functionscss

CustomSiliconSolutions,Inc.
20091Ver.
1.
2–July2009cssCustomSiliconSolutions,Inc.
CSS555(C)MicropowerTimer(SPICEModel)OverviewTheanalogsectionoftheCSS555(andCSS555C)containsamicro-powerversionofthestandard555Timer.
ItisdesignedinanadvancedCMOSprocessandincludesprogrammableoptionssoitcanbeconfiguredtofittheintendedapplication.
Speed,power,triplevelsandcountersettingsarestoredinaninternalEEPROM.
Toassistboardleveldesign,aSPICEmodelforCSS's555Timerhasbeendeveloped.
Itprovidesasimulationcapabilityatthecomponentlevel.
Tomakeitportableacrossmanysimulators,itusesonlyLevel1devicemodels.
ThecomparatorsaremodeledwithBLMgainblockstoreducesimulationtime.
Evenwiththesesimplifications,themodelprovidesaverygoodrepresentationoftherealIC.
TheCSS555Timermodelincludes:1)SupplyCurrent(overVDD)2)InputSwitchLevels3)PropagationDelayTime4)OutputDrive(overtemperatureandVDD)5)ESDClampDiodesandPadCapacitanceThepowerandtriplevelsettingschangetheIC'ssupplycurrent,propagationdelayandtriplevelparameters.
SeparateSPICEmodelfilesareprovidedforeachcombinationofpowerandtriplevelsettings.
Thetableshownbelow(Table1)liststheavailablemodels.
SPICEModelPowerSettingTripLevelsCSS555CSS555CMicroStandardCSS555_uPwr_StdLevCSS555C_uPwr_StdLevMicroLowVoltageCSS555_uPwr_LowVLevCSS555C_uPwr_LowVLevLowStandardCSS555_LowPwr_StdLevCSS555C_LowPwr_StdLevLowLowVoltageCSS555_LowPwr_LowVLevCSS555C_LowPwr_LowVLevTable1BlockDiagramAsimplifiedblockdiagramofa555timerisshown,forreference,inFigure1.
Figure1ApplicationNoteAN555-2ControlVoltage_+Comp_+CompRSQQResetFlipFlopVDDTriggerOutputThresholdVHVLR1R2R3ResetVSS156482Discharge37555TimerCustomSiliconSolutions,Inc.
20092Ver.
1.
2–July2009CSS555(C)MicropowerTimer(SPICEModel)CSS555SPICEModelTheCSS555SPICEmodelhasbeenstructuredtoprovideanaccuratemodelthatrunsquicklyandiscompatiblewithmostSPICEsimulators.
ItincludestheanalogportionoftheICandcoversmostoftheimportantelectricalcharacteristics,includingsupplycurrent,inputlevels,outputdriveandpropagationdelay.
Themodelcanbeusedoverawideoperatingrange:VDD=1.
2Vto5.
5V,Temperature=-40°Cto+85°C.
Mostofthedeviceparametersarevalidacrossthisrange.
Modelsfordifferentconfigurationsettings(power&triplevels)arealsoprovided,alongwithtablestoadjustthemodelsforworst-caseconditions.
AsummaryofthefunctionsandparametersincludedinthemodelisshowninTable2.
AschematicofthemodelisprovidedinAppendixA.
ModelSummaryParameter/FunctionSymbolDescriptionComponentsSupplyCurrentIDD0Supplycurrent(Standbymode)R3,R4,R5,IBIAS,RBIAS,D8(A/B/C)ResetInputLevelsVRSTCMOSinputswitchlevelMP4,MN4,Level1modelsTripLevelsVTRIG,VCTRLResistordivider(1/3,2/3or10%,90%ofVDD)R3,R4,R5InputComparatorsVoltagelimitersIdealvoltagegainblocksRC+DiodeclampsEC1,EC2(AV=1000)R1,DL1P,DL1N&R2,DL2P,DL2NRSFlipFlopCMOSNORGates(NOR2&NOR3)RCdelayinfeedbackpathLevel1MOSFETmodelsRNR2D,CNR2D,RNR3D,CNR3DDelayTimetDLYDelayfromTRIG/VTinputstoTimerOutputR1,CD1;R2,CD2;RDLY,CDLYTimerOutputDriverVOL,VOHCMOSoutputdriverMP3,MN3,Level1modelsDischargeOutputVDISOpendrainoutputMN7,Level1modelESDClampDiodesInputprotectiondiodes,Pins2-7DN2–DN7,DP2–DP7Note:Componentandmodelvaluesarecenteredforoperationat3.
0V,25°CTable2SupplyCurrent(IDD0)Thestaticsupplycurrentismadeupofthreecomponents:thetriplevelresistordivider,thecomparatorsandthebiascurrentgenerator(forthecomparators).
Thefirstcomponentislinearlyproportionaltothesupplyvoltageandmodeledbythedivider(R3,R4,&R5).
ThelasttwocomponentsarelumpedtogetherandmodeledbyaDCcurrentsource(IBIAS)andaresistor(RBIAS)acrossthesupply.
DiodesD8A,D8BandD8ChavebeenincludedtoshuntIBIAStoVSSwhenVDDislessthan0.
5V.
ThevaluesofIBIASandRBIASdependonthepowersetting.
Thisrelativelysimplemodelprovidesareasonablyaccuratemodelofthestaticsupplycurrentforsupplyvoltagesfrom0Vto5.
5V.
Thebiascurrentgeneratoristemperaturecompensatedtokeepthecomparatorspeedrelativelyconstantovertemperature.
Sincethesupplycurrentisaweakfunctionoftemperature(andnotlinear),theCSS555modeldoesnotincludeatemperaturecoefficientforIDD0.
Formostapplications,thetransientorswitchingcurrentisasmallcomponentofthetotalcurrent.
TheRSflip-flopandoutputdriverareincludedinthiscomponent.
Thecurrentrequiredtodriveoff-chipdevicesisoftenthedominantcomponentoftheswitchingcurrentandisincludedinthemodelbywayoftheoutputdriver(MOSFETMP3).
ResetInputTheResetinputdrivesaCMOSinverter.
ItismodeledbyMP4andMN4.
TheratioofthePchandNchW/L'sprovideaswitchlevelofapproximatelyxVDD.
Inputleakagecurrent(IL)intherealdeviceistypicallylessthan1nAandcanbepositiveornegative.
SinceIListypicallynegligibleandmaybeeitherpolarity,itisnotincludedinthemodel.
TripLevels(VCTRL&VTRIG)ThestandardupperandlowertriplevelsarexVDDandxVDD.
TheyaregeneratedbytheresistordividerformedbyR3,R4andR5.
Tominimizepower,eachresistoris2.
5M.
Togainmore"headroom"forthecomparatorsatlowsupplylevels,thetriplevelscanbechangedto0.
9xVDDand0.
1xVDD.
Forthisconfiguration,R3,R4andR5arechangedto750K,6.
0Mand750Krespectively.
(Thetotalseriesresistanceremains7