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LT3071 Datasheet(PDF) 22 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) 22 Page - Linear Technology

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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*
1
5.43
2.71
2
2.71
1.36
*Traceresistanceismeasuredinmilliohms/in


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