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?f2011fsemtechfcorporation 1 SC508/SC508a ecospeed ? dc-dc buck controller with integrated ldo features powerfsystem inputfvoltageff4.5vftof46v integratedfbootstrapfswitch fixedf5vfldofoutputff200ma 1%freferenceftolerancef f -40ftof+85fc selectablefinternal/externalfbiasfpowerfsupply ecospeed ? farchitecturefwithfpseudo-fxedffre - quencyfadaptivefon-timefcontrol logicfinputfandfoutputfcontrolf independentfcontrolfenfforffldofandfswitcherf programmablefsoft-startftime programmablefv in fuvlofthreshold powerfgoodfoutput selectablefp ower-savefmode programmablef ultrasonicfpower-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 aec-q100fqualifedfavailablefinfSC508a applications ofcefautomationfandfcomputing networkingfandftelecommunicationfequipment point-of-loadfpowerfsuppliesfandfmodulefreplacement automotivefapplications ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? description thefSC508/SC508afisfafsynchronousfecospeed ? f buckfreg - ulatorfwhichfincorporatesfsemtechsfadvanced,fpatentedf adaptivef`on-timefcontrolfarchitectureftofprovidefexcel - lentflight-loadfefciencyfandffastftransientfresponse.ffitf featuresfanfintegratedfbootstrapfswitchfandfaffxedf5vf ldofinfaf3fxf3(mm)fpackage.ffthefdevicefisfhighlyfefcientf andfusesfminimalfpcbfarea.ffthefSC508afisfavailablefforf automotivefapplicationsfandfisfqualifedftofaec-q100.ff thefSC508fsupportsfusingfstandardfcapacitorftypesfsuchfasf 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,fselectablefpower-savefmodes,fandfpro - grammablefultrasonicfpower-save.ffthefdevicefalsofprovidesf separatefenablefinputsfforfthefpwmfcontrollerfandfldofasf wellfasfafpowerfgoodfoutputfforfthefpwmfcontroller.ffoutputf voltagefrangefisf0.6ftof5v,fwithfoutputfvoltagesfgreaterfthanf 5vfsupportedfusingfadditionalfcomponents.ff thefinputfvoltagefcanfrangeffromf5vftof46v.ffthefwidef inputfvoltagefrange,fprogrammableffrequency,fandfinte - gratedf5vfldofmakefthefdevicefextremelyffexiblefandf easyftofusefinfafbroadfrangefoffapplications.ffsupportfisf providedfforfmulti-cellfbatteryfsystemsfinfadditionftoftra - ditionalfdcfpowerfsupplyfapplications. revisionf4.1 power management typical application circuit vdda pgood en ton enl 1 f vext or vldo pgnd agnd vldo enable ldo rton sc 508 sc 508 a 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 3 . 3 o
SC508/SC508a 2 pin confguration ordering information marking information 508 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. 3)f aec-q100fqualifed. yyww = date code xxxx = semtech lot number mlpq-ut20 device package SC508ultrt (1)(2) mlpq-ut20 SC508aultrt (1)(2)(3) mlpq-ut20 SC508evb evaluationfboard SC508aevb evaluationfboard 508 a yyww xxxx yyww = date code xxxx = semtech lot number
SC508/SC508a 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 +50 lxftofpgndf(v)f(transientff100ns)f f. . . . . . . . . -2f tof +50 dh,fbstftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +55 dh,fbstftoflxf(v) f f. . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +6 dlftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3f tof +6 vinftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3ftof+50 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. . . . . . . . . . f 0.3f tof +6 vddaftofvddpf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3fftof+0.3 agndftofpgndf(v) f. . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3ftof+0.3f esdfprotectionflevel (1)f (kv) f. . . . . . . . . . . . . . . . . . . . . . . . . . . . f2 recommended operating conditions inputfvoltagef(v)f f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5f tof 46 vddaftofagnd,fvddpftofpgndf(v) f. . . . . . . . . . . f 4.5f 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(vddafpinf+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 SC508/SC508a 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 fultrasonicfpower-savefoperationf enf=f5v,fr psv f=f115k?,fv fb f>f600mv 2.4 forcedfcontinuousfmodefoperationff operatingff sw f=f220khz,fpsvf=fvdda,fnofload 