-startfoffpwmfregulator soft-startfisfachievedfinfthefpwmfregulatorfbyfusingfanf internalfvoltageframpfasfthefreferencefforftheffbfcompara - tor.f fthef voltagef rampf isf generatedf usingf anf internalf chargefpumpfwhichfdrivesfthefreferenceffromfzeroftof 500mvfinf1.2mvfincrements,fusingfanfinternalf500khzf oscillator.fwhenftheframpfvoltagefreachesf500mv,fthef rampfisfignoredfandftheffbfcomparatorfswitchesfoverftofaf fixedf 500mvf threshold.f duringf soft-startf thef outputf voltageftracksfthefinternalframp,fwhichflimitsfthefstart-upf inrushfcurrentfandfprovidesfafcontrolledfsoft-startfprofle.ff typicalfsoft-startframpftimefisf850s. duringf soft-startf thef regulatorf turnsf offf thef low-sidef mosfetfonfanyfcyclefiffthefinductorfcurrentffallsftofzero,f regardlessfoffthefpsavefsetting.ffthisfpreventsfnegativef inductorfcurrent,fallowingfthefdeviceftofstartfintofafpre- biasedfoutput. power good output thefpgoodf(powerfgood)foutputfisfanfopen-drainfoutputf whichfrequiresfafpull-upfresistor.ffwhenfthefvoltagefatfthef fbfpinfisf10%fbelowfthefnominalfvoltage,fpgoodfisfpulledf low.fitfremainsflowfuntilftheffbfvoltagefreturnsfabovef-8%f offnominal.fduringfstart-upfpgoodfisfheldflowfandfwillf notfbefallowedftoftransitionfhighfuntilfthefpgoodfstart- upf delayf fimef hasf passedf andf soft-startf isf completedf (whenfv fb freachesf500mv).ffthefdelayftimefstartingffromf enfgoingfhighfisftypicallyf2msfforfvddaf=f5vfandf1msfforf vddaf=f3.3v. pgoodfwillftransitionflowfifftheffbfvoltagefexceedsf+20%f offnominalf(600mv),fwhichfisfalsofthefover-voltagefshut - downfthreshold.fpgoodfalsofpullsflowfiffthefenfpinfisflowf whenfvddafisfpresent. output over-voltage protection ovpf(over-voltagefprotection)fbecomesfactivefasfsoonfasf thefdevicefisfenabled.fthefovpfthresholdfisfsetfatf500mvf +f20%f(600mv).ffwhenfv fb fexceedsfthefovpfthreshold,fdlf latchesfhighfandftheflow-sidefmosfetfisfturnedfon.fdlf remainsfhighfandfthefcontrollerfremainsfoffuntilfthefenf inputfisftoggledforfvddafisfcycled.ftherefisfaf5sfdelayf builtfintofthefovpfdetectorftofpreventffalseftransitions.f pgoodfisfalsoflowfafterfanfovpfevent. outputfunder-voltagefprotection whenfv fb ffallsf25%fbelowfitsfnominalfvoltagef(fallsftof 375mv)fforfeightfconsecutivefclockfcycles,fthefswitcherfisf shutfoffandfthefdhfandfdlfdrivesfarefpulledflowftoftri- statef thef mosfets.fthef controllerf staysf offf untilf enf isf toggledforfvddafisfcycled. vddafuvlofandfpor thefvddafunder-voltageflock-outf(uvlo)fcircuitryfinhib - itsfswitchingfandftri-statesfthefdh/dlfdriversfuntilfvddaf risesfabovef2.9v.ffwhenfvddafexceedsf2.9v,fanfinternalf porf(power-onfreset)fresetsftheffaultflatchfandfthefsoft- startfcounterfandfthenfthefsc418fbeginsfthefsoft-startf applications information (continued)
