LT3071EUFD datasheet(21/28 Pages) LINER

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LT3071

3071f

ApplicAtions inFormAtion

Withahighinputvoltage,aproblemcanoccurwherethe

removalofanoutputshortwillnotallowtheoutputvolt-

agetorecover.Otherregulatorswithcurrentlimitfoldback

alsoexhibitthisphenomenon,soitisnotuniquetothe

LT3071.Theloadlineforsuchaloadmayintersectthe

outputcurrentcurveattwopoints:normaloperationand

theSOArestrictedloadcurrentsettings.Acommonsitu-

ationisimmediatelyaftertheremovalofashortcircuit,

butwithastaticload≥1A.Inthissituation,removalofthe

loadorreductionofIOUTto<1Awillclearthiscondition

andallowVOUTtoreturntonormalregulation.

Reverse Voltage

TheLT3071incorporatesacircuitthatdetectsifVINde-

creasesbelowVOUT.Thisreverse-voltagedetectorhas

atypicalthresholdofabout(VIN–VOUT)=–6mV.Ifthe

thresholdisexceeded,thisdetectorcircuitturnsoffthe

drivetotheinternalNMOSpasstransistor,therebyturning

offtheoutput.Theoutputpullslowwiththeloadcurrent

dischargingtheoutputcapacitance.Thiscircuit’sintent

istolimitandpreventback-feedcurrentfromOUTtoIN

iftheinputvoltagecollapsesduetoafaultoroverload

condition.

Thermal Considerations

The LT3071’s maximum rated junction temperature of

125°Climitsitspowerhandlingcapabilityandisdomi-

natedbytheoutputcurrentmultipliedbytheinput/output

voltagedifferential:

IOUT•(VIN–VOUT)

TheLT3071’sinternalpowerandthermallimitingcircuitry

protectitunderoverloadconditions.Forcontinuousnor-

malloadconditions,donotexceedthemaximumjunction

temperatureof125°C.Givecarefulconsiderationtoall

sourcesofthermalresistancefromjunctiontoambient.

Thisincludesjunctiontocase,case-to-heatsinkinterface,

heatsinkresistanceorcircuitboardtoambientasthe

applicationdictates.Also,consideradditionalheatsources

mountedinproximitytotheLT3071.TheLT3071isasurface

mountdeviceandassuch,heatsinkingisaccomplished

byusingtheheatspreadingcapabilitiesofthePCboard

and its copper traces. Surface mount heat sinks and

platedthrough-holescanalsobeusedtospreadtheheat

generated by power devices. Junction-to-case thermal

resistanceisspecifiedfromtheICjunctiontothebottom

ofthecasedirectlybelowthedie.Thisisthelowestresis-

tancepathforheatflow.Propermountingisrequiredto

ensurethebestpossiblethermalflowfromthisareaofthe

packagetotheheatsinkingmaterial.Notethattheexposed

padiselectricallyconnectedtoGND.

Table3liststhermalresistanceasafunctionofcopper

areainafixedboardsize.Allmeasurementsweretaken

instillairona4-layerFR-4boardwith1ozsolidinternal

planesand2oztop/bottomexternaltraceplaneswitha

totalboardthicknessof1.6mm.PCBlayers,copperweight,

boardlayoutandthermalviasaffecttheresultantthermal

resistance.Forfurtherinformationonthermalresistance

andhighthermalconductivitytestboards,refertoJEDEC

standard JESD51, notably JESD51-12 and JESD51-7.

Achievinglowthermalresistancenecessitatesattention

todetailandcarefulPCBlayout.

Table 3, UFD Plastic Package, 28-Lead QFN

COPPER AREA

BOARD AREA

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

TOPSIDE* BACK SIDE

2500mm2

30°C/W

1000mm2

2500mm2

32°C/W

225mm2

2500mm2

33°C/W

100mm2

2500mm2

35°C/W

*Deviceismountedontopside

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