13 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=f30v,fv out f=f3fv,ffr ton f=f600k?,ffvddaf=f5v 1530 1700 1870 ns on-timefaccuracy;fffforcedfcontinuousfmode v in f=f4.5ftof10v,fv out f=f3fv,ffr ton f=f600k?,fvddaf=f5v 15 % minimumfon-time 80 ns minimumfof-time 250 ns ultrasonicffrequency minimumfswitchingffrequency,fr psv f=f115k? 25 khz 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 SC508/SC508a 5 electrical characteristics (continued) parameter conditions min typ max units fault protection i lim fsourcefcurrent 9 10 11 a i lim fsourcefcurrentftemperaturefcoefcient 0.41 %/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=f46v +11 a fbfinputfbiasfcurrent fbf=fvddaforfagnd -1 +1 a psvfinputfbiasfcurrent psvf=fvdda 5 16 a psvf < f1.5v 1 a linear regulator vldofaccuracy vldofloadf=f10ma 4.875 5.0 5.125 v currentflimit vldofff4.5vf(typ) 200 vldoftofvoutfswitch-overfthreshold f(4) -130 +130 mv vldoftofvoutfnon-switch-overfthreshold f(4) -500 +500 mv vldoftofvoutfswitch-overfresistance vldof=fvoutf=f5v 2.0 ? vldofdropfoutfvoltage v in ftofvldo,ffldofloadf=f50ma 1.2 v
SC508/SC508a 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 1.0 2.0 ? r dh_pull-down,fff vddpf=f5v 0.6 1.2 ? 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 45 ns bootstrapfswitchfresistance 16 ? low-side driver (dl, vddp, pgnd) peakfcurrent vddpf=f5v,ffdlfsourcing 2 a vddpf=f5v,ffdlfsinking 4 onfresistance r dl_pull-up, ffvddpf=f5v 1.0 2.1 ? r dl_pull-down,ff vddpf=f5v 0.50 0.86 ? 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.fthefenlfvoltagefisfcomparedftofanfinternalfreference. (2)f forfupsvfandffcmfoperation,fthefvddafandfvddafsupplyfcurrentfincludesfthefdh/dlfcurrentfrequiredftofdrivefthefexternalfmosfets. (3)f thefenlfpinfwillfenablefthefldofwithf0.8vftypical.ffthefenlfpinfv in fulvoffunctionfwillfdisablefthefswitcherfunlessfthefenlfpinfexceedsfthefv in f uvlofthresholdfwhichfisftypicallyf1.56v. (4)f thefswitch-overfthresholdfisfthefmaximumfvoltagefdiferentialfbetweenfthefvldofandfvoutfpinsfwhichfensuresfthatfvldofwillfinternallyf switch-overftofvout.fthefnon-switch-overfthresholdfisfthefminimumfvoltagefdiferentialfbetweenfthefvldofandfvoutfpinsfwhichfensuresf thatfvldofwillfnotfswitch-overftofvout.
SC508/SC508a 7 typical characteristics (continued) efciency vs load psave mode 15 v 28 v 42 v 20 30 40 50 60 70 80 90 100 0 1 2 3 4 5 6 7 8 e f f i c i e n c y ( % ) i out ( adc ) efciency vs load forced continuous mode 15 v 28 v 20 30 40 50 60 70 80 90 100 0 1 2 3 4 5 6 7 8 e f f i c i e n c y ( % ) i out ( adc ) 42 v efciency vs load ultrasonic psave mode 15 v 28 v 20 30 40 50 60 70 80 90 100 0 1 2 3 4 5 6 7 8 e f f i c i e n c y ( % ) i out ( adc ) 42 v efciency vs load 5v output 15 v 28 v 20 30 40 50 60 70 80 90 100 e f f i c i e n c y ( % ) i out ( adc ) 42 v 0 1 2 3 4 5 6 inductor : cyntec pcmb 104 e - 4 r 7 ms high side mosfet : alpha omega ao 4440 low side mosfet : fairchild fmds 5352 efciency vs load 12v output 28 v 20 30 40 50 60 70 80 90 100 e f f i c i e n c y ( % ) i out ( adc ) 42 v 0 1 2 3 4 5 6 inductor : cyntec pcmb 104 e - 4 r 7 ms low side mosfet : fairchild fmds 5352 high side mosfet : fairchild fmds 5352 load regulation 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 i out ( adc ) v o u t ( v ) 15 v in fcm 42 v in psave 28 v in upsave v out f5v,finternalf5vfbias,fpower-save,f250khzffrequency v out f12v,fexternalf5vfbias,fpower-save,f400khzffrequency v out f1.8v,fexternalf5vfbias,f220khzffrequency v out f1.8v,fexternalf5vfbias,f220khzffrequency v out f1.8v,fexternalf5vfbias,f220khzffrequency v out f1.8v,fexternalf5vfbias,f220khzffrequency
