Datasheet

52.0
52.025
0.0
1.0
2.5
–0.5
0.5
1.5
3.0
2.0
3.5
52.075 52.1 52.125
t (s)
(V)
52.15 52.175
VXSDAIN
Delay: 72.258
VXSDAOUT
54.0 54.2
0.0
1.0
2.5
–0.5
0.5
1.5
3.0
2.0
3.5
54.4 54.6 54.8
t (s)
(V)
55.0 55.2 55.4 55.6 55.8 56.0
VXSDAOUT
VXSDAIN
Rise-TimeAccelerators
READYDigitalOutput
ENLowCurrentDisable
TCA4311
www.ti.com ...........................................................................................................................................................................................SCPS173–DECEMBER2008
10-kΩpullupresistorsand100pFequivalentcapacitanceonbothsidesofthepart.BycomparisonwithFigure2,
theV
CC
=3.3Vcurveshowsthatincreasingthecapacitancefrom50pFto100pFresultsinat
PHL
increasefrom
55nsto75ns.Largeroutputcapacitancestranslatetolongerdelays(upto150ns).Usersmustquantifythe
differenceinpropagationtimesforarisingedgeversusafallingedgeintheirsystemsandadjustsetupandhold
timesaccordingly.
Figure1.Input-OutputConnectiont
PLH
Figure2.Input-OutputConnectiont
PHL
Onceconnectionhasbeenestablished,rise-timeacceleratorcircuitsonallfourSDAandSCLpinsareactivated.
TheseallowtheusertochooseweakerDCpullupcurrentsonthebus,reducingpowerconsumptionwhilestill
meetingsystemrise-timerequirements.Duringpositivebustransitions,theTCA4311switchesin2mA(typical)
ofcurrenttoquicklyslewtheSDAandSCLlinesoncetheirDCvoltagesexceed0.6V.Usingageneralruleof
20pFofcapacitanceforeverydeviceonthebus(10pFforthedeviceand10pFforinterconnect),choosea
pullupcurrentsothatthebuswillriseonitsownatarateofatleast1.25V/µstoguaranteeactivationofthe
accelerators.
Forexample,assumeanSMBussystemwithV
CC
=3V,a10-kΩpullupresistorandequivalentbuscapacitance
of200pF.Therise-timeofanSMBussystemiscalculatedfrom(V
IL(MAX)
–0.15V)to(V
IH(MIN)
+0.15V),or0.65
Vto2.25V.IttakesanRCcircuit0.92timeconstantstotraversethisvoltagefora3Vsupply;inthiscase,0.92
×(10kΩ×200pF)=1.84µs.Thus,thesystemexceedsthemaximumallowedrise-timeof1µsby84%.
However,usingtherise-timeaccelerators,whichareactivatedataDCthresholdofbelow0.65V,theworst-case
rise-timeis:(2.25V–0.65V)×200pF/1mA=320ns,whichmeetsthe1µsrise-timerequirement.
ThispinprovidesadigitalflagwhichislowwheneitherENisloworthestart-upsequencedescribedearlierin
thissectionhasnotbeencompleted.READYgoeshighwhenENishighandstart-upiscomplete.Thepinis
drivenbyanopendrainpull-downcapableofsinking3mAwhileholding0.4Vonthepin.Connectaresistorof
10kΩtoV
CC
toprovidethepullup.
GroundingtheENpindisconnectsthebackplanesidefromthecardside,disablestherise-timeaccelerators,
drivesREADYlow,disablesthebusprechargecircuitryandputsthepartinanear-zerocurrentstate.Whenthe
pinvoltageisdrivenallthewaytoV
CC
,thepartwaitsfordatatransactionsonboththebackplaneandcardsides
tobecomplete(asdescribedintheStart-Upsection)beforereconnectingthetwosides.
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