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CircuitDescription

Thenetworkdriver(Figure1)consistsofapulldownsection(Q1,R1,C1,R5)andapullupsection(Q2,R2,C2,R6).

Athirdtransistorandsurroundingcomponents(Q3,C4,R7)formastrongpullupsectiontosupplyextrapowerfor

devicessuchasEEPROMs,ortemperaturesensors.This"strong-pullup"functionisnotdiscussedinthisdocument.Of

thethreetransistors,amaximumofoneisconductingatanytime.Whenthereisno1-Wirecommunication("idle"

state),allthreetransistorsarenonconducting.

TheseriespathofR4,R1andR3providesthestandard1-WirepulluptoVCC.Withthiscircuit,thetotalpullup

resistanceisapproximately1kΩ.Thisvalueapplieswhenthe1-Wirelineisidle.SinceR4isconnectedtothedrainof

Q1,thecurrentflowingthroughitwhenQ1isconductingdoesnotaffectthelowlevelvoltageonthe1-Wireline.R4

+R1+R3togetherwiththeloador"weight"ofthe1-Wirenetworkdeterminethespeedatwhichthevoltageonthe1-

Wirerisesto5V.LoweringR4isnotrecommendedsinceitwouldraisethelow-levelvoltageonthe1-Wire,whichis

undesirable.TheSchottky-diodesD1andD2eliminatespikesfromESDhitsorcrosscouplingfromothercabling

nearbybyconductingthemtoGNDandVCC,respectively.R3limitstheESDcurrentandprotectsD1andD2.

Aspecialcharacteristicofthisdriveristheproperlineterminationofthe1-Wirecableonthemasterend.The

category5unshieldedtwisted-pairdatacable,whichisrecommendedfor1-Wireapplications,hasacharacteristic

impedanceofapproximately100Ω.LineterminationisaccomplishedthroughR1orR2inserieswithR3whenQ1or

Q2isconducting.C3inserieswithR1andR3provideanAC-coupledterminationforpresencepulses.Toadaptthis

drivertoadifferentimpedance,R1andR2needtobechangedaccordingly.

Allthreesectionsofthisdriverareslew-ratecontrolledwhentheassociatedtransistoristurnedon.R5andC1limit

theslewratewhenthedriverpullsthe1-Wirelinelow,e.g.,atthebeginningofatimeslotoraresetpulse.R6and

C2limittheslewratewhenthedyna