SC508/SC508a 8 typical characteristics (continued) start-up en input enf(5v/div) v inf =f42v,fv out f=f1.8v,fi out f=f0a,fpsave time (1m??? i?? ???i?? ??i?? pgoodf(5v/div) lxf(50v/div) pre-bias start- up en input enf(5v/div) v inf =f42v,fv out f=f1.8v,fi out f=f0a,fpsave time (1m??? i?? ???i?? ??i?? v out f(1v/div) pgoodf(5v/div) start-up ss ramp-up ssf(5v/div) v inf =f42v,fv out f=f1.8v,fi out f=f0a,fpsave,fstartupfusingfenfinput time (1m??? i?? ???i?? ??i?? shutdown en input enlf(5v/div) v inf =f5v,fv out f=f1.8v,fi out f=f1a,fpsave time (1m??? i?? ???i?? ??i?? pgoodf(5v/div) lxf(50v/div) v out f(1v/div) over-current automatic restart v inf =f42v,fv out f=f1.8v,fexternalfloadf=f15a time (20m??? i?? ???i?? ??i?? i out f(10a/div) lxf(50v/div) start-up, shutdown enl input (v in uvlo) enlf(5v/div) v inf =f42v,fv out f=f1.8v,fi out f=f1a,fpsave time (4m??? i?? ???i?? ??i?? v out f(1v/div) v out f(1v/div) v out f(1v/div) pgoodf(5v/div) lxf(50v/div) pgoodf(5v/div) lxf(50v/div) lxf(50v/div) v out f(1v/div) ssf(5v/div)
SC508/SC508a 9 typical characteristics (continued) switching psave v out f(50mv/div) v inf =f42v,fv out f=f1.8v,fi out f=f0a time (10m??? i?? ???i?? ??i?? dlf(5v/div) lxf(50v/div) transient response psave v out f(50mv/div) v inf =f42v,fv out f=f1.8v,fi out f=f0aftof8aftof0a time (100??? i?? ???i?? ??i?? lxf(50v/div) switching upsave v outf f(50mv/div) v inf =f42v,fv out f=f1.8v,fi out f=f0a time (20 ???i?? ??i?? dlf(5v/div) transient response upsave v inf =f42v,fv out f=f1.8v,fi out f=f0aftof8aftof0a time (100 ???i?? ??i?? lxf(50v/div) i out f(5a/div) v out f(50mv/div) transient response fcm v out f(50mv/div) v inf =f42v,fv out f=f1.8v,fi out f=f0aftof8aftof0a time (100 ???i?? ??i?? i out f(5a/div) lxf(50v/div) switching fcm v out f(50mv/div) v inf =f42v,fv out f=f1.8v,fi out f=f0a time (4 ???i?? ??i?? lxf(50v/div) dlf(5v/div) lxf(50v/div) i out f(5av/div)
SC508/SC508a 10 rlim 100 nf rpsv ( 2 ) 100 n f sc 508 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 36 v to 1 . 8 v @ 8 a ctop * pad 20 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 / 50 v murata grm 32 er 71 h 106 ka 12 l www . murata . com www . panasonic . com cout 330 f / 6 m w sanyo 2 tpf 330 m 6 www . vishay . com l 1 1 . 8 h vishay ihlp 4040 ezer 1 r 8 m 01 key component? www . aosmd . com q 1 ao 4440 alpha omega ao 4440 www . fairchildsemi . com q 2 fdms 5352 fairchild fdms 5352 6 . 81 k w css 3 . 9 nf cbst ( 1 ) ( 2 ) rpsv : use 115 k w for ultrasonic operation . remove rpsv for power - save operation . connect 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 : rbst 3 . 3 w detailed application circuit
SC508/SC508a 11 pin descriptions pin # pin name pin function 1 fb feedbackfinputfforfswitchingfregulatorffconnectftofanfexternalfresistorfdividerffromfoutputffusedftofpro - gramfthefoutputfvoltage. 2 vout switcherfoutputfvoltagefsensefpinffalsofthefinputftofthefinternalfswitch-overfmosfetfbetweenfvoutfandf vldo.ffthefvoltagefatfthisfpinfmustfnotfexceedfthefvddafpin.ffforfoutputfvoltagesfupftof5vfconnectfthisfpinf directlyf tof thef switcherf output.f forf outputf voltagesf exceedingf 5vf connectf thisf pinf tof thef switcherf outputf throughfafresistorfdivider.ff 3 vdda supplyfinputfforfinternalfanalogfcircuitsfffconnectftofexternalf5vfsupplyforfconnectftofvldoffalsofthef sensefinputfforfvddafunderfvoltageflockoutf(vddafuvlo). 4 vldo outputfoffthef5vfldoffthefvoltagefatfthisfpinfmustfnotfexceedfthefvoltagefatfthefvddafpin. 5 vin inputfsupplyfvoltageffconnectftofthefsamefsupplyfusedfforfthefhigh-sidefmosfet.ffconnectfaf100nffcapaci - torffromfthisfpinftofagndftofflterfhighffrequencyfnoise. 6 ss soft-startfffconnectftanfexternalfcapacitorftofagndftofprogramfthefsoftfstartfandfautomaticfrecoveryftime. 7 nc nofconnection 8 bst bootstrapfpinffconnectfaf100nffminimumfcapacitorfandfseriesfresistorffromfbstftoflxftofdevelopftheffoat - ingfvoltagefforfthefhigh-sidefgatefdrive.ffaf3.3fohmfresistorfisfrecommended. 9 dh high-sidefgatefdrivefoutput 10 lx switchingf(phase)fnode 11 pgnd powerfgroundfforfthefdlfandfdhfdriversfandftheflow-sidefexternalfmosfet. 12 dl low-sidefgatefdrivefoutput 13 vddp 5vfsupplyfinputfforfthefdhfandfdlfgatefdrivesffconnectftofthefsamef5vfsupplyfusedfforfvdda. 14 psv power-savefprogrammingfinputffconnectfafresistorftofagndftofsetfafminimumf(ultrasonic)fpower-saveffre - quency f ffoatfpinftofselectfpower-savefwithfnofminimumffrequencyffpullfupftofvddaftofdisablefpower- 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
