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APPLICATIONINFORMATION
TPS794xx
GNDEN NR
在 输出
V
在
V
输出
0.1
µ
F
0.01
µ
F
2.2
µ
F
ExternalCapacitorRequirements
tps79401,tps79418
tps79425,tps79428
tps79430,tps79433
SLVS349D–NOVEMBER2001–REVISEDOCTOBER2004
thetps794xxfamilyoflow-落后(ldo)regulatorshasbeenoptimizedforuseinnoise-sensitiveequipment.
thedevicefeaturesextremelylowdropoutvoltages,highpsrr,ultralowoutputnoise,lowquiescentcurrent,
andenableinputtoreducesupplycurrentstolessthan1µawhentheregulatoristurnedoff.
atypicalapplicationcircuitisshowninfigure22.
figure22.typicalapplicationcircuit
a0.1-µforlargerceramicinputbypasscapacitor,connectedbetweeninandgndandlocatedclosetothe
tps794xx,isrequiredforstability.itimprovestransientresponse,noiserejection,andripplerejection.一个
高等级的-valueinputcapacitormaybenecessaryiflarge,快-上升-timeloadtransientsareanticipatedorthedevice
islocatedseveralinchesfromthepowersource.
likemostlowdropoutregulators,thetps794xxrequiresanoutputcapacitorconnectedbetweenoutandgnd
tostabilizetheinternalcontrolloop.theminimumrecommendedcapacitanceis2.2µf.any2.2µforlarger
ceramiccapacitorissuitable.
theinternalvoltagereferenceisakeysourceofnoiseinanldoregulator.thetps794xxhasannrpinwhich
isconnectedtothevoltagereferencethrougha250-k
Ω
internalresistor.the250-k
Ω
internalresistor,在
conjunctionwithanexternalbypasscapacitorconnectedtothenrpin,createsalow-passfiltertoreducethe
voltagereferencenoiseand,因此,thenoiseattheregulatoroutput.inorderfortheregulatortooperate
合适的,thecurrentflowoutofthenrpinmustbeataminimum,becauseanyleakagecurrentcreatesanir
dropacrosstheinternalresistorthuscreatinganoutputerror.因此,thenrcapacitormusthaveminimal
leakagecurrent.thebypasscapacitorshouldbenomorethan0.1µftoensurethatitisfullychargedduringthe
quickstarttimeprovidedbytheinternalswitchshowninthefunctionalblockdiagram.
forexample,thetps79428exhibitsonly32µv
RMS
ofoutputvoltagenoiseusinga0.1-µfceramicnrcapacitor
anda2.2-µfceramicoutputcapacitor.notethattheoutputstartsupslowerasthebypasscapacitance
increasesduetotherctimeconstantatthenrpinthatiscreatedbytheinternal250-k
Ω
resistorandexternal
电容.
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