sc418 19 cycle.fthefswitcherfwillfshutfoffiffvddaffallsfbelowf2.7v.fff vddpfdoesfnotfhavefulvofprotection. notefthatfthefvddafuvlofwillfnotfstopfmosfetfswitchingf untilfthefvddafvoltageffallsftof2.7v.ffduringfthisftimefthef gatefdriverfvoltagesfwillftrackfthefvddafsupply.ffnotfallf mosfetsfwillfoperateforfswitchfefectivelyfatfdriveflevelsf off2.7v.ffforfthisfreason,fitfisfnotfrecommendedftofrelyfonf vddafuvloftofshutdownfthefswitcherfunlessfthefmosfetsf arefcapablefoffoperatingfwithf2.7vfdrive. ldo regulator thefldofoutputfisfprogrammableffromf0.75vftof5.25vf usingfexternalfresistors.ftheffeedbackfpinf(fbl)fforfthef ldofisfregulatedftof750mv.ffthefldofenablefpinf(enl)f providesfindependentfcontrol.fthefldofvoltagefcanfbef usedftofprovidefthefbiasfvoltagefforfthefswitchingfregula - tor.ffwhenfafseparatefsourcefisfusedfasfthefbiasfsupply,fthef ldofcanfbefprogrammedftofprovidefafdiferentfvoltage.ff thefexternalfresistorfconnectionsfarefshownfinffiguref10. vldo to fbl pin r ldo 2 r ldo 1 figure 10 vldo resistor divider thefldofoutputfvoltagefisfsetfbyftheffollowingfequation. ? ? 1 ? u 2 ldo 1 ldo r r 1 mv 750 vldo afminimumfcapacitancefoff1ffreferencedftofagndfisf normallyfrequiredfatfthefoutputfoffthefldofforfstability.ffiff thefldofisfprovidingfbiasfpowerftofthefdevice,fthenfaf minimumf0.1ffcapacitorfreferencedftofagndfisfrequired,f alongfwithfafminimumf1ffcapacitorfreferencedftofpgndf tofflterfthefgatefdrivefpulses.ffreferftofthefpcbflayoutf guidelinesfsection.fff enl pin and vin uvlo thefenlfpinfalsofactsfasfthefv in funder-voltageflockoutfforf thefswitcher.ffthefv in fuvlofvoltagefisfprogrammablefviafaf resistorfdividerfatfthefvin,fenlfandfagndfpins.ffthefv in f uvloffunctionfhasfaftypicalfthresholdfoff2.6vfonfthefv inf risingfedge.fftheffallingfedgefthresholdfisf2.4v. timingfisfimportantfwhenfdrivingfenlfwithflogicfandfnotf implementingfv in fuvlo.ffthefenlfpinfmustftransitionffromf highftoflowfwithinf2fswitchingfcyclesftofavoidfthefpwmf outputfturningfof.ffiffenlfgoesfbelowfthefv in fuvlofthresh - oldfandfstaysfabovef1v,fthenfthefswitcherfwillfturnfoffbutf thefldofwillfremainfon. notefthatfitfisfpossibleftofoperatefthefswitcherfwithfthef ldofdisabled,fbutfthefenlfpinfmustfbefbelowftheflogicf lowfthresholdf(0.4vfmaximum),fotherwisefthefv in fuvlof functionfwillfdisablefthefswitcher. theftablefbelowfsummarizesftheffunctionfoffthefenlfandf enfpins,fwithfrespectftofthefrisingfedgefoffenl.ff en enl ldo status switcher status low low,ff2.6v on of high high,f>f2.6v on on figuref11fshowsfthefenlfvoltagefthresholdsfandftheirf efectfonfldofandfswitcherfoperation. agnd enl low threshold ( min 0 . 4 v ) 2 . 6 v 2 . 4 v ldo on ldo on ldo off vin uvlo hysteresis enl voltage switcher on if en = high switcher on if en = high switcher off by vin uvlo figure 11 enl thresholds applications information (continued)
sc418 20 applications information (continued) enl logic control of pwm operation whenfthefenlfinputfexceedsfthefvinfuvlofthresholdfoff 2.6v,finternalflogicfchecksfthefpgoodfsignal.ffiffpgoodfisf high,fthefswitcherfisfalreadyfrunningfandfthefldofwillfstartf withoutfafectingfthefswitcher.ffiffpgoodfisflow,fthefdevicef disablesfpwmfswitchingfuntilfthefldofoutputfhasfreachedf 90%foffitsffnalfvalue.ffthisfdelayfpreventsfthefadditionalf currentfneededfbyfthefdhfandfdlfgatefdrivesffromfover - loadingfthefldofatfstart-up. infsomefcasesfitfisfdesirableftofusefthefv in fuvloffeaturefforf thefswitcherfwithoutfusingfthefldo.ffthisfcanfbefdonefbyf connectingftheffblfpinfdirectlyftofvdda.ffthisfdisablesfthef ldo,fallowingfthefenlfpinftofbefusedfonlyfforfvinfuvlofforf thefswitcher. ldo start-up beforefldofstart-up,fthefdevicefchecksfthefstatusfoffthef followingf signalsf tof ensuref properf operationf canf bef maintained. enlfpin vldofoutput v in finputfvoltage whenfthefenlfpinfisfhighfandfv in fisfavailable,fthefldofwillf beginfstart-up.ffduringfthefinitialfphase,fwhenfvldofisf nearf zero,f thef ldof initiatesf af current-limitedf start-upf (typicallyf115ma)ftofchargefthefoutputfcapacitor.ffwhenf v ldo fhasfreachedf90%fofftheffnalfvaluef(asfsensedfatfthef fblfpin),fthefldofcurrentflimitfisfincreasedftof200mafandf thefldofoutputfisfquicklyfdrivenftofthefnominalfvalue.ff thefldofstart-upfisfshownfinffiguref12. f v vldo final 90 % of v vldo final constant current startup voltage regulating with ~ 200 ma current limit figure 12 ldo start-up 1. 2. 3. ldo switch-over operation thefsc418fincludesfafswitch-overffunctionfforfthefldo.f thefswitch-overffunctionfisfdesignedftofincreasefefciencyf byfusingfthefmorefefcientfdc-dcfconverterftofpowerfthef ldofoutput,favoidingftheflessfefficientfldofregulatorf whenfpossible.fthefswitch-overffunctionfconnectsfthef vldofpinfdirectlyftofthefvoutfpinfthroughfanfinternalf switch.ffwhenfthefswitch-overfisfcompletefthefldofisf turnedfof,fwhichfreducesfoperatingfpowerfloss.fiffthefldof outputfisfusedftofbiasfthefsc418,fthenfafterfswitch-overf thefdevicefisfself-poweredffromfthefswitchingfregulatorf withfthefldofturnedfof. afterfthefswitcherfcompletesfsoft-start,fthefswitch-overf logicfwaitsfforf32fswitchingfcyclesfbeforefitfstartsfthef switch-over.ftherefareftwofmethodsfoffcompletingfthef switch-overfoffv ldo ftofv out .f inftheffrstfmethod,fthefldofisfalreadyfinfregulationfwhenf thefdc-dcfconverterfisfenabled.fasfsoonfasfthefpgoodf outputfgoesfhigh,fthef32fcyclefcountfisfstarted.fthefvolt - agesfatfthefvldofandfvoutfpinsfarefthenfcompared;fiffthef twofvoltagesfarefwithinf300mvfoffeachfother,fthefvldof pinfconnectsftofthefvoutfpinfusingfanfinternalfswitch,f andfthefldofisfturnedfof. infthefsecondfmethod,fthefdc-dcfconverterfisfalreadyf runningfandfthefldofisfenabled.ffinfthisfcasefthef32fcyclef countfisfstartedfasfsoonfasfthefldofreachesf90%foffitsffnalf value.ffatfthisftime,fthefvldofandfvoutfpinsfarefcompared,f andfiffwithinf300mvfthefswitch-overfoccursfandfthefldof isfturnedfof. switch-over limitations on vout and vldo becausefthefinternalfswitch-overfcircuitfalwaysfcomparesf thefvoutfandfvldofpinsfatfstart-up,ftherefareflimitationsf onfpermissiblefcombinationsfoffvoutfandfvldo.ffconsiderf thefcasefwherefvoutfisfprogrammedftof1.5vfandfvldofisf programmedftof1.8v.fafterfstart-up,fthefdevicefwouldf connectfvoutftofvldofandfdisablefthefldo,fsinceftheftwof voltagefarefwithinfthef300mvfswitch-overfwindow.fftof avoidfunwantedfswitch-over,fthefminimumfdifferencef betweenfvoutfandfvldofshouldfbef500mv.f