SC508/SC508a 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 vldo switchover mux a y b 5 v ldo vldo bst fb comparator vdda lx en vldo switchover comparator 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
SC508/SC508a 13 synchronous buck converter thefSC508fisfafstepfdownfsynchronousfdc-dcfbuckfcon - trollerfwithfanfinternalf5vfldo.ffitfprovidesfefcientfopera - tionfinfafspacefsavingf3x3f(mm)f20-pinfpackage.ffthef programmablefoperatingffrequencyfrangefupftof1mhzf enablesfoptimizingfthefconfgurationfforfpcbfareafandf efficiency.ffforfautomotivefapplications,fthefSC508afisf qualifedftofaec-q100.ff thefcontrollerfusesfafpseudo-fixedffrequencyfadaptivef on-timefcontrol.ffthisfallowsffastftransientfresponsefwhichf permitsfthefusefoffsmallerfoutputfcapacitors. input voltage requirements thefSC508frequiresftwofinputfsuppliesfforfnormalfopera - tion:ffv in fandfvdda/vddp.ffv in foperatesfoverfthefwidef rangefoff5vftof46v.ffvddafandfvddpfrequirefaf5vfsupplyf whichfcanfbeffromfanfexternalfsourceforffromfthefinternalf ldo.ffvddafandfvddpfarefderivedffromfthefsamefsourcef voltage.ff psuedo-fxed frequency adaptive on-time control thefpwmfcontrolfmethodfusedfbyfthefSC508fisfpseudo- 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 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
SC508/SC508a 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 thefSC508fusesfanfexternalfresistorftofsetfthefon-timefwhichf indirectlyfsetsftheffrequency.ffthefon-timefcanfbefpro - grammedftofprovidefanfoperatingffrequencyfoffupftof1mhzf usingfafresistorfbetweenftheftonfpinfandfground.fthefresis - torfvaluefis selected by the following equation. 28 7 ,1 6: ,1 28 7 28 7 ,1 21 72 1 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 notefthatfthefon-timefvariationfincreasesftoftypicallyf15%f atfinputfvoltagesffromf4.5ftof10v.ffinfmostfapplicationsfthisf doesfnotfsignifcantlyfafectfoverallfperformance.ff f 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 infsomefapplicationsfafsmallfcapacitorfctopfisfplacedfinf parallelfwithfr1ftofprovidefaflargerfripplefsignalffromfvoutf toftheffbfpin.ffinfthesefapplications,fthefoutputfvoltagef voutfisfcalculatedfaccordingftoftheffollowingfequationfinf 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 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 applications information (continued)
SC508/SC508a 15 iffthefinternalfldofisfusedfforfbiasfpower,fthefldofswitch- overffunctionfmustfbefinhibitedfbyfselectingfthefresistorf dividerf sof thatf thef voltagef atf thefvoutf pinf doesf notf exceedf4v;ffthisfwillffinhibitfthefvldofswitch-overffunc - tion.fffiffthefSC508fbiasfpowerfisffromfanfexternalf5vfsupplyf andfthefldofisfdisabledfbyfgroundingfthefenlfpin,fthef voltagefatfthefvoutfpinfisfnotflimitedftof4vfandfcanfbefasf highfasfthefvddafsupplyfvoltage.ff notefthatfthefvoutfpinfhasfanfinternalf500k w fresistorf connectedftofagnd.fftofminimizefthefefectfoffthisfresistorf onfthefresistorfdividerfratio,fthefmaximumfrecommendf valuefforfresistorfr v2 finffiguref4fisf10k 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 thefSC508foperatesfthefswitcherfinfforcedfcontinuousf 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 uniformffrequencyfacrossftheffullfloadfrangefwithfthef 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 programmable ultrasonic power-save operation thefdevicefprovidesfprogrammablefultrasonicfpower-savef operationf atf lightf loads;f thef minimumf operatingf fre - quencyfisfprogrammedfbyfconnectingfafresistorffromfpsvf tofagnd.ffthefSC508fusesfthefpsvfresistorftofsetfanfinter - nalftimerfthatfmonitorsftheftimefbetweenfconsecutivef high-sidefgatefpulses.ffifftheftimefexceedsfthefprogrammedf timer,fdlfdrivesfhighftofturnftheflow-sidefmosfetfon.ffthisf drawsfcurrentffromfv out fthroughfthefinductor,fforcingfbothf v out fandfv fb ftoffall.ffwhenfv fb fdropsftofthef600mvfthresh - 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 againffallsfbelowfthef600mvfthreshold,forfthefinternalf timerffforcesfanotherfdlfturn-on.f applications information (continued)
