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RegulatorProtection
THERMALINFORMATION
P
D
最大值
V
i(avg)
V
o(avg)
I
o(avg)
V
i(avg)
x I
(q)
(3)
TPS72501
tps72515,tps72516
tps72518,tps72525
SLVS341D–MAY2002–REVISEDMARCH2004
applicationinformation(持续)
thetps725xxpasselementhasabuilt-inbackdiodethatsafelyconductsreversecurrentwhentheinput
voltagedropsbelowtheoutputvoltage(e.g.,duringpowerdown).currentisconductedfromtheoutputtothe
inputandisnotinternallylimited.ifextendedreversevoltageisanticipated,externallimitingmightbe
适合的.
thetps725xxalsofeaturesinternalcurrentlimitingandthermalprotection.duringnormaloperation,这
tps725xxlimitsoutputcurrenttoapproximately1.6a.whencurrentlimitingengages,theoutputvoltagescales
backlinearlyuntiltheovercurrentconditionends.whilecurrentlimitingisdesignedtopreventgrossdevice
失败,careshouldbetakennottoexceedthepowerdissipationratingsofthepackage.ifthetemperatureofthe
deviceexceeds165
°
c,热的-protectioncircuitryshutsitdown.oncethedevicehascooleddowntobelow
145
°
c,regulatoroperationresumes.
TheamountofheatthatanLDOlinearregulatorgeneratesisdirectlyproportionaltotheamountofpowerit
dissipatesduringoperation.allintegratedcircuitshaveamaximumallowablejunctiontemperature(t
J
最大值)
abovewhichnormaloperationisnotassured.asystemdesignermustdesigntheoperatingenvironmentsothat
theoperatingjunctiontemperature(t
J
)doesnotexceedthemaximumjunctiontemperature(t
J
最大值).thetwo
mainenvironmentalvariablesthatadesignercanusetoimprovethermalperformanceareairflowandexternal
heatsinks.thepurposeofthisinformationistoaidthedesignerindeterminingtheproperoperatingenvironment
foralinearregulatorthatisoperatingataspecificpowerlevel.
ingeneral,themaximumexpectedpower(p
d(最大值)
)consumedbyalinearregulatoriscomputedas:
在哪里:
•
V
i(avg)
istheaverageinputvoltage.
•
V
o(avg)
istheaverageoutputvoltage.
•
I
o(avg)
istheaverageoutputcurrent.
•
I
(q)
isthequiescentcurrent.
formosttildoregulators,thequiescentcurrentisinsignificantcomparedtotheaverageoutputcurrent;
因此,thetermv
i(avg)
xI
(q)
canbeneglected.theoperatingjunctiontemperatureiscomputedbyaddingthe
ambienttemperature(t
一个
)andtheincreaseintemperatureduetotheregulator'spowerdissipation.这
temperatureriseiscomputedbymultiplyingthemaximumexpectedpowerdissipationbythesumofthethermal
resistancesbetweenthejunctionandthecase(r
θ
JC
),thecasetoheatsink(r
θ
CS
),andtheheatsinktoambient
(r
θ
SA
).thermalresistancesaremeasuresofhoweffectivelyanobjectdissipatesheat.典型地,thelargerthe
设备,themoresurfaceareaavailableforpowerdissipationandthelowertheobject'sthermalresistance.
figure20illustratesthesethermalresistancesfor(一个)asot223packagemountedinajedeclow-kboard,和
(b)addpakpackagemountedonajedechigh-kboard.
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