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PowerDissipationandJunctionTemperature
P
D
(
最大值
)
T
J
最大值
T
一个
R
JA
(1)
P
D
V
在
V
输出
I
输出
(2)
ProgrammingtheTPS79301AdjustableLDORegulator
V
输出
V
REF
1
R
1
R
2
(3)
R
1
V
输出
V
ref
1
R
2
(4)
C
1
(3 x 10
7
) x (r
1
R
2
)
(r
1
x R
2
)
(5)
tps79301,tps79318
tps79325,tps79328,tps793285
tps79330,tps79333,tps793475
SLVS348H–JULY2001–REVISEDOCTOBER2004
applicationinformation(持续)
Specifiedregulatoroperationisassuredtoajunctiontemperatureof125
°
c;themaximumjunctiontemperature
shouldberestrictedto125
°
cundernormaloperatingconditions.thisrestrictionlimitsthepowerdissipationthe
regulatorcanhandleinanygivenapplication.toensurethejunctiontemperatureiswithinacceptablelimits,
calculatethemaximumallowabledissipation,p
d(最大值)
,andtheactualdissipation,p
D
,whichmustbelessthanor
equaltoP
d(最大值)
.
themaximumpowerdissipationlimitisdeterminedusingequation1:
在哪里:
•
T
J
maxisthemaximumallowablejunctiontemperature.
•
R
θ
JA
isthethermalresistancejunction-至-ambientforthepackage(seethedissipationratingstable).
•
T
一个
istheambienttemperature.
theregulatordissipationiscalculatedusingequation2:
powerdissipationresultingfromquiescentcurrentisnegligible.excessivepowerdissipationtriggersthethermal
protectioncircuit.
TheoutputvoltageoftheTPS79301adjustableregulatorisprogrammedusinganexternalresistordivideras
showninfigure23.theoutputvoltageiscalculatedusingequation3:
在哪里:
•
V
REF
=1.2246vtyp(theinternalreferencevoltage)
resistorsr1andr2shouldbechosenforapproximately50-µadividercurrent.lowervalueresistorscanbe
usedforimprovednoiseperformance,butthesolutionconsumesmorepower.higherresistorvaluesshouldbe
avoidedasleakagecurrentinto/outoffbacrossr1/r2createsanoffsetvoltagethatartificiallyin-
creases/decreasesthefeedbackvoltageandthuserroneouslydecreases/increasesv
输出
.therecommended
designprocedureistochooser2=30.1k
Ω
tosetthedividercurrentat50µa,c1=15pfforstability,andthen
calculater1usingequation4:
inordertoimprovethestabilityoftheadjustableversion,itissuggestedthatasmallcompensationcapacitorbe
placedbetweenoutandfb.forvoltages<1.8v,thevalueofthiscapacitorshouldbe100pf.forvoltages
>1.8v,theapproximatevalueofthiscapacitorcanbecalculatedasshowninequation5:
thesuggestedvalueofthiscapacitorforseveralresistorratiosisshowninthetablebelow.ifthiscapacitoris
notused(suchasinaunity-gainconfiguration)orifanoutputvoltage<1.8vischosen,thentheminimum
recommendedoutputcapacitoris4.7µfinsteadof2.2µf.
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