SC508/SC508a 16 becausef thef periodf betweenf on-timesf isf limitedf tof af maximumfvalue,fafminimumfoperatingffrequencyfisfmain - tained.fffiguref6fshowsfultrasonicfpower-savefoperation. fb ripple voltage ( v fb ) fb threshold 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 6 ultrasonic power-save operation thefequationfforfdeterminingfthefr psv fresistorfvaluefisf shownfnext.ffthefdesiredfminimumffrequencyfisff swmin . swmin psv f pf 350 1 r u power-save mode operation thefdevicefprovidesfpower-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 byfthefoutputfcapacitor.ffiffthefinductorfcurrentfdoesfnotf reachfzerofonfanyfswitchingfcycle,fthefcontrollerfimmedi - atelyfexitsfpower-savefandfreturnsftofforcedfcontinuousf mode.f figuref 7f 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 7 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 figuref8fshowsftypicalfwaveformsfforfthefsmartfpower- saveffeature. applications information (continued)
SC508/SC508a 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 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 8 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 notefthatfthefldofenablefpinf(enl)fcanfalsofdisablefthef switchingfregulatorfthroughfthefv in fuvloffunction.ffreferf tofthefenlfpinfandfv in fuvlofsection.ff currentflimitfprotection thefSC508ffeaturesfprogrammablefcurrentflimiting,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 finffiguref9. time i peak i load i lim i n ? u c t o r c u r r e n t figure 9 valley current limit applications information (continued)
SC508/SC508a 18 thefcurrentflimitfschematicfwithfthefr lim fresistorfisfshownf inffiguref10. 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 10 valley current limit settingfthefvalleyfcurrentflimitftof10afresultsfinfafpeakf inductorfcurrentfoff10afplusfpeakfripplefcurrent.finfthisf situationfthefaveragefcurrentfthroughfthefinductorfisf10af plusfone-halffthefpeak-to-peakfripplefcurrent. thefr lim fvaluefisfcalculatedfbyfthefnextfequation. $ , 5'6 5 /, 0 21 /, 0 u fff thefinternalf10afcurrentfsourcefisftemperaturefcompen - satedfatf4100ppmfinforderftofprovideftrackingfwithfthef rds on . soft-startfoffpwmfregulator thefSC508fhasfafprogrammablefsoft-startftimefthatfisfcon - trolledfbyfanfexternalfcapacitorfatfthefssfpin.fduringfthef soft-startftime,fthefcontrollerfsourcesf3affromfthefssfpinf tof chargef thef capacitor.f duringf thef start-upf processf (figuref11),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. 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 (1b*22' afterfthefpowerfgoodfstart-upfdelayftimefisfcompleted,f thefssfpinfisfinternallyfpulledfupftofthefvddafsupply. thefsoft-startfcyclefandfpowerfgoodftimingfcanfbefseenfinf theffiguref11.ff en ss fb pgood css charging current 3 ua v ss = 1 . 5 v vout in regulation v ss = 3 . 35 v t ss t pgood figure 11 soft-start cycle and power good timing applications information (continued)
SC508/SC508a 19 pre-bias start-up SC508fcanfsupportfsoft-startfwithfanfoutputfpre-bias.fthef ssframpftimefisfthefsamefasfafnormalfstart-upfwhenfthef outputfvoltagefstartsffromfzero.funderfafpre-biasfstart-up,f thefdhfandfdlfdriversfinhibitfswitchingfuntilf40%foffthef rampfatfthefssfpinfequalsfthefpre-biasffbfvoltageflevel.f pre-biasf start-upf isf achievedf byf turningf offf thef lowerf mosfetfwhenfthefinductorfcurrentfreachesfzerofduringf thef soft-startf cycle.fthisf methodf preventsf thef outputf voltageffromfdecreasing. power good output thefpgoodf(powerfgood)foutputfisfanfopen-drainfoutputf whichfrequiresfafpull-upfresistor.ffduringfstart-up,fpgoodf isfheldflowfandfisfnotfallowedftoftransitionfhighfuntilfthef outputfvoltagefisfinfregulationfandfthefssfpinfhasfreachedf 67%foffvdda.fftheftimeffromfenfgoingfhighftofpgoodf goingfhighfisftypicallyf11msfforfcssf=f10nf.ff 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 offfwhilefthefdevicefgoesfthroughfthefautomaticffaultf recoveryfcycle.ffwhenfthefautomaticfrecoveryfcyclefisf