sc418 21 applications information (continued) itfisfnotfrecommendedftofusefthefswitch-overffeaturefforf anf outputf voltagef off 3.3vf orf lessf sincef thisf doesf notf providefsufcientfvoltagefforfthefgate-sourcefdriveftofthef internalfp-channelfswitch-overfmosfet.ff itfisfnotfrecommendedftofusefthefswitch-overffeaturefinf casesfwherefthefdevicefisfpoweredffromf3.3vfwhichfisfini - tiallyfderivedffromfthefldo.ffiffthefswitch-overfwasfused,f thenfthef3.3vfswitcherfoutputfwouldfpassfthroughfthef switch-overfmosfetftofpowerfthefvdda/vddpfpins.ffthef rds (on) foffthefswitch-overfdevice,ftypicallyf2.2fohmsfatf 3.3v,fwillfcausefafvoltagefdropfacrossfthefdevice.ffthef vdda/vddpf pinsf wouldf bef typicallyf 200mvf orf moref belowf thef 3.3vf rail,f duef tof thef irf dropf causedf byf thef vdda/vddpfcurrent.ffiffthisfvoltagefdropfbecomesflargef enough,fthefvddafvoltagefwillfdropfnearfthefvddafuvlof threshold,fcausingfthefdeviceftofshutdown.ff switch-over mosfet parasitic diodes thefswitch-overfmosfetfcontainsfparasiticfdiodesfthatf arefinherentftofitsfconstruction,fasfshownfinffiguref13. switchover mosfet parasitic diode parasitic diode vdda v ldo v out switchover control figure 13 switch-over mosfet parasitic diodes itfisfimportantftofpreventfforwardfbiasfoffthesefdiodes.ff theffollowingftwofconditionsfmustfbefsatisfedfinforderfforf thefparasiticfdiodesftofstayfof. vddaf > ffv ldof fffff vddaf > ffv out f iffeitherfv ldo forffv out fisfhigherfthanfvdda,fthenfthefrespec - tivefdiodefwillfturnfonfandfthefsc418foperatingfcurrentf willfthenffowfthroughfthisfdiode.ffthisfhasfthefpotentialfoff damagingfthefdevice. notefthatfiffthefvddafsupplyfisf3.3vfthenfbothfthefldof andf switchingf regulatorf outputsf aref limitedf tof 3.3vf maximum.fftryingftofobtainfhigherfvoltagesfsuchfasf5vf ? ? willfforward-biasfonefoffthefparasiticfdiodesfandfcouldf damagefthefdevice. using the internal ldo to bias the sc418 theffollowingfstepsfmustfbeffollowedfwhenfusingfthef internalfldoftofbiasfthefdevice. connectf vddaf andf vddpf tof vldof beforef enablingfthefldo. anyfexternalfloadfonfvldofshouldfnotfexceedf 40mafuntilfthefldofvoltagefhasfreachedf90%foff fnalfvalue. whenfthefswitch-overffeaturefisfusedfandfthefvdda/vddpf powerfcomesffromfv out ,fthefenfandfenlfinputsfmustfbef usedfcarefully.ffdofnotfconnectfthefenfpinfdirectlyftofvddaf orfanotherfsupplyfvoltage.ffiffthisfisfdone,fdrivingfthefenlf pinf lowf (tof agnd)f willf turnf offf thef ldof andf thef ldof switch-overfmosfet,fbutfthefswitcherfwillfcontinuefoper - ating.ffv out fwillffeedfintofthefldofoutputfandfthefvdda/ vddpffsuppliesfthroughfthefinternalfparasiticfdiode.ffthisf canfpotentiallyfdamagefthefdevice,fandfalsofpreventsfthef switcherffromfshuttingfoffuntilfthefvddafsupplyfdropsf belowfthefvddafuvlofthreshold.ffforfthesefapplicationsfaf dedicatedflogicfsignalfisfrequiredftofdrivefenflowfandf disablefthefswitcher.ffthisfsignalfcanfbefcombinedfwithf thefenlfsignalfiffneeded. 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 ? ? ? ? ? ?
