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capacitorrecommendationsforthepth08080wwide-outputadjust
InputCapacitor
outputcapacitors(optional)
CeramicCapacitors
TantalumCapacitors
CapacitorTable
PTH08080W
SLTS235A–FEBRUARY2005–REVISEDMARCH2005
POWERMODULES
theminimumrecommendedinputcapacitoris100µfofcapacitance.whenv
O
>3.4v,the100-µfelectrolytic
capacitancemustberatedfor650-marmsripplecurrent.forv
O
<3.4v,theripplecurrentratingmustbeat
least500marms.theripplecurrentratingofelectrolyticcapacitorsisamajorconsiderationwhentheyareused
attheinput.
whenspecifyingregulartantalumcapacitorsforuseattheinput,aminimumvoltageratingof2
×
(maximumdc
voltage+acripple)ishighlyrecommended.thisisstandardpracticetoensurereliability.polymer-tantalum
capacitorsarenotaffectedbythisrequirement.
forimprovedripplereductionontheinputbus,ceramiccapacitorscanalsobeaddedtocomplementthe
requiredelectrolyticcapacitance.
nooutputcapacitanceisrequiredfornormaloperation.不管怎样,applicationswithloadtransients(sudden
changesinloadcurrrent)canbenefitbyaddingexternaloutputcapacitance.a100-µfelectrolyticorceramic
capacitorcanbeusedtoimprovetransientresponse.addinga100-µfnonceramiccapacitorallowsthemodule
tomeetitstransientresponsespecification.ahigh-qualitycomputer-gradeelectrolyticcapacitorshouldbe
足够的.
electrolyticcapacitorsshouldbelocatedclosetotheloadcircuit.thesecapacitorsprovidedecouplingoverthe
frequencyrange,2khzto150khz.aluminumelectrolyticcapacitorsaresuitableforambienttemperatures
above0
°
c.foroperationbelow0
°
c,tantalumoros-con-typecapacitorsarerecommended.whenusingoneor
morenonceramiccapacitors,thecalculatedequivalentesrshouldbenolowerthan10m
Ω
(14m
Ω
usingthe
生产者'smaximumesrforasinglecapacitor).alistofpreferredlow-等效串联电阻-typecapacitorsareidentifiedin
table3,therecommendedcapacitortable.
above150khz,theperformanceofaluminumelectrolyticcapacitorsbecomeslesseffective.tofurtherimprove
thereflectedinputripplecurrent,ortheoutputtransientresponse,multilayerceramiccapacitorsmustbeadded.
ceramiccapacitorshavelowesrandtheirresonantfrequencyishigherthanthebandwidthoftheregulator.
WhenplacedattheoutputtheircombinedESRisnotcriticalaslongasthetotalvalueofceramiccapacitance
doesnotexceed200µf.也,topreventtheformationoflocalresonances,donotexceedthemaximum
numberofcapacitorsspecifiedinthecapacitortable.
additionaltantalum-typecapacitorscanbeusedatboththeinputandoutput,andarerecommendedfor
applicationswheretheambientoperatingtemperaturecanbelessthan0
°
c.theavxtps,sprague
593d/594/595,andkemett495/t510/t520capacitorsseriesaresuggestedovermanyothertantalumtypesdue
totheirratedsurge,powerdissipation,andripplecurrentcapability.asacaution,manygeneral-目的
tantalumcapacitorshaveconsiderablyhigheresr,reducedpowerdissipation,andlowerripplecurrent
能力.thesecapacitorsarealsolessreliableastheyhavelowerpowerdissipationandsurgecurrentratings.
TantalumcapacitorsthatdonothaveastatedESRorsurgecurrentratingarenotrecommendedforpower
产品.whenspecifyingos-conandpolymertantalumcapacitorsfortheoutput,theminimumesrlimitis
encounteredwellbeforethemaximumcapacitancevalueisreached.
thecapacitortable,table3,identifiesthecharacteristicsofcapacitorsfromanumberofvendorswith
acceptableesrandripplecurrent(rms)比率.therecommendednumberofcapacitorsrequiredatboththe
inputandoutputbusesisidentifiedforeachcapacitortype.thisisnotanextensivecapacitorlist.电容
fromothervendorsareavailablewithcomparablespecifications.thoselistedareforguidance.thermsrating
andesr(at100khz)arecriticalparametersnecessarytoensurebothoptimumregulatorperformanceandlong
capacitorlife.
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