completed,fthefdevicefwillfattemptfafnewfsoft-startfcycle.ff atfthefstartfoffthefsoft-startfcycle,fthefdlfoutputfwillfgof lowfforftypicallyf30usecfwhilefthefcontrollerfinitializesfthef 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 thefSC508fincludesfanfautomaticfrecoveryffeaturef(hiccupf modefuponffault).ffiffthefswitcherfoutputfisfshutfdownfduef tofaffaultfcondition,fthefdevicefusesfthefssfcapacitorfasfaf timer.ffuponffaultfdetectionfthefssfpinfisfpulledflowfandf thenfbeginsfchargingfthroughfthefinternalf3 afcurrentf source.ffwhenfthefssfcapacitorfreachesf67%foffvdda,fthef ssfpinfisfagainfpulledflow,fafterfwhichfthefssfcapacitorf beginsfanotherfchargingfcycle.ffthefssfcapacitorfwillfbef usedfforf15fcyclesfoffchargingffromf0ftof67%foffvdda.ff duringfthesefcyclesfthefswitcherfisfofffandftherefisfnof mosfetfswitching.ff duringfthef16thfssfchargingfcycle,fthefnormalfsoft-startf routinefisfimplementedfandfthefmosfetsfbeginfswitch - ing.ffswitchingfcontinuesfuntilfthefpowerfgoodfstart-upf delayftimefisfreached.ffiffthefswitcherfoutputfisfstillfinfaf faultfcondition,fthefswitcherfwillfagainfshutfdownfandf forcef15fcyclesfoffssfchargingfbeforefattemptingfanotherf soft-start.f f fthef longf delayf betweenf soft-startf cyclesf reducesf thef averagef powerf lossf inf thef powerf components.fff thefautomaticfrecoveryftimingfisfshownfinffiguref12. 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 12 automatic recovery timing ifftheffaultfwasfdueftofanfover-voltagefcondition,fthefdlf outputfwillfremainfhighfduringfthef15fssfchargingfcycles.ff forfallfotherffaults,fthefdlfoutputfwillfremainflow.ffinfallf cases,fduringfthef16thfssfchargingfcycle,fdlfwillfdriveflowf forftypicallyf30usecfasfthefcontrollerfinitializesfafsoft-startf cycle.f applications information (continued)
SC508/SC508a 20 applications information (continued) vdda uvlo and por thefvddafunder-voltageflock-outf(uvlo)fcircuitryfinhib - itsfswitchingfandftri-statesfthefdh/dlfdriversfuntilfvddaf risesfabovef3.9v.ffwhenfvddafexceedsf3.9v,fanfinternalf porf(power-onfreset)fresetsftheffaultflatchfandfthefsoft- startfcircuitryfandfthenfthefSC508fisfreadyftofbeginfafsoft- startfcycle.fthefswitcherfwillfshutfoffiffvddaffallsfbelowf 3.6v.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 fuvloffunctionfhasfaftypicalfthresholdfoff1.56vf onfthefv inf risingfedge.fftheffallingfedgefthresholdfisf1.52v. timingfisfimportantfwhenfdrivingfenlfwithflogicfandfnotf usingfthefv in fuvlofcapability.ffthefenlfpinfmustftransitionf fromfhighftoflowfwithinf2fswitchingfcyclesftofavoidfthef pwmfoutputfturningfof.ffiffenlfgoesfbelowfthefv in fuvlof thresholdfandfstaysfabovef1v,fthenfthefswitcherfwillfturnf offbutfthefldofwillfremainfon. notefthatfitfisfpossibleftofoperatefthefswitcherfwithfthef ldofdisabled,fbutfthefenlfpinfmustfbefbelowftheflogicf lowfthresholdf(0.4vfmaximum),fotherwisefthefv in fuvlof functionfwillfdisablefthefswitcher. thefnextftablefsummarizesftheffunctionfoffthefenlfandfenf pins,fwithfrespectftofthefrisingfedgefoffenl.ff en enl ldo status switcher status low low,ff1.56v on of high high,f>f1.56v on on figuref13fshowsfthefenlfvoltagefthresholdsfandftheirf efectfonfldofandfswitcherfoperation. agnd enl low threshold ( min 0 . 4 v ) 1 . 56 v 1 . 52 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 13 enl thresholds enl logic control of pwm operation whenfthefenlfinputfexceedsfthefv in fuvlofthresholdfoff 1.56v,finternalflogicfchecksfthefpgoodfsignal.ffiffpgoodf isfhigh,fthefswitcherfisfalreadyfrunningfandfthefldofwillf startfwithoutfafectingfthefswitcher.ffiffpgoodfisflow,fthef devicefdisablesfpwmfswitchingfuntilfthefldofoutputfhasf reachedf90%foffitsffnalfvalue.ffthisfdelayfpreventsfthef additionalfcurrentfneededfbyfthefdhfandfdlfgatefdrivesf fromfoverloadingfthefldofatfstart-up. ldo start-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(typicallyf20ma).ffthisfprotectsfthefldoffromfthermalf damagefiffthefvldofpinfisfshortedftofground.ffasfv ldo f exceedsf1v,fthefstart-upfcurrentfgraduallyfincreasesftof 1. 2. 3.
