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A900mV25FWHighPSRRCMOSVoltageReferenceDedicatedtoImplantableMicro-DevicesYumuHu,MohumudSowanPolyStimNeurotechno!ogyLaboratory,DepartmentofElectricalEngineering,EcolePolytechniquedeMontrCal.yamu.hu/mohamad.sawan@polymtl.caAbstractInthispaper,wedescribeanultralow-voltagehighPSRRbandgapvoltagereferencecircuitdedicatedtoimplantablebiomedicalapplications.Thecircuittakesadvantageofthelowgate-sourcevoltageofweakinversionCMOStransistor.Andbyusingnegativefeedbacktogeneratearegulatedsupply,thepowersupplyrejectionratio(PSRR)oftheproposedcircuitcanbeincreasedupto82dBat10KHz.Althoughthecircuitisdesignedforworkingunder900mVsupplyvoltage,thepreliminarysimulationresultshowsthatitisabletoworkaslowas600mV.Thetemperaturecoefficientisaround80ppm/OC,whilethecircuitconsumes25pWofpower.1.IntroductionInrecentyears,therehasbeenasteadygrowthinproductionofelectronicsforbiomedicalpurposes,specificallyintheareaofintegratedcircuit(IC)designforimplantablebiomedicalsystems[I].VoltagereferencesaresubcircuitsnecessarilypresentedinmanysuchapplicationsthatemployA/D,D/Aconverters,voltageregulatorsandinstrumentationcircuits.Thereferencecircuitsarerequiredtoprovideaprecise,stableandtemperature-insensitivereferencevoltagetothosecircuits.WiththedownscalingofCMOStechnologyandthewidelyusageoflowpoweranaloganddigitalcircuitry,thedemandoflowvoltagereferenceisgrowingfast,especiallyinmobilebattery-operatedproductssuchascellularphones,laptopsandcamerarecorders.Thelowvoltageelectroniccircuitsarealsodesiredinthebiomedicalimplantableapplications.Inrespectthat.usinginterconnectwireswillpotentiallycauseinfectionatthepointswheretheybreaktheskin,andimplantedbatteryneedtoberenewalperiodicallyifitservesasalongtermmonitoringdevice,normallyinsuchapplicationsthepowerisrequiredtobeprovidedbyatranscutaneouslinkviahumanbody'sskin[I].SMI,FigureI.BasicmodelofatranscutaneouslinkofimplantablebiomedicalsystemFigure1presentsthebasicmodelofthetranscutaneouslink.Afterafull-waverectifier,inductivecoupledvoltagesignalisrectifiedandneedtobefurtherregulatedbyaregulator.UnliketheDCinputvoltagefromabattery,therectifiedvoltage(V,JcontainsmanyACcomponents,whichinvolvetheripplesstemfromthecharginganddischargingofthestoragecapacitorandalsocouplednoisethroughthecapacitor.Furthermore,thelowersupplyvoltagecausesinthePSRRgettingworse.Therefore,thePSRRofthereferencecircuitneedtobeimprovedsignificantlyoverconventionallowvoltagebandgapreferences.Ontheotherhand,therequirementoftemperaturedependencyofthecircuitsisrelaxedsincetheenvironmenttemperatureofthededicateapplicationisnotvariedasawideastheotherapplications.Thepaperisorganizedasfollows:Theprincipleoflowvoltagereferencecircuitanditslimitationarebrieflydescribedinthesection11.Thenweproposeanimprovedreferencecircuit.AlsotheanalysisofPSRRandthetemperaturedependencyoftheproposedcircuitarepresentedinthesection111.InthesectionIV,simulationresultsofthecircuitandconclusionwillbegivenout.11.Current-ModeVoltageReferenceCircuitTraditionalbandgapreferences(BGR)inCMOStechnologyaddtheforwardbiasvoltageacrossasubstrate-verticalPNPBITwithavoltagethatisproportionaltoabsolutetemperature(PTAT)toproduceanoutputthatisindependentoftemperatureinthefirstorder.TheoutputvoltageoftheBGRisnearlythesamevoltageofthebandgapofsilicon,around1.25V.ThistypeBGRbecomesimpossiblewhenitssupplyvoltagehasbeendecreasedtobelow1VinthemodemICcircuits.SomeworkshavebeendescribedtoproposelowvoltageBGR[2][3].Theyarebasedonacurrentmodetocompensatethevariationoftemperature.Thecorecircuitisillustratedinfigure2.ThePTATcurrent(I,)isgeneratedbyasinglefeedbackloop,whichequals.whereKrepresentsthearearatioofthetwotransistorsQ,andQ2.Byparallelizingtwomatchingresistorsatthetwoinputsoftheerroramplifierrespectively,acomplementarya7803-7761-31031S17.0002003IEEE1-373toabsolutetemperature(CTAT)current(Iz)isgenerated,whichisTherefore,theoutputreferencevoltageiswrittenasInV,In’=(‘t--“).R,nVIn(K)R,R,(3)ProvidingtheresistanceR,,R2andR,havethesametemperaturedependency,iftheratioofRIandR2satisfies(4)theoutputvoltageV,,(willbetemperatureindependentinthefirst-orderanditsvaluecanbearbitrarilyadjustedbythevalueofR,ratherthanthefixedoutputvoltage(1.25V)intheconventionalBGR.VinVLIFigure2.CurrentmodeBGRHowever,theBGRinthefigure2sufferslowprecisionandlowPSRRduetotheemploymentofsimplecurrentmirror.Thiscanbesolvedtosomeextentby,usingacascodecurrentmirror.Nevertheless,ifthesupplyvoltageisdecreasedbelow900mV,andsincethebase-emittervoltageofabipolartransistorisaround600mVattheroomtemperature,thecascodedtransistorsarepronetooperateinthetrioderegion,whichwillnotimprovetheprecisionandPSRRassuited.111.ImprovedVoltageReferenceToallowlowvoltageoperation,weproposeusingPMOStransistorsthatworkinweakinversiontoreplacethesubstrate-verticalPNPbipolartransistors(Q,,Q2infigure2).Byproperlysizethetransistors,thegate-sourcevoltageofthemcanbedecreasedto250mVatroomtemperature,whichsavesfewhundredsmilli-voltstoaccommodatethecascodingtransistor.Furthermore,theincreasedheadroomallowsustoaddapre-regulatortoimprovethePSRR[4].Inaddition,thelowerg