LTC3541 datasheet(14/20 Pages) LINER

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LTC3541

3541fa

APPLICATIO S I FOR ATIO

Extraconsiderationmustbegiventotheuseofceramic

capacitors.Ceramiccapacitorsaremanufacturedwitha

varietyofdielectrics,eachwithdifferentbehavioracross

temperature and applied voltage. The most common

dielectricsusedareZ5U,Y5V,X5RandX7R.TheZ5U

andY5Vdielectricsaregoodforprovidinghighcapaci-

tancesinasmallpackage,butexhibitlargevoltageand

temperaturecoefficientsasshowninFigures8and9.

Whenusedwitha2Vregulator,a1µFY5Vcapacitorcan

loseasmuchas75%ofitsinitialcapacitanceoverthe

operatingtemperaturerange.TheX5RandX7Rdielectrics

resultinmorestablecharacteristicsandareusuallymore

suitableforuseastheoutputcapacitor.TheX7Rtypehas

betterstabilityacrosstemperature,whiletheX5Risless

expensiveandisavailableinhighervalues.Inallcases,

theoutputcapacitanceshouldneverdropbelow1µFor

instabilityordegradedperformancemayoccur.

EFFICIENCY CONSIDERATIONS

Generally,theefficiencyofaregulatorisequaltotheout-

putpowerdividedbytheinputpowertimes100%.Itis

oftenusefultoanalyzeindividuallosstermstodetermine

whichtermsarelimitingefficiencyandwhatifanychange

wouldyieldthegreatestimprovement.Efficiencycanbe

expressedas:

Efficiency=100%–(L1+L2+L3+...)

whereL1,L2,etc.aretheindividuallosstermsasaper-

centageofinputpower.

Althoughalldissipativeelementsinthecircuitproduce

losses,threemainsourcestypicallyaccountforthemajority

ofthelossesintheLTC3541circuits:VINquiescentcurrent,

I2RlossesandlossacrossVLDOoutputdevice.When

operatingwithboththebuckandVLDOactive(ENBUCK

andENVLDOequaltologichigh),VINquiescentcurrent

lossandlossacrosstheVLDOoutputdevicedominate

theefficiencylossatlowloadcurrents,whereastheI2R

lossandlossacrosstheVLDOoutputdevicedominate

theefficiencylossatmediumtohighloadcurrents.At

lowloadcurrentswiththepartoperatingwiththelinear

regulator(ENBUCKequaltologiclow,ENVLDOequalto

logichigh),efficiencyistypicallydominatedbytheloss

acrossthelinearregulatoroutputdeviceandVINquiescent

current.Inatypicalefficiencyplot,theefficiencycurveat

verylowloadcurrentscanbemisleadingsincetheactual

powerlostisoflittleconsequence.

1.TheVINquiescentcurrentlossinthebuckisduetotwo

components:theDCbiascurrentasgivenintheElectrical

Characteristicsandtheinternalmainswitchandsynchro-

nousswitchgatechargecurrents.Thegatechargecurrent

resultsfromswitchingthegatecapacitanceoftheinternal

powerswitches.Eachtimethegateisswitchedfromhigh

tolowtohighagain,apacketofcharge,dQ,movesfrom

VINtoground.TheresultingdQ/dtisthecurrentoutof

VINthatistypicallylargerthantheDCbiascurrentand

Dc BIAS VOLTAGE (V)

3541 F08

–20

–40

–60

–80

–100

X5R

Y5V

BOTH cAPAcITORS ARE 1

µF,

10V, 0603 cASE SIZE

Figure 8. Change in Capacitor vs Bias Voltage

TEMPERATURE (

°c)

–50

–100

–80

–60

–40

–20

X5R

Y5V

–25

3541 F09

BOTH cAPAcITORS ARE 1

µF,

10V, 0603 cASE SIZE

Figure 9. Change in Capacitor vs Temperature

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