Choosing an Appropriate Pull-up/Pull-down …

1 ApplicationReportSLVA485 October2011 ChoosinganAppropriatePull- :opendrain(opencollectorforbipolaroutput s)orpush- pull (alsoknownastotempole). , ,opendrainoutputsprovidemoreflexibilityt oadesignerastheycanbepulled-uptoanyvolta gefoundinthesystem,whichcanbeusefulwhent heyserveasinputstoa processorwhichmightrequirea (PG)andLowBattery(LBO)onswitchingregulat ors,resetandPowerFail(PFO)onsupplyvoltag esupervisors(SVS),andLowBattery,PowerFai l, pull -uporpull-downresistor,thefactorstha tshouldbeconsideredwhenselectinga pull -uporpull-downresistor,andhowtocalcu latea (PGFloatingHigh)..22 TypicalPGOutputEquivalentCircuit(PGLow). .33 ResetOutputEquivalentCircuit(ResetFloati ngLow)..54 ResetOutputEquivalentCircuit(ResetHigh). . October2011 ChoosinganAppropriatePull-up/ pull -downRe sistorforOpenDrainOutputsSubmitDocumenta tionFeedbackCopyright 2011,TexasInstrumentsIncorporated_+Compa ratorILKGQ1 (OFF)PGVPGIPull-upRPull-upENIENVOUTI nside the criticalvoltagewithina systemandoutputsa resetsignalif theopendrainoutputin needofa somevoltagein powerconverter,a PGoutputis routinelyusedtodrivetheenableinputofa PGoutputis lowif thechip s outputvoltageis belowa theLBIpinvoltagedropsbelowa whethera pull -upora pull -downresistoris ,theTPS62067step-downconverterhasa PGoutputthatit driveslowif thechip s outputvoltageis notin ,PGneedsapull-upresistorthatpullsthePGpi nhighwhenthechipallowsit tofloat,indicatingthatpoweris ,theTL7759 SVShasa resetoutputthatit driveshighwhenit detectsa supplyvoltagedropbelowa lowwhenthatvoltageis , s leakagecurrent,theleakagecurrentoftheinp utpinthattheopendrainoutputis connectedto,thevoltagethattheoutputis beingpulledupto,thehighorlowvoltagelogic levelofboththeoutputpinandtheinputpintha ttheoutputis connectedto, pull -upresistoris neededandhowtocalculatea rangeforit.

2 ThesecondexampleusestheTL7759 SVSanddemonstrateswhena pull -downresistoris neededandhowtocalculatea ,Vout, throughapull-upresistor,RPull-up, (PGFloatingHigh)Thecircuitin Figure1 is analyzedtofindthemaximumvalueforRPull-up , whenpoweris goodandQ1is off,thedatasheetspecifiesthatthereis someleakagecurrentthroughit. Thisvalueisfoundin thedatasheetasILKG, ,thevoltageonthePGpinandonthesubsequentE Ninputis less2 ChoosinganAppropriatePull-up/ pull -downRe sistorforOpenDrainOutputsSLVA485 October2011 SubmitDocumentationFeedbackCopyright 2011,TexasInstrumentsIncorporated_+Compa ratorIOLQ1 (ON)PGVPGIPull-upRPull-upENIENVOUTI nside the ForthecalculationofthemaximumvalueofRPul l-up, themaximumvalueofILKGis usedbecauseitwouldresultin thelargestvoltagedropacrossRPull-up. Also,assumingthatthePGoutputfeedstheENin putonanotherchip,therewillalsobea foundin thedatasheetofthesubsequentpartandis labeledIENin Figure1. Equation1 showshowtocalculateIPull-upusingKirchhof f s Law.

3 (1)Tocalculatethemaximumvalueofthepull-u presistor,Equation2 setsthevoltageatthePGpin,VPG,equaltothes ubsequentchip s ENpin s VIH. VIHis theminimumvoltagethatis specifiedtobereadasalogichigh.(2)Thisval ueis a maximumbecausechoosinga largerresistorwouldresultin a largervoltagedropacrossRPull-upwhichwoul dcauseVPGtobelowerthantheminimumvalueofV IH. Inotherwords,thesubsequentchipwouldnotre cognizethePGvoltageasbeinga showsthesamecircuitasanalyzedabovewhenQ1 is onandPGis belowregulationandpoweris (PGLow)WhenfindingtheminimumvalueforRPul l-up, it is assumedthatQ1is turnedonasshownin Figure2, soVPGis ,Q1hasa resistance,RDSon, on,thePGvoltagemustbesufficientlylowtore gisterasa thepull-upresistor,thecurrentlabeledIOLi s foundin theICdatasheetasthetestcurrentforVOL, thetestconditioninthedatasheetgivesknown performance, ,buttheycouldyielda VOLhigherthanitsspecifiedmaximum.(Inthec aseoftheTPS62067usedin theexample,itsVOLtestcurrentanditsabsolu temaximumPGsinkcurrentareequal,butthisis notalwaysthecase.)

