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LT3071 Datasheet(PDF) 21 Page - Linear Technology

Part No. LT3071
Description  5A, Low Noise, Programmable Output, 85mV Dropout Linear Regulator with Analog Margining
Download  28 Pages
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT3071 Datasheet(HTML) 21 Page - Linear Technology

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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
2500mm2
2500mm2
30°C/W
1000mm2
2500mm2
2500mm2
32°C/W
225mm2
2500mm2
2500mm2
33°C/W
100mm2
2500mm2
2500mm2
35°C/W
*Deviceismountedontopside


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