sc418 22 applications information (continued) 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=f1.05vf + f4% f sw f=f250khz loadf=f10afmaximum frequency selection selectionfoffthefswitchingffrequencyfrequiresfmakingfaf trade-offbetweenfthefsizefandfcostfoffthefexternalfflterf componentsf (inductorf andf outputf capacitor)f andf thef powerfconversionfefciency.ff thefdesiredfswitchingffrequencyfisf250khz.ff afresistor,fr 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 = 318 ns at 13.2v in , 1.05v out , 250khz substitutingfforfr ton fresultsfinftheffollowingfsolution. r ton = 154.9k?, 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%ftof50%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 infthisfexamplefthefinductorfripplefcurrentfisfsetfequalftof 50%foffthefmaximumfloadfcurrent.ffthusfripplefcurrentf willfbef50%fxf10aforf5a.f ftoffndfthefminimumfinductancefneeded,fusefthefv in fandf t on fvaluesfthatfcorrespondftofv inmax .f h 77 . 0 a 5 ns 318 ) 05 . 1 2 . 13 ( l p u af slightlyf largerf valuef off 0.88hf isf selected.fthisf willf decreasefthefmaximumfi ripple ftof4.4a. notefthatfthefinductorfmustfbefratedfforfthefmaximumfdcf loadfcurrentfplusf1/2foffthefripplefcurrent. thefripplefcurrentfunderfminimumfv in fconditionsfisfalsof checkedfusingftheffollowingfequations. ns 384 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 25 . 4 h 088 ns 384 ) 05 . 1 8 . 10 ( i vinmin _ ripple p u
sc418 23 applications information (continued) capacitor selection thefoutputfcapacitorsfarefchosenfbasedfonfrequiredfesrf andfcapacitance.ffthefmaximumfesrfrequirementfisfcon - trolledfbyfthefoutputfripplefrequirementfandfthefdcftoler - ance.fthefoutputfvoltagefhasfafdcfvaluefthatfisfequalftof thefvalleyfoffthefoutputfripplefplusf1/2foffthefpeak-to-peakf ripple.ffchangefinfthefoutputfripplefvoltagefwillfleadftofaf changefinfdcfvoltagefatfthefoutput.ff thefdesignfgoalfisfforfthefoutputfvoltagefregulationftofbef 4%funderfstaticfconditions.ffthefinternalf500mvfrefer - enceftolerancefisf1%.fallowingf1%ftoleranceffromftheffbf resistorfdivider,fthisfallowsf2%ftolerancefdueftofv out fripple.ff sincefthisf2%ferrorfcomesffromf1/2foffthefripplefvoltage,f thefallowablefripplefisf4%,forf42mvfforfaf1.05vfoutput. thef maximumf ripplef currentf off 4.4af createsf af ripplef voltagefacrossfthefesr.ffthefmaximumfesrfvaluefallowedf isfshownfbyftheffollowingfequations. a 4 . 4 mv 42 i v esr ripplemax ripple max esr max = 9.5 m? thef outputf capacitancef isf chosenf tof meetf transientf requirements.fafworst-casefloadfrelease,ffromfmaximumf loadf tof nof loadf atf thef exactf momentf whenf inductorf currentfisfatfthefpeak,fdeterminesfthefrequiredfcapaci - tance.ffiffthefloadfreleasefisfinstantaneousf(loadfchangesf fromfmaximumftofzerofinf sc418 24 applications information (continued) meetf thef twof designf criteriaf off minimumf 379ff andf maximumf9m ?fesr ,fselectftwofcapacitorsfratedfatf220ff andf15m ?fesr . itfisfrecommendedfthatfanfadditionalfsmallfcapacitorfbef placedfinfparallelfwithfc out finforderftofflterfhighffrequencyf switchingfnoise. 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 capacitorf acrossf thef upperf feedbackf resistor,f asf shownfinffiguref14.ffthisfcapacitorfshouldfbefleftfunpopu - latedf unlessf itf canf bef confirmedf thatf double-pulsingf exists.ffaddingfthefc top fcapacitorfwillfcouplefmorefripplef intoffbftofhelpfeliminatefthefproblem.fanfoptionalfcon - nectionfonfthefpcbfshouldfbefavailablefforfthisfcapacitor.ff v out to fb pin r 2 r 1 c top figure 14 capacitor coupling to fb pin 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 whenfapplicationsfusefceramicfoutputfcapacitors,fthefesrf isfnormallyftoofsmallftofmeetfthefpreviouslyfstatedfesrf criteria.finfthesefapplicationsfitfisfnecessaryftofaddfafsmallf virtualfesrfnetworkfcomposedfofftwofcapacitorsfandfonef resistor,fasfshownfinffiguref15.