4 Equation3 usesKirchhoff s Law,theIC s testcurrent,andtheleakagecurrentofthesub sequentENinputtocalculatethecurrentthrou ghthepull-upresistor.(3)Thevoltageacross thepull-upresistoris equaltoVoutminusVPG. Thedatasheetgivesa maximumvalueforVPG( ),butit couldbe0 V whichwouldresultin a highercurrentflowingthroughthepull-upres istorthanif it ,Equation4 calculatestheminimumpull-upresistance.(4 )3 SLVA485 October2011 ChoosinganAppropriatePull-up/ pull -downRe sistorforOpenDrainOutputsSubmitDocumenta tionFeedbackCopyright 2011, a minimumvaluebecausechoosinganyvaluelower forRPull-upcausesa highercurrentthanthetestconditioncurrent toflowin a currenthigherthanthetestconditioncurrent flowsthroughQ1,thevoltagedropacrossQ1is andEquation2 yielda low,themaximumallowedVPGvoltagedependson whetherVOLis greaterthanorlessthanVIL, thesubsequentENpin s VOLis greaterthanVIL, thenspecifiedperformanceis notpossible,becausethemaximumallowedvolt age,VIL, is lessthantheensuredhighestvoltageonthePGp in,VOL.

5 Inthiscase,theminimumresistorvaluecalcul atedinEquation4 shouldbesignificantlyincreasedtomaintain plentyofmarginin thePGvoltagein ordertoachievea sufficientlylowvoltageatthePGoutputtoreg isterasa desired,thesecalculationsassumethatVOLis givesvaluesfromthePGoutputIC s datasheet(inthiscasetheTPS62067[SLVS833A ]),andTable2 givesvaluesfortheENinputfromthatIC s datasheet(alsoa TPS62067[SLVS833A])thatareusedtocalculat ea ,theTPS62067 s (PG)IOL1 mAILKG(max)100nAVOL(max) (EN)VIH(min) (max)1000nAVIL(max) ,useIEN, ILKG, andEquation1 tofindthecurrentthroughthepull-upresisto r,IPull-up.(5)NowthatIPull-uphasbeencalc ulated,themaximumvalueforRPull-upis foundbyusingEquation2. Anoutputvoltage,Vout, is usedasanexample.(6)Thenextstepis is utilizedtofindthecurrentthroughthepull-u presistor.(7)Withthisvalue,Equation4 findstheminimumvalueforRPull-up.(8)4 ChoosinganAppropriatePull-up/ pull -downRe sistorforOpenDrainOutputsSLVA485 October2011 SubmitDocumentationFeedbackCopyright 2011,TexasInstrumentsIncorporated_+Compa ratorIOLQ1 (OFF)uPRPull-downIRVCCI nside the ,therangeofpull-upresistorvaluesis(9)4 CalculatingthePull-downResistorRangeFigu re3 showsanIC s resetoutputconnectedtogroundthrougha pull -downresistor,RPull-down.

6 Theresetpinis thenconnectedtotheresetinputofa (ResetFloatingLow)Thecircuitin Figure3 is analyzedtofindthemaximumvalueforRPull-do wn, whenresetis lowandQ1is off,thedatasheetspecifiesthatthereis someleakagecurrentthroughit. Thisvalueisfoundin thedatasheetasIOL, thelow-leveloutputcurrentforreset,andit createsa , alsocurrentflowingoutoftheprocessor s resetinput,andthisvalueisfoundin thedatasheetoftheprocessorastheresetinpu tleakagecurrent,labeledIRin s Lawtocalculatethecurrentthroughthepull-d ownresistor,IPDL, whenresetis pulleddown.(10)VILis foundin s LawtocalculatethemaximumvalueforRpull-do wn.(11)Figure4 showsthesamecircuitasanalyzedin Figure3 whenQ1is onandresetis belowthespecifiedthreshold,andtheprocess orneedstobein October2011 ChoosinganAppropriatePull-up/ pull -downRe sistorforOpenDrainOutputsSubmitDocumenta tionFeedbackCopyright 2011,TexasInstrumentsIncorporated_+Compa ratorIOHQ1 (ON)uPRPull-downIRVCCI nside the (ResetHigh)UsingFigure4 tofindtheminimumvalueforRPull-down, it is assumedthatQ1is turnedonsothatVRESET isshortedtoVCC.

