LT3071EUFD datasheet(22/28 Pages) LINER

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LT3071

3071f

ApplicAtions inFormAtion

Calculating Junction Temperature

Example:Givenanoutputvoltageof0.9V,aninputvoltage

rangeof1.2V±5%,aBIASvoltageof2.5V,amaximumout-

putcurrentof4Aandamaximumambienttemperatureof

50°C,whatwillthemaximumjunctiontemperaturebe?

Thepowerdissipatedbythedeviceequals:

IOUT(MAX)•(VIN(MAX)–VOUT)+(IBIAS–IGND)•VOUT

+IGND•VBIAS

where:

IOUT(MAX)=4A

VIN(MAX)=1.26V

IBIASat(IOUT=4A,VBIAS=2.5V)=6.91mA

IGNDat(IOUT=4A,VBIAS=2.5V)=0.87mA

thus:

P=4A•(1.26V–0.9V)+(6.91mA–0.87mA)•0.9V

+0.87mA•2.5V=1.448W

With the QFN package soldered to maximum copper

area,thethermalresistanceis30°C/W.Sothejunction

temperatureriseaboveambientequals:

1.448Wat30°C/W=43.44°C

Themaximumjunctiontemperatureequalsthemaximum

ambienttemperatureplusthemaximumjunctiontempera-

tureriseaboveambientor:

TJMAX=50°C+43.44°C=93.44°C

Applications that cannot support extensive PCB space

forheatsinkingtheLT3071requireaderatingofoutput

currentorincreasedairflow.

Paralleling Devices for Higher IOUT

MultipleLT3071smaybeparalleledtoobtainhigheroutput

current.Thisparallelingconceptborrowsfromthescheme

employedbytheLT3080.

Toaccomplishthisparalleling,tietheREF/BYPpinsof

theparalleledregulatorstogether.Thiseffectivelygives

anaveragedvalueofmultiple600mVreferencevoltage

sources.TietheOUTpinsoftheparalleledregulatorsto

thecommonloadplanethroughasmallpieceofPCtrace

ballastoranactualsurfacemountsenseresistorbeyond

theprimaryoutputcapacitorsofeachregulator.There-

quiredballastisdependentupontheapplicationoutput

voltageandpeakloadcurrent.Therecommendedballast

isthatvaluewhichcontributes1%toloadregulation.For

example,twoLT3071regulatorsconfiguredtooutput1V,

sharinga10Aloadrequire2mΩofballastateachoutput.

TheKelvinSENSEpinsconnecttotheregulatorsideof

theballastresistorstokeeptheindividualcontrolloops

fromconflictingwitheachother(seeFigures8and9).

Keepthisballasttraceareafreeofsoldertomaintaina

controlledresistance.

Table4showsasimpleguidelineforPCBtraceresistance

asafunctionofweightandtracewidth.

Table 4. PC Board Trace Resistance

WEIGHT (Oz)

100 MIL WIDTH*

200 MIL WIDTH*

5.43

2.71

1.36

*Traceresistanceismeasuredinmilliohms/in

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