sc418 25 applications information (continued) ff r 1 r 2 fb pin c c c out l low - side high - side c l r l figure 15 virtual esr ramp current thisfnetworkfcreatesfaframpfvoltagefacrossfc l ,fanalogousf toftheframpfvoltagefgeneratedfacrossfthefesrfoffafstan - dardfcapacitor.ffthisframpfisfthenfcapacitivelyfcoupledfintof theffbfpinfviafcapacitorfc c . dropout performance thefoutputfvoltagefadjustfrangefforfcontinuous-conduc - tionf operationf isf limitedf byf thef fixedf 250nsf (typical)f 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 - tionsfitftripsfwhenftheffeedbackfpinfisf500mv,f + f 1%. thefon-timefpulseffromfthefsc418finfthefdesignfexamplef isfcalculatedftofgivefafpseudo-fxedffrequencyfoff250khz.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.f thefswitchingffrequencyfalsofvariesfwithfloadfcurrentfasfaf resultfoffthefpowerflossesfinfthefmosfetsfandfthefinduc -
sc418 26 applications information (continued) tor.ffforfafconventionalfpwmfconstant-frequencyfcon - verter,f asf loadf increasesf thef dutyf cyclef alsof increasesf slightlyftofcompensatefforfirfandfswitchingflossesfinfthef mosfetsfandfinductor.ffafadaptivefon-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 asfwithfanyfswitch-modefconverter,fgoodfpcbflayoutfisf essentialftofachievingfhighfperformance.fftheffollowingf guidelinesfwillfprovidefanfoptimumfpcbflayout. thefdeviceflayoutfcanfbefconsideredfinffourfparts. groundingfforfpgndfandfagnd powerfcomponentsf low-noisefanalogfcircuits bypassfcapacitors grounding for pgnd and agnd afgroundfplaneflayerfforfpgndfisfrecommendedf tofminimizefthefefectsfoffswitchingfnoise,fresis - tiveflosses,fandftofmaximizefheatfremovalffromf thefpowerfcomponents.f afseparatefgroundfplaneforfislandfshouldfbefusedf forfagndfandfallfassociatedfcomponents.fthef agndfislandfshouldfavoidfoverlappingfswitchingf signalsfonfotherflayersf(dh/dl/bst/lx). connectfpgndfandfagndftogetherfwithfafzerof ohmfresistorforfcopperftrace.f makefthefconnec - tionfnearfthefagndfandfpgndfpinsfoffthefic.f f f power components usef short,f widef connectionsf betweenf thef powerf components. inputfcapacitorsfandfhigh-sidefmosfets high-sidefandflow-sidefmosfetsfandfinductorf ( lxf connection).f f usef widef copperf tracesf tof providefhighfcurrentfcarryingfcapacityfandfforf heatfdissipation.ff inductorfandfoutputfcapacitors. ? ? ? ? ? ? ? ? ? ? allfpgndfconnectionsf fthefinputfcapacitors,f low-sidefmosfets,foutputfcapacitors,fandfthef pgndfpinfoffthefsc418.ff anfinnerflayerfgroundfplanefisfrecom - mended.ff eachfpowerfcomponentfrequiresfafshort,f lowfimpedancefconnectionftofthefpgndff plane. placefviasftofthefpgndfplanefdirectlyfnearf thefcomponentfpins. usefshortfwideftracesfforfthefpinfconnectionsf fromfthefsc418f(lx,fdh,fdlfandfbst).fdofnotf routeftheseftracesfnearfthefsensitiveflow-noisef analogfsignalsf(fb,ffbl,fton,fv out ). low-noise analog circuits low-noisefanalogfcircuitsfarefsensitivefcircuitsfthatfaref referencedftofagnd.ffdueftoftheirfhighfimpedancefandf sensitivityftofnoise,fitfisfimportantfthatfthesefcircuitsfbef keptfawayffromfthefswitchingfsignals.f usefafplaneforfsolidfareafforfagnd.ffplacefallf componentsf connectedf tof agndf abovef thisf area.ff usefshortfdirectftracesfforfthefagndfcon - nectionsftofallfcomponents.f placefviasftofthefagndfplanefdirectlyfnearf thefcomponentfpins. properfroutingfoffthefv out fsenseftracefisfessentialf sincefitffeedsfintoftheffbfresistorfdivider.ffnoisef onftheffbfwaveformfwillfcausefinstabilityfandf multiplefpulsing.f connectfthefv out fsenseftracefdirectlyftof thefoutputfcapacitorforfafceramicfbypassf capacitor.f routefthisftracefoverftofthefvoutfpin,f carefullyfavoidingfallfswitchingfsignalsf andfpowerfcomponents.ff routefthisftracefinfafquietflayerfiffpossible.ff routefthisftracefawayffromfthefswitch - ingftracesfandfcomponents,fevenfiffthef tracefisflonger.favoidfshorterftracefroutingf throughfthefpowerfswitchingfarea. iffafbypassfcapacitorfisfusedfatftheficfsidef offthef v out fsensef trace,fitfshouldfbefplacedf nearftheffbfresistorfdivider. ? ? ? ? ? ? ? ? ? ? ? ? ? ?