7 Inreality,Q1hasa saturationvoltagethatwillcauseVRESET tobelowerthanVCC. VRESET mustbehighenoughtobereadasa logichighbytheconnectedcircuitry s valuefortheresethigh-leveloutputvoltage, VOH, atsometestcurrentIOH. Thisgivesknownperformance, ,buttheycouldyielda s Law,theIC s testcurrent,andtheleakagecurrentofthesub sequentresetinputtocalculatethecurrentth roughthepull-downresistor.(12)Aftercalcu latingthevalueforIPDH, Equation13is usedtofindtheminimumpull-downresistorval ue.(13)Inthecasewhereresetis high,theminimumallowedVRESET voltagedependsonwhetherVOHis greaterthanorlessthanVIH, thesubsequentresetinputpin s VOHis lowerthanVIH, thenspecifiedperformanceis notpossible,becausetheminimumrequiredvol tage,VIH, is greaterthanthespecifiedlowestvoltageonth eresetpin,VOH. Inthiscase,theminimumresistorvaluecalcul atedin Equation13shouldbesignificantlyincreased tomaintainplentyofmarginin theresetvoltagein ordertoachievea sufficientlyhighvoltageattheresetoutputt oregisterasa desired,thesecalculationsassumethatVOHis givesvaluesfromtheresetoutputIC s datasheet(inthiscasetheTL7759),whileTabl e4 givestypicalvaluesforIRandVILthatarefoun din theresetinputIC s (Output)VCC5 VIOL(max)1 AIOH(min)4 VIOH8 mA6 ChoosinganAppropriatePull-up/ pull -downRe sistorforOpenDrainOutputsSLVA485 October2011 SubmitDocumentationFeedbackCopyright 2011, (Input)VIL(max) (max)100nAVIH(min)3 VFirst,useIOL, IR, andIPDL aswellasEquation10tofinda valueforIPDL.

8 (14)NowuseEquation11tocalculatethemaximu mvalueforthepull-downresistor.(15)Thenex tstepis touseEquation12tofindIPDH.(16)Lastly,Equ ation13is utilizedtofindtheminimumpull-downresisto rvalue.(17)Thisfinalcalculationyieldsa rangeforthepull-downresistorof(18)6 OtherSelectionConsiderationsTheaboveexam plesarecalculatedusingparametersfromthed atasheetthatensureperformance,suchastheo utputlowvoltageata thecurrentsunkbythePGpinis lowerthanthetestconditioncurrent,thenthe voltagedropacrossthePGpin s FETis however,thecurrentsunkbythePGpinis higherthanthetestconditioncurrent,thenth evoltageatthePGpincouldbehigherthanspeci fied, ,thetestconditioncurrentis alwaysusedforthemaximumallowedcurrentin theabovecalculationsbecauseitspecifiesa ,butthentheoutputvoltageatthePGorresetpi nis nolongerspecifiedtofallin ,firsttheonresistance,orRDSon, oftheinternalFETis calculatedbyusingthespecifiedmaximumPGou tput-lowvoltage,VOL, anditstestcurrent,IOL.

9 (19)ThentheoutputvoltageatthePGpinis justRDSontimesthenewsinkcurrent.(20)This voltageshouldremainbelowVILin ordertobereadasa ,a graphofthePGorresetvoltageversuscurrentm aybeshownin ,liketheaboveRDSoncalculation, ,thereareotherfactorstoconsiderwhenselec tinga resistornearthelowendoftherangeis ,if theminimumpull-downresistorof500 is usedandthesaturationvoltage7 SLVA485 October2011 ChoosinganAppropriatePull-up/ pull -downRe sistorforOpenDrainOutputsSubmitDocumenta tionFeedbackCopyright 2011, V,thepull-downresistordropsVCC 1 V acrossit. IfVCCis 5 V,the4 V dropacrossRPull-downproduces8 mAofcurrentanda resulting32mWpowerlossin thepull-downresistorplusanadditional8 mWin theIC s ,VCCtimesIsink,mightbeverysignificantfor someapplications,in whichcasea , higherimpedancenetwhichis moresusceptibletopickingupnoisefromother nearbysignalsontheboardthathappentocoupl etoit. Thisis especiallya concernforlengthyopendrainoutputsroutedo vera secondconcernforlargeresistorvaluesis d together,fromthedownstreaminputpins,orfr omnearbytracesthatcreatea capacitorin a calculatedusingcircuitanalysisandsomepar ametersfromthepart s correctlyrecognizedbythesubsequentchip s theactualvalueselected,whichallows,forex ample,thereuseofa resistorvaluealreadyin October2011 SubmitDocumentationFeedbackCopyright 2011,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)

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