chapter5.ppt

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1、DigitalIntegratedCircuitsADesignPerspectiveTheInverterJanM.RabaeyAnanthaChandrakasanBorivojeNikolicJuly30,2002TheCMOSInverter:AFirstGlanceVinVoutCLVDDCMOSInverterPolysiliconInOutVDDGNDPMOS2lMetal1NMOSContactsNWellTwoInvertersConnectinMetalSharepowerandgroundAbutcellsCMOSInverterFirst-OrderDCAn

2、alysisVOL=0VOH=VDDVM=f(Rn,Rp)VDDVDDVin5VDDVin50VoutVoutRnRpCMOSInverter:TransientResponsetpHL=f(Ron.CL)=0.69RonCLVoutVoutRnRpVDDVDDVin5VDDVin50(a)Low-to-high(b)High-to-lowCLCLVoltageTransferCharacteristicPMOSLoadLinesVDSpIDpVGSp=-2.5VGSp=-1VDSpIDnVin=0Vin=1.5VoutIDnVin=0Vin=1.5Vin=VDD+VGSpIDn=

3、-IDpVout=VDD+VDSpVoutIDnVin=VDD+VGSpIDn=-IDpVout=VDD+VDSpCMOSInverterLoadCharacteristicsCMOSInverterVTCSwitchingThresholdasafunctionofTransistorRatio1001010.80.911.11.21.31.41.51.61.71.8MV(V)Wp/WnDeterminingVIHandVILVOHVOLVinVoutVMVILVIHAsimplifiedapproachInverterGainGainasafunctionofVDDGain=-1

4、SimulatedVTCImpactofProcessVariations00.511.522.500.511.522.5Vin(V)Vout(V)GoodPMOSBadNMOSGoodNMOSBadPMOSNominalPropagationDelayCMOSInverterPropagationDelayApproach1CMOSInverterPropagationDelayApproach2CMOSInvertersPolysiliconInOutMetal1VDDGNDPMOSNMOS1.2mm=2lTransientResponsetp=0.69CL(Reqn+Req

5、p)/2?tpLHtpHLDesignforPerformanceKeepcapacitancessmallIncreasetransistorsizeswatchoutforself-loading!IncreaseVDD(????)DelayasafunctionofVDDDeviceSizing(forfixedload)Self-loadingeffect:IntrinsiccapacitancesdominateNMOS/PMOSratiotpLHtpHLtpb=Wp/WnImpactofRiseTimeonDelayInverterSizingInverterChainC

6、LIfCLisgiven:Howmanystagesareneededtominimizethedelay?Howtosizetheinverters?Mayneedsomeadditionalconstraints.InOutInverterDelayMinimumlengthdevices,L=0.25mmAssumethatforWP=2WN=2Wsamepull-upandpull-downcurrentsapprox.equalresistancesRN=RPapprox.equalrisetpLHandfalltpHLdelaysAnalyzeasanRCnetworkt

7、pHL=(ln2)RNCLtpLH=(ln2)RPCLDelay(D):2WWLoadforthenextstage:InverterwithLoadLoad(CL)DelayAssumptions:noload->zerodelayCLtp=kRWCLRWRWWunit=1kisaconstant,equalto0.69InverterwithLoadLoadDelayCintCLDelay=kRW(Cint+CL)=kRWCint+kRWCL=kRWC