Datasheet

ResistorPull-UpValueSelection
SystemsWithDisparateSupplyVoltages
TCA4311
SCPS173–DECEMBER2008........................................................................................................................................................................................... www.ti.com
Thesystempullupresistorsmustbestrongenoughtoprovideapositiveslewrateof1.25V/µsontheSDAand
SCLpins,inordertoactivatetheboostpullupcurrentsduringrisingedges.ChoosemaximumresistorvalueR
usingtheformula:
R≤(V
CC(MIN)
–0.6)(800,000)/C
whereRisthepullupresistorvalueinohms,V
CC(MIN)
istheminimumV
CC
voltageandCistheequivalentbus
capacitanceinpicofarads(pF).
Inaddition,regardlessofthebuscapacitance,alwayschooseR≤16kΩforV
CC
=5.5Vmaximum,R≤24kΩ
forV
CC
=3.6Vmaximum.Thestart-upcircuitryrequireslogichighvoltagesonSDAOUTandSCLOUTto
connectthebackplanetothecard,andthesepullupvaluesareneededtoovercometheprechargevoltage.
Inlarge2-wiresystems,theV
CC
voltagesseenbydevicesatvariouspointsinthesystemcandifferbyafew
hundredmillivoltsormore.ThissituationiswellmodeledbyaseriesresistorintheV
CC
line,asshownin
Figure15.ForproperoperationoftheTCA4311,makesurethatV
CC(BUS)
≥V
CC(TCA4311)
–0.5V.
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Not Recommended For New Designs