sc418 27 applications information (continued) allfcomponentsfconnectedftoftheffbfpinfmustf beflocatedfnearfthefpin.fftheffbftracesfshouldfbef keptfsmallfandfnotfroutedfnearfanyfnoisyfswitch - ingfconnectionsforfpowerfcomponents. placefthefr ilim fresistorfnearfthefic.ffforfanfaccu - ratefi lim fcurrentfsensefconnection,froutefthefr ilim f tracefdirectlyftofthefdrainfofftheflow-sidefmosfetf (lx).ffusefanfinnerfroutingflayerfiffneeded.fff placefthefr ton fresistorfnearftheftonfpin.ffroutef r tonf toftheftonfpinfandftofagndfusingfshortf tracesfandfavoidfallfswitchingfsignals. forfthefldofcomponents theffblfresistorsfshouldfbefplacedfnearf theffblfpin.ffthefareafofftheffblftracesf shouldfbefminimized,favoidingfallfswitch - ingfconnections.f theftopffblfresistorfmustfroutefdirectlyf tofthefldofoutputfcapacitor.fffthefldof loadfcurrentfshouldfnotffowfthroughfthisf trace.fff bypass capacitors thefdevicefrequiresfbypassfcapacitorsfforftheffollowingf pins.ff vddaf pinf withf respectf tof agnd.f fthisf 0.1ff minimumfcapacitorfmustfbefplacedfandfroutedf closeftoftheficfpins,fonfthefsameflayerfasfthefic. vddpfwithfrespectftofpgnd.ffthisf1ffminimumf capacitorfmustfbefplacedfandfroutedfcloseftof theficfpinsfandfonfthefsameflayerfasfthefic. bstfpinfwithfrespectftoflx.fthisf0.1ffminimumf capacitorfmustfbefplacedfnearfthefic,fonfeitherf sidefoffthefpcb.ffusefshortftracesfforfthefroutingf betweenfthefcapacitorfandfthefic. vldofpinfwithfrespectftofagnd.f thisfisfthefldofoutputfcapacitor. thefplacementfoffthisfcapacitorfisfnotf critical,fhoweverfforfroutingfitfisfimportantf thatftheftopffblfresistorfconnectsfdirectlyf tofthisfcapacitor,fandfthatfthefldofloadf currentfdoesfnotffowfthroughfthisftrace. iffvldofisfnotfusedftofpowerfthefdevicef thenfafminimumf1ffcapacitorfreferencedf tofagndfisfrequired.ff iffvldofisfusedftofpowerfthefdevicefthenf ? ? ? ? ? ? ? ? ? ? ? ? ? ? thefvddafandfvddpfcapacitorsfalsof functionfasfthefldofoutputfcapacitor.ff forfvdda,fafminimumf0.1ffcapacitorfref - erencedftofagndfisfrequired.ffforfvddp,f afminimumf1ffcapacitorfreferencedftof pgndfisfrequired.f vinf pinf withf respectf tof agnd.f thisf 0.1ff minimumfcapacitorfmustfbefplacedfandfroutedf closef tof thef icf pins.f fthisf capacitorf providesf noisefflteringfforfthefinputftofthefinternalfldo. ?
sc418 28 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 .
sc418 29 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
sc418 30 contact information semtechfcorporation powerfmanagementfproductsfdivision 200fflynnfroad,fcamarillo,fcaff93012 phone:ff(805)f498-2111ffffax:ff(805)f498-3804 www.semtech.com ?fsemtechf2010 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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