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OutputNoise
V
N
10.7
V
RMS
V
V
输出
(1)
BoardLayoutRecommendationstoImprovePSRRandNoisePerformance
InternalCurrentLimit
关闭
DropoutVoltage
Startup
tps79901,tps79915,tps79916,tps79918
tps79925,tps79927,tps79928,tps799285
tps79929,tps79930,tps79932,tps79933
SBVS056C–JANUARY2005–REVISEDMAY2005
inmostldos,thebandgapisthedominantnoisesource.ifanoisereductioncapacitor(c
NR
)isusedwiththe
tps799xx,thebandgapdoesnotcontributesignificantlytonoise.instead,noiseisdominatedbytheoutput
resistordividerandtheerroramplifierinput.tominimizenoiseinagivenapplication,usea0.01µfnoise
reductioncapacitor;fortheadjustableversion,smallervalueresistorsintheoutputresistordividerreducenoise.
Aparallelcombinationthatgives2µAofdividercurrentwillhavethesamenoiseperformanceasafixedvoltage
版本.tofurtheroptimizenoise,equivalentseriesresistanceoftheoutputcapacitorcanbesetto
approximately0.2
Ω
.thisconfigurationmaximizesphasemargininthecontrolloop,reducingtotaloutputnoise
byupto10%.
noisecanbereferredtothefeedbackpoint(fbpin)suchthatwithc
NR
=0.01µftotalnoiseisapproximately
givenbyEquation1:
thetps79901adjustableversiondoesnothavethenoise-reductionpinavailable,soultra-lownoiseoperationis
notpossible.noisecanbeminimizedaccordingtotheaboverecommendations.
toimproveacperformancesuchaspsrr,outputnoise,andtransientresponse,itisrecommendedthatthe
boardbedesignedwithseparategroundplanesforV
在
andV
输出
,witheachgroundplaneconnectedonlyatthe
gndpinofthedevice.inaddition,thegroundconnectionforthebypasscapacitorshouldconnectdirectlytothe
gndpinofthedevice.
thetps799xxinternalcurrentlimithelpsprotecttheregulatorduringfaultconditions.duringcurrentlimit,这
outputwillsourceafixedamountofcurrentthatislargelyindependentofoutputvoltage.forreliableoperation,
thedeviceshouldnotbeoperatedincurrentlimitforextendedperiodsoftime.
thepmospasselementinthetps799xxhasabuilt-inbodydiodethatconductscurrentwhenthevoltageat
outexceedsthevoltageatin.thiscurrentisnotlimited,soifextendedreversevoltageoperationis
预期,externallimitingmaybeappropriate.
theenablepin(en)isactivehighandiscompatiblewithstandardandlowvoltagettl-cmoslevels.当
shutdowncapabilityisnotrequired,encanbeconnectedtoin.
thetps799xxusesapmospasstransistortoachievelowdropout.当(v
在
–V
输出
)islessthanthedropout
电压(v
做
),thepmospassdeviceisinitslinearregionofoperationandtheinput-至-outputresistanceisthe
R
ds,在
ofthepmospasselement.becausethepmosdevicebehaveslikearesistorindropout,v
做
将
approximatelyscalewithoutputcurrent.
aswithanylinearregulator,psrrandtransientresponsearedegradedas(v
在
–V
输出
)approachesdropout.
ThiseffectisshowninFigure18通过Figure20inthe
TypicalCharacteristics
部分.
fixedvoltageversionsofthetps799xxuseaquick-startcircuittofast-chargethenoisereductioncapacitor,
C
NR
,ifpresent(看
FunctionalBlockDiagrams
,Figure1).thisallowsthecombinationofverylowoutputnoise
andfaststart-uptimes.thenrpinishighimpedancesoalowleakagec
NR
capacitormustbeused;大多数
ceramiccapacitorsareappropriateinthisconfiguration.
notethatforfasteststartup,v
在
shouldbeappliedfirst,thentheenablepin(en)drivenhigh.ifenistiedtoin,
startupwillbesomewhatslower.refertoFigure25和Figure26inthe
TypicalCharacteristics
部分.这
快-startswitchisclosedforapproximately135µs.toensurethatc
NR
isfullychargedduringthequick-开始
时间,a0.01µforsmallercapacitorshouldbeused.
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