Datasheet

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SwitchingFrequency(RT)
RT(kW) +
46000
ƒ
s(kHz)–35.9
(4)
180°OutofPhaseSynchronization(SYNC)
V
I(SYNC)
V
O(PH)
TPS54550
SLVS623A–MARCH2006–REVISEDAPRIL2006
TheRTpincontrolstheSYNCpinfunctions.Ifthe
RTpinisfloatingorgrounded,SYNCisanoutput.If
TheTPS54550hasaninternaloscillatorthat
theswitchingfrequencyhasbeenprogrammedusing
operatesattwicethePWMswitchingfrequency.The
aresistorfromRTtoAGND,thenSYNCfunctionsas
internaloscillatorfrequencyiscontrolledbytheRT
aninput.
pin.GroundingtheRTpinsetsthePWMswitching
frequencytoadefaultfrequencyof250kHz.FloatingTheinternalvoltagerampchargingcurrentincreases
theRTpinsetsthePWMswitchingfrequencytolinearlywiththesetfrequencyandkeepsthefeed
500kHz.forwardmodulatorconstant(Km=8)regardlessof
thefrequencysetpoint.
ConnectingaresistorfromRTtoAGNDsetsthe
frequencyaccordingtoEquation4(seeFigure13).
Table2.SwitchingFrequency,SYNCandRTPins
SWITCHINGFREQUENCYSYNCPINRTPIN
250kHz,internallysetGeneratesSYNCoutputsignalAGND
500kHz,internallysetGeneratesSYNCoutputsignalFloat
Externallysetto250kHzto700kHzTerminatetoquietgroundwith10-kΩR=215kΩto69kΩ
resistor.
ExternallysynchronizedfrequencySynchronizationSignalUse110kΩwhenRTfloatsand237kΩwhenRTis
groundedandusingthesyncoutsignalofanother
TPS54550.SetRTresistorequalto90%to110%of
externalsynchronizationfrequency.
WhenoperatingthetwoTPS54550devices180°out
ofphase,thetotalRMSinputcurrentisreduced,
decreasingtheamountofinputcapacitanceneeded
TheSYNCpinisconfigurableasaninputorasan
andincreasingefficiency.
output,perthedescriptionintheprevioussection.
Whenoperatingasaninput,theSYNCpinisa
WhensynchronizingaTPS54550toanexternal
falling-edgetriggeredsignal(seeFigure3and
signal,thetimingresistorontheRTpinmustbeset
Figure4).Whenoperatingasanoutput,thesignal's
sothattheoscillatorisprogrammedtorunat90%to
fallingedgeisapproximately180°outofphasewith
110%ofthesynchronizationfrequency.
therisingedgeofthePHpins.Thus,twoTPS54550
devicesoperatinginasystemcanshareaninputNOTE:Donotusesynchronizationinputfordesigns
capacitoranddrawripplecurrentattwicethewithoutputvoltages>10V.
frequencyofasingleunit.
Figure3.SYNCInputWaveform
9
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