SC508/SC508a 21 applications information (continued) 80ma.f fwhenfv ldo f reachesf 4.5v,f thef ldof currentf limitf increasesftof200mafandfthefldofoutputfrisesfquicklyftof 5.0v.ffthefldofstart-upfproflefisfshownfinffiguref14. f 5 v 4 . 5 v 20 ma constant current voltage regulating with 200 ma current limit 1 . 0 v vldo increasing current f figure 14 ldo start-up ldo switch-over operation thefSC508fincludesfafswitch-overffunctionfforfthefldo.f thefswitch-overffunctionfisfdesignedftofincreasefefciencyf byfusingfthefmorefefcientfdc-dcfconverterftofpowerfthef ic,favoidingftheflessfefcientfldofregulatorfwhenfpossible.f thefswitch-overffunctionfconnectsfthefvldofpinfdirectlyf tofthefvoutfpinfthroughfanfinternalfswitch.ffwhenfthef switch-overf isf completef thef ldof isf turnedf off,f whichf reducesfoperatingfpowerfloss.fiffthefldofoutputfisfusedftof biasfthefSC508,fthenfafterfswitch-overfthefdevicefisfself- poweredf fromf thef switchingf regulatorf withf thef ldof turnedfof. afterfthefswitcherfcompletesfsoft-startfandfthefpgoodf delay,fthefswitch-overflogicfwaitsfforf32fswitchingfcyclesf beforefitfstartsfthefswitch-over.ftherefareftwofmethodsfoff completingfthefswitch-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-overfmosfetfparasiticfdiodes thefswitch-overfmosfetfcontainsfparasiticfdiodesfthatf arefinherentftofitsfconstruction,fasfshownfinffiguref15. switchover mosfet parasitic diode parasitic diode vdda vldo vout switchover control figure 15 switch-over mosfet parasitic diodes itfisfimportantftofpreventfforwardfbiasfoffthesefdiodes.ff theffollowingftwofconditionsfmustfbefsatisfedfinforderfforf thefparasiticfdiodesftofstayfof. vddaf > ffv ldof fffff vddaf > ffv out f iffeitherfv ldo forffv out fisfhigherfthanfvdda,fthefrespectivef diodefwillfturnfonfandfthefSC508foperatingfcurrentfwillf thenffowfthroughfthisfdiode.ffthisfhasfthefpotentialfoff damagingfthefdevice. usingfthefinternalfldoftofbiasfthefSC508 theffollowingfstepsfmustfbeffollowedfwhenfusingfthef internalfldoftofbiasfthefdevice. connectf vddaf andf vddpf tof vldof beforef enablingfthefldo. duringfthefinitialfstart-upfthefldo,fwhenfthef ldofoutputfisflessfthanf1v,fthefexternalfloadf shouldfnotfexceedf10ma.ffabovef1v,fanyfexter - nalfloadfonfvldofshouldfnotfexceedf40mafuntilf thefldofvoltagefhasfreachedf4.5v.ff whenfthefswitch-overffeaturefisfusedfandfthefvdda/vddpf powerfcomesffromfthefvoutfpin,ffthefenfandfenlfinputsf mustfbefusedfcarefully.ffdofnotfconnectfthefenfpinfdirectlyf tofvddaforfanotherfsupplyfvoltage.ffiffthisfisfdone,fdrivingf ? ? ? ?
SC508/SC508a 22 applications information (continued) thefenlfpinflowf(tofagnd)fwillfturnfoffthefldofandfthef ldofswitch-overfmosfet,fbutfthefswitcherfwillfcontinuef operating.ffv out fwillffeedfintofthefldofoutputfandfthef vdda/vddpffsuppliesfthroughfthefinternalfparasiticfdiode.ff thisfcanfpotentiallyfdamagefthefdevice,fandfalsofpreventsf thefswitcherffromfshuttingfoffuntilfthefvddafsupplyfdropsf belowfthefvddafuvlofthreshold.ffforfthesefapplicationsfaf dedicatedflogicfsignalfisfrequiredftofdrivefenflowfandf disablefthefswitcher.ffthisfsignalfcanfbefcombinedfwithf thefenlfsignalfiffneeded,fasflongfasfthefenfpinfdoesfnotf exceedfabsolutefmaximumfratings. ldo usage at low input voltage applicationsfrequiringfsteady-stateforftransientfoperationf atflowfinputfvoltagesf(v in fbelowf6.5v)fmayfusefthefinternalf ldoftofbiasfthefvdda/vddpfpinsfwithinflimitations.fftheref areflimitationsftofbothfstartupfandfnormalfoperationfasf explainedfbelow. whenfstartingfupfusingfthefinternalfldo,fswitcherfopera - tionfisfinhibitedfuntilfthefldofoutputfreachesf4.5v.ffduringf thisftime,fthefldofstart-upfisfimplementedfusingfafcurrentf source.ffatflowfv in fitfisfimportantftofnotfapplyfanfexternalf loadftofthefldo,finforderftofallowfthefldofoutputftofreachf thef4.5vfthresholdfandfallowfswitchingftofbegin.ffff oncefswitchingfbegins,fldofoperationftransitionsffromf current-sourcef operationf tof voltagef regulation.f fthef minimumfoperatingfv in fisfthenflimitedfbyfthefrds on foffthef internalfldofmosfet.ffthefcurrentfrequiredftofpowerfthef SC508fandfexternalfmosfetfgatesfcausesfafvoltagefdropf fromfthefv in fpinftofthefvldofpin.ffthefvldofpinfmustfstayf abovef4.5v,fotherwisefthefldofcontrolfwillfrevertfbackftof current-sourcefoperation,fcausingfmorefvoltagefdropfatf thefldofoutput.ffthefrds on foffthefldofmosfetfatflowfv in fisf typicallyf24fohmsfatf25c.fffffffffffff 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=f28vf + f10% v out f=f1.8vf + f4% f sw f=f220khz loadf=f8afmaximum 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. out in on ton v 28pf 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 = 266 ns at 30.8v in , 1.8v out , 220khz substitutingfforfr ton fresultsfinftheffollowingfsolution. r ton = 156k?, use r ton = 154k? ? ? ? ? ? ? ? ?
