Datasheet
OPERATION
Start-Up
ConnectionCircuitry
InputtoOutputOffsetVoltage
PropagationDelays
TCA4311
SCPS173–DECEMBER2008...........................................................................................................................................................................................
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WhentheTCA4311firstreceivespoweronitsV
CC
pin,eitherduringpower-uporduringliveinsertion,itstartsin
anundervoltagelockout(UVLO)state,ignoringanyactivityontheSDAandSCLpinsuntilV
CC
risesabove
2.5V.
Duringthistime,the1Vprechargecircuitryisalsoactiveandforces1Vthrough100-kΩnominalresistorstothe
SDAandSCLpins.BecausetheI/Ocardisbeingpluggedintoalivebackplane,thevoltageonthebackplane
SDAandSCLbussesmaybeanywherebetween0VandV
CC
.PrechargingtheSCLandSDApinsto1V
minimizestheworst-casevoltagedifferentialthesepinswillseeatthemomentofconnection,therefore
minimizingtheamountofdisturbancecausedbytheI/Ocard.
OncetheTCA4311comesoutofUVLO,itassumesthatSDAINandSCLINhavebeeninsertedintoalive
systemandthatSDAOUTandSCLOUTarebeingpoweredupatthesametimeasitself.Therefore,itlooksfor
eitherastopbitorbusidleconditiononthebackplanesidetoindicatethecompletionofadatatransaction.
Wheneitheroneoccurs,thepartalsoverifiesthatboththeSDAOUTandSCLOUTvoltagesarehigh.Whenall
oftheseconditionsaremet,theinput-to-outputconnectioncircuitryisactivated,joiningtheSDAandSCLbusses
ontheI/Ocardwiththoseonthebackplane,andtherisetimeacceleratorsareenabled.
Oncetheconnectioncircuitryisactivated,thefunctionalityoftheSDAINandSDAOUTpinsisidentical.Alow
forcedoneitherpinatanytimeresultsinbothpinvoltagesbeinglow.Forproperoperation,logiclowinput
voltagesshouldbenohigherthan0.4VwithrespecttothegroundpinvoltageoftheTCA4311.SDAINand
SDAOUTenteralogichighstateonlywhenalldevicesonbothSDAINandSDAOUTreleasehigh.Thesameis
trueforSCLINandSCLOUT.Thisimportantfeatureensuresthatclockstretching,clocksynchronization,
arbitrationandtheacknowledgeprotocolalwayswork,regardlessofhowthedevicesinthesystemaretiedto
theTCA4311.
Anotherkeyfeatureoftheconnectioncircuitryisthatitprovidesbidirectionalbuffering,keepingthebackplane
andcardcapacitancesisolated.Becauseofthisisolation,thewaveformsonthebackplanebusseslookslightly
differentthanthecorrespondingcardbuswaveforms,asdescribedhere.
Whenalogiclowvoltage,V
LOW1
,isdrivenonanyoftheTCA4311'sdataorclockpins,theTCA4311regulates
thevoltageontheothersideofthechip(callitV
LOW2
)toaslightlyhighervoltage,asdirectedbythefollowing
equation:
V
LOW2
=V
LOW1
+75mV+(V
CC
/R)×100
whereRisthebuspullupresistanceinohms(Ω).Forexample,ifadeviceisforcingSDAOUTto10mVwhere
V
CC
=3.3VandthepullupresistorRonSDAINis10kΩ,thenthevoltageonSDAIN=10+75+(3.3/10000)×
100=118mV.SeetheTypicalPerformanceCharacteristicssectionforcurvesshowingtheoffsetvoltageasa
functionofV
CC
andR.
Duringarisingedge,therise-timeoneachsideisdeterminedbythecombinedpullupcurrentoftheTCA4311
boostcurrentandthebusresistorandtheequivalentcapacitanceontheline.Ifthepullupcurrentsarethesame,
adifferenceinrise-timeoccurswhichisdirectlyproportionaltothedifferenceincapacitancebetweenthetwo
sides.ThiseffectisdisplayedinFigure1forV
CC
=3.3Vanda10-kΩpullupresistoroneachside(50pFonone
sideand150pFontheother).Sincetheoutputsidehaslesscapacitancethantheinput,itrisesfasterandthe
effectivet
PLH
isnegative.
Thereisafinitepropagationdelay,t
PHL
,throughtheconnectioncircuitryforfallingwaveforms.Figure2shows
thefallingedgewaveformsforthesameV
CC
,pullupresistorsandequivalentcapacitanceconditionsasusedin
Figure1.AnexternalNMOSdevicepullsdownthevoltageonthesidewith150pFcapacitance;theTCA4311
pullsdownthevoltageontheoppositeside,withadelayof55ns.Thisdelayisalwayspositiveandisafunction
ofsupplyvoltage,temperatureandthepullupresistorsandequivalentbuscapacitancesonbothsidesofthe
bus.TheTypicalPerformanceCharacteristicssectionshowst
PHL
asafunctionoftemperatureandvoltagefor
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