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DETAILEDDESCRIPTION
UndervoltageLockout(UVLO)SlowStartEnable(SSENA)andInternalSlow
T
SS_INTERNAL(ms)
+
1.15k
ƒ
s(kHz)
(3)
5 µA
Disabled
Enabled
C
SS
320 kΩ
125 kΩ
R1
R2
Input Voltage Supply
1 kΩ
ExtendingSlowStartTime
R1 +
VIN(start) 1 kW
1.24 V
* 1kW
(1)
VIN(stop) +
(R1 ) 1 kW) 1.02 V
1 kW
(2)
TPS54550
SLVS623A–MARCH2006–REVISEDAPRIL2006
Start
Theundervoltagelockout(UVLO)systemhasan
internalvoltagedividerfromVINtoAGND.TheOncetheSSENApinvoltageexceeds0.5V,the
defaultsforthestart/stopvaluesarelabeledVINandTPS54550startsoperation.TheTPS54550hasan
giveninTable1.TheinternalUVLOthresholdcaninternaldigitalslowstartthatrampsthereference
beoverriddenbyplacinganexternalresistordividervoltagetoitsfinalvaluein1150switchingcycles.
fromVINtoground.TheinternaldividervaluesareTheinternalslowstarttime(10%-90%)is
approximately320kΩforthehigh-sideresistorandapproximatedbythefollowingexpression:
125kΩforthelow-sideresistor.Thedividerratio
(andthereforethedefaultstart/stopvalues)isquite
accurate,buttheabsolutevaluesoftheinternal
resistorsmayvaryasmuchas15%.Ifhighaccuracy
OncetheTPS54550deviceisinnormalregulation,
isrequiredforanexternallyadjustedUVLO
theSSENApinishigh.IftheSSENApinispulled
threshold,selectlowervalueexternalresistorstoset
belowthestopthresholdof0.5V,switchingstops
theUVLOthreshold.Usinga1-kΩresistorforthe
andtheinternalslowstartresets.Ifanapplication
low-sideresistorR2(seeFigure1)isrecommended.
requirestheTPS54550tobedisabled,useopen
UndernocircumstancesshouldtheUVLOpinbe
drainoropencollectoroutputlogictointerfacetothe
connecteddirectlytoVIN.
SSENApin(seeFigure2).TheSSENApinhasan
internalpull-upcurrentsource.Donotuseexternal
Table1.Start/StopVoltageThreshold
pull-upresistors.
STARTVOLTAGESTOPVOLTAGE
THRESHOLDTHRESHOLD
VIN(Default)4.493.69
UVLO1.241.02
Figure2.InterfacingtotheSSENAPin
Figure1.CircuitUsingExternalUVLOFunction
TheequationsforselectingtheUVLOresistorsare:
Inapplicationsthatuselargevaluesofoutput
capacitance,theremaybeaneedtoextendtheslow
starttimetopreventthestartupcurrentfromtripping
thecurrentlimit.Thecurrentlimitcircuitisdesigned
todisablethehigh-sideMOSFETandresetthe
internalvoltagereferenceforashortamountoftime
Forapplicationswhichrequireanundervoltagelock
whenthehigh-sideMOSFETcurrentexceedsthe
out(UVLO)thresholdgreaterthan4.49V,external
currentlimitthreshold.Iftheoutputcapacitanceand
resistorsmaybeimplemented(seeFigure1)to
loadcurrentcausethestartupcurrenttoexceedthe
adjustthestartvoltagethreshold.Forexample,an
currentlimitthreshold,thepowersupplyoutputwill
applicationneedinganUVLOstartvoltageof
notreachthedesiredoutputvoltage.Toextendthe
approximately7.8VusingEquation1,R1is
slowstarttimeandtoreducethestartupcurrent,an
calculatedtotheneareststandardresistorvalueof
externalcapacitorcanbeaddedtotheSSENApin.
5.36kΩ.UsingEquation2,theinputvoltagestop
Theslowstartcapacitanceiscalculatedusingthe
thresholdiscalculatedas6.48V.
followingequation:
C
SS
(μF)=5.55x10
–3
T
ss
(ms)
8
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