Datasheet

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PWMControlandFeedForward
Dead-timeControl
LowSideGateDriver(LSG)
ThermalShutdown
TPS54550
SLVS623A–MARCH2006–REVISEDAPRIL2006
Theminimumontimeisdesignedtobe180ns.
Duringtheinternalslow-startinterval,theinternal
Signalsfromtheerroramplifieroutput,oscillator,and
referencerampsfrom0Vto0.891V.Duringthe
currentlimitcircuitareprocessedbythePWM
initialslow-startinterval,theinternalreference
controllogic.Referringtotheinternalblockdiagram,
voltageisverysmall,resultinginacoupleofskipped
thecontrollogicincludesthePWMcomparator,
pulsesbecausetheminimumontimecausesthe
PWMlatch,andtheadaptivedead-timecontrollogic.
actualoutputvoltagetobeslightlygreaterthanthe
Duringsteady-stateoperationbelowthecurrentlimit
presetoutputvoltageuntiltheinternalreference
threshold,thePWMcomparatoroutputandoscillator
rampsup.
pulsetrainalternatelyresetandsetthePWMlatch.
OncethePWMlatchisreset,thelow-sidedriverand
integratedpull-downMOSFETremainonfora
Adaptivedead-timecontrolpreventsshoot-through
minimumdurationsetbytheoscillatorpulsewidth.
currentfromflowingintheintegratedhigh-side
Duringthisperiod,thePWMrampdischargesrapidly
MOSFETandtheexternallow-sideMOSFETduring
tothevalleyvoltage.Whentherampbeginsto
theswitchingtransitionsbyactivelycontrollingthe
chargebackup,thelow-sidedriverturnsoffandthe
turnontimesofthedrivers.Thehigh-sidedriver
high-sideFETturnson.ThepeakPWMramp
doesnotturnonuntilthevoltageatthegateofthe
voltagevariesinverselywithinputvoltagetomaintain
low-sideMOSFETisbelow1V.Thelow-sidedriver
aconstantmodulatorandpowerstagegainof8V/V.
doesnotturnonuntilthevoltageatthegateofthe
AsthePWMrampvoltageexceedstheerrorhigh-sideMOSFETisbelow1V.
amplifieroutputvoltage,thePWMcomparatorresets
thelatch,thusturningoffthehigh-sideFETand
turningonthelow-sideFET.Thelow-sidedriver
LSGistheoutputofthelow-sidegatedriver.The
remainsonuntilthenextoscillatorpulsedischarges
100-mAMOSFETdriveriscapableofprovidinggate
thePWMramp.
driveformostpopularMOSFETssuitableforthis
Duringtransientconditions,theerroramplifieroutput
application.UsetheSWIFTDesignerSoftwareTool
canbebelowthePWMrampvalleyvoltageorabove
tofindthemostappropriateMOSFETforthe
thePWMpeakvoltage.Iftheerroramplifierishigh,
application.ConnecttheLSGpindirectlytothegate
thePWMlatchisneverresetandthehigh-sideFET
ofthelow-sideMOSFET.Donotuseagateresistor
remainsonuntiltheoscillatorpulsesignalsthe
astheresultingturn-ontimemaybetooslow.
controllogictoturnthehigh-sideFEToffandthe
internallow-sideFETanddriveron.Thedevice
operatesatitsmaximumdutycycleuntiltheoutput
Thedeviceusesthethermalshutdowntoturnoffthe
voltagerisestotheregulationsetpoint,setting
MOSFETdriversandcontrollerifthejunction
VSENSEtoapproximatelythesamevoltageasthe
temperatureexceeds+165°C.Thedeviceis
internalvoltagereference.Iftheerroramplifieroutput
restartedautomaticallywhenthejunction
islow,thePWMlatchiscontinuallyresetandthe
temperaturedecreasesto7°Cbelowthethermal
high-sideFETdoesnotturnon.Theinternallow-side
shutdowntrippointandstartsupundercontrolofthe
FETandlow-sidedriverremainonuntiltheVSENSE
slow-startcircuit.
voltagedecreasestoarangethatallowsthePWM
comparatortochangestates.TheTPS54550is
capableofsinkingcurrentthroughtheexternal
low-sideFETuntiltheoutputvoltagereachesthe
regulationsetpoint.
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