SC508/SC508a 23 applications information (continued) 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%fxf8aforf4a.f ftoffndfthefminimumfinductancefneeded,fusefthefv in fandf t on fvaluesfthatfcorrespondftofv inmax .f + $ qv / u af slightlyf smallerf valuef off 1.8hf isf selected.fthisf willf increasefthefmaximumfi ripple ftof4.3a. notefthatfthefinductorfmustfbefratedfforfthefmaximumfdcf loadfcurrentfplusf1/2foffthefripplefcurrent. thefripplefcurrentfunderfminimumfv in fconditionsfisfalsof checkedfusingftheffollowingfequations. qv qv 9 9 5 s) 7 ,10, 1 28 7 72 1 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. thef maximumf ripplef currentf off 4.3af createsf af ripplef voltagefacrossfthefesr.ffthefmaximumfesrfvaluefallowedf isfshownfbyftheffollowingfequations. $ p 9 , 9 (65 5,33/(0$; 5,33/ ( 0$; esr max p 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 SC508/SC508a 24 applications information (continued) causefafpeakfvoltagefonfthefcapacitorfaccordingftofthef followingfequation. 2 out 2 peak 2 ripplemax out min v v i 2 1 i l cout ? 1 ? u assumingfafpeakfvoltagefv peak foff1.98f(180mvfrisefuponf loadfrelease),fandfaf8afloadfrelease,fthefrequiredfcapaci - tancefisfshownfbyfthefnextfequation. 2 2 2 min 1.80 1.98 4.3 2 1 8 h 1.8 cout ? 1 ? u p cout min = 272f iffthefloadfreleasefisfrelativelyfslow,fthefoutputfcapacitancef canfbefreduced.ffatfheavyfloadsfduringfnormalfswitching,f whenftheffbfpinfisfabovefthef600mvfreference,fthefdlf outputfisfhighfandftheflow-sidefmosfetfisfon.ffduringfthisf time,fthefvoltagefacrossfthefinductorfisfapproximatelyf f -v out .ffthisfcausesfafdown-slopeforffallingfdi/dtfinfthef inductor.ffiffthefloadfdi/dtfisfnotffasterfthanfthef-di/dtfinf thefinductor,fthenfthefinductorfcurrentfwillftendftoftrackf theffallingfloadfcurrent.ffthisfwillfreducefthefexcessfinduc - tivefenergyfthatfmustfbefabsorbedfbyfthefoutputfcapaci - tor,fthereforefafsmallerfcapacitancefcanfbefused.f theffollowingfcanfbefusedftofcalculatefthefneededfcapaci - tancefforfafgivenfdi load /dt.ffpeakfinductorfcurrentfisfshownf byfthefnextfequation. i lpk = i max + 1/2 x i ripplemax i lpk = 8 + 1/2 x 4.3 = 10.2a dt dl current load of change of rate load i max = maximum load release = 10a out pk load max out lpk lpk out v v 2 dt dl i v i l i c u u u example s a 5 . 2 dt dl load p thisfwouldfcausefthefoutputfcurrentftofmoveffromf10aftof zerofinf4sfasfshownfbyftheffollowingfequation. 1.8 1.98 2 s 1 2.5 8 1.8 10.2 h 1.8 10.2 c out u p u u p u c out = 198f notefthatfc out fisfmuchfsmallerfinfthisfexample,f198ff comparedftof272ffbasedfonfafworst-casefloadfrelease.ftof meetf thef twof designf criteriaf off minimumf 272ff andf maximumf16.7m ?fesr ,fselectfafcapacitorfratedfatf330ff andf6m ?fesr . itfisfrecommendedfthatfanfadditionalfsmallfcapacitorfwithf afvaluefoff1ftof10ffbefplacedfinfparallelfwithfc out finforderf tofflterfhighffrequencyfswitchingfnoise. 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 shownfinffiguref16.ffthisfcapacitorfshouldfbefleftfunpopu - latedf unlessf itf canf bef confirmedf thatf double-pulsingf
SC508/SC508a 25 applications information (continued) 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 ' '
SC508/SC508a 26 applications information (continued) figuref18fshowsfthefequivalentfcircuitfforfcalculatingfthef magnitudefoffthefripplefcontributionfatftheffbfpinfdueftofc l .f 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.ffsincefthefSC508ffregu - 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
SC508/SC508a 27 applications information (continued) 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 - tionsfitftripsfwhenftheffeedbackfpinfisf600mv,f + f 1%. thefon-timefpulseffromfthefSC508finfthefdesignfexamplef 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.f thefswitchingffrequencyfalsofvariesfwithfloadfcurrentfasfaf resultfoffthefpowerflossesfinfthefmosfetsfandfthefinduc - 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
SC508/SC508a 28 applications information (continued) 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 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 andfthefpgndfpinfoffthefSC508.ff anfinnerflayerfgroundfplanefisfrecommended.ff eachfpowerfcomponentfrequiresfafshort,flowf impedancefconnectionftofthefpgndffplane. placefviasftofthefpgndfplanefdirectlyfnearfthef componentfpins. usefshortfwideftracesfforfthefpinfconnectionsf fromfthefSC508f(lx,fdh,fdlfandfbst).fdofnotf routeftheseftracesfnearfthefsensitiveflow-noisef analogfsignalsf(fb,ffbl,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 ? ? ? ? ? ? ? ? ? ? ? ? ?
SC508/SC508a 29 applications information (continued) 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. 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. ? ? ? ? ?
SC508/SC508a 30 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 .
SC508/SC508a 31 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
SC508/SC508a 32 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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