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THERMALINFORMATION
P
D
最大值
V
i(avg)
V
o(avg)
I
o(avg)
V
i(avg)
x I
(q)
(1)
一个
B
C
一个
B
C
T
J
一个
R
θ
JC
T
C
B
R
θ
CS
T
一个
C
R
θ
SA
(一个)
(b)
To–263 包装
to–220 包装
tps75801,tps75815
tps75818,tps75825
TPS75833
SLVS330D–JUNE2001–REVISEDMARCH2004
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.
figure21illustratesthesethermalresistancesfor(一个)ato-220packageattachedtoaheatsink,和(b)一个
至-263packagemountedonajedechigh-kboard.
figure21.thermalresistances
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