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APPLICATIONINFORMATION
V
在
V
输出
TPS793xx
GNDEN NR
在 输出
V
在
V
输出
0.1
µ
F
0.01
µ
F
2.2
µ
F
ExternalCapacitorRequirements
BoardLayoutRecommendationtoImprovePSRRandNoisePerformance
tps79301,tps79318
tps79325,tps79328,tps793285
tps79330,tps79333,tps793475
SLVS348H–JULY2001–REVISEDOCTOBER2004
thetps793xxfamilyoflow-落后(ldo)regulatorshasbeenoptimizedforuseinnoise-敏感的
电池-operatedequipment.thedevicefeaturesextremelylowdropoutvoltages,highpsrr,ultralowoutput
噪音,lowquiescentcurrent(170µatypically),andenable-inputtoreducesupplycurrentstolessthan1µa
whentheregulatoristurnedoff.
atypicalapplicationcircuitisshowninfigure22.
figure22.typicalapplicationcircuit
a0.1-µforlargerceramicinputbypasscapacitor,connectedbetweeninandgndandlocatedclosetothe
tps793xx,isrequiredforstabilityandimprovestransientresponse,noiserejection,andripplerejection.一个
高等级的-valueinputcapacitormaybenecessaryiflarge,快-上升-timeloadtransientsareanticipatedorthedevice
islocatedseveralinchesfromthepowersource.
likemostlowdropoutregulators,thetps793xxrequiresanoutputcapacitorconnectedbetweenoutandgnd
tostabilizetheinternalcontrolloop.theminimumrecommendedcapacitanceis2.2µf.any2.2-µforlarger
ceramiccapacitorissuitable,providedthecapacitancedoesnotvarysignificantlyovertemperature.ifload
currentisnotexpectedtoexceed100ma,a1.0-µfceramiccapacitorcanbeused.
theinternalvoltagereferenceisakeysourceofnoiseinanldoregulator.thetps793xxhasannrpinwhich
isconnectedtothevoltagereferencethrougha250-k
Ω
internalresistor.the250-k
Ω
internalresistor,在
conjunctionwithanexternalbypasscapacitorconnectedtothenrpin,createsalowpassfiltertoreducethe
voltagereferencenoiseand,因此,thenoiseattheregulatoroutput.inorderfortheregulatortooperate
合适的,thecurrentflowoutofthenrpinmustbeataminimum,becauseanyleakagecurrentcreatesanir
dropacrosstheinternalresistorthuscreatinganoutputerror.因此,thebypasscapacitormusthave
minimalleakagecurrent.thebypasscapacitorshouldbenomorethan0.1-µftoensurethatitisfullycharged
duringthequickstarttimeprovidedbytheinternalswitchshownintheFunctionalBlockDiagrams
asanexample,thetps79328exhibitsonly32µv
RMS
ofoutputvoltagenoiseusinga0.1-µfceramicbypass
capacitoranda2.2-µfceramicoutputcapacitor.notethattheoutputstartsupslowerasthebypasscapacitance
increasesduetotherctimeconstantatthenrpinthatiscreatedbytheinternal250-k
Ω
resistorandexternal
电容.
toimproveacmeasurementslikepsrr,outputnoise,andtransientresponse,itisrecommendedthattheboard
bedesignedwithseparategroundplanesforV
在
andV
输出
,witheachgroundplaneconnectedonlyatthegnd
pinofthedevice.inaddition,thegroundconnectionforthebypasscapacitorshouldconnectdirectlytothegnd
pinofthedevice.
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