LTC3541 datasheet(13/20 Pages) LINER

13 / 20 page

LTC3541

3541fa

APPLICATIO S I FOR ATIO

resistanceofCOUT.ΔILOADalsobeginstochargeordis-

chargeCOUT,whichgeneratesafeedbackerrorsignal.The

regulatorloopthenactstoreturnVOUTtoitssteady-state

value.DuringthisrecoverytimeVOUTcanbemonitored

forovershootorringingthatwouldindicateastability

problem.Foradetailedexplanationofswitchingcontrol

looptheoryseeApplicationNote76.

Asecond,moreseveretransientiscausedbyswitching

inloadswithlarge(>1

µF)supplybypasscapacitors.The

dischargedbypasscapacitorsareeffectivelyputinparal-

lelwithCOUT,causingarapiddropinVOUT.Noregulator

candeliverenoughcurrenttopreventthisproblemifthe

loadswitchresistanceislowanditisdrivenquickly.The

onlysolutionistolimittherisetimeoftheswitchdrive

so that the load rise time is limited to approximately

(25•CLOAD).Thus,a10µFcapacitorchargingto3.3V

wouldrequirea250

µsrisetime,limitingthecharging

currenttoabout130mA.

VLDO/LINEAR REGULATOR

Adjustable Output Voltage

TheLTC3541LVOUToutputvoltageissetbytheratiooftwo

externalresistorsasshowninFigure7.Thedeviceservos

LVOUTtomaintaintheLFBpinvoltageat0.4V(referenced

toground).Thus,thecurrentinR1isequalto0.4V/R1.

Forgoodtransientresponse,stability,andaccuracy,the

currentinR1shouldbeatleast2

µA,thusthevalueof

R1shouldbenogreaterthan200k.ThecurrentinR2is

thecurrentinR1plustheLFBpinbiascurrent.Sincethe

LFBpinbiascurrentistypically<10nA,itcanbeignored

intheoutputvoltagecalculation.Theoutputvoltagecan

becalculatedusingtheformulainFigure8.Notethatin

shutdowntheoutputisturnedoffandthedividercurrent

willbezeroonceCOUTisdischarged.

TheLTC3541VLDOandlinearregulatorloopsoperate

atarelativelyhighgainof–3.5

µV/mAand–3.4µV/mA

respectively,referredtotheLFBinput.Thus,aloadcur-

rentchangeof1mAto300mAproducesa1.05mVdrop

attheLFBinputfortheVLDOandaloadcurrentchange

of1mAto30mAproducesa0.1mVdropattheLFBinput

forthelinearregulator.Tocalculatethechangereferred

totheoutputsimplymultiplybythegainofthefeedback

network(i.e.,1+R2/R1).Forexample,toprogramthe

outputfor1.2VchooseR2/R1=2.Inthisexample,an

outputcurrentchangeof1mAto300mAproduces1.05mV

•(1+2)=3.15mVdropattheoutput.

SincetheLFBpinisrelativelyhighimpedance(depending

ontheresistordividerused),straycapacitanceatthispin

shouldbeminimized(<10pF)topreventphaseshiftinthe

erroramplifierloop.Additionally,specialattentionshould

begiventoanystraycapacitancesthatcancoupleexternal

signalsontotheLFBpinproducingundesirableoutput

ripple.ForoptimumperformanceconnecttheLFBpinto

R1andR2withashortPCBtraceandminimizeallother

straycapacitancetotheLFBpin.

Output Capacitance and Transient Response

TheLTC3541isdesignedtobestablewithawiderangeof

ceramicoutputcapacitors.TheESRoftheoutputcapaci-

toraffectsstability,mostnotablywithsmallcapacitors.

A minimum output capacitor of 2.2

µF with an ESR of

0.05

Ωorlessisrecommendedtoensurestability.The

LTC3541VLDOisamicropowerdeviceandoutputtran-

sientresponsewillbeafunctionofoutputcapacitance.

Largervaluesofoutputcapacitancedecreasethepeak

deviationsandprovideimprovedtransientresponsefor

largerloadcurrentchanges.Notethatbypasscapacitors

usedtodecoupleindividualcomponentspoweredbythe

LTC3541willincreasetheeffectiveoutputcapacitorvalue.

HighESRtantalumandelectrolyticcapacitorsmaybeused,

butalowESRceramiccapacitormustbeinparallelatthe

output.ThereisnominimumESRormaximumcapacitor

sizerequirement.

Figure 7. Programming the LTC3541

( )

LVOUT

LTc3541

3541 F07

cOUT

VOUT = 0.4V 1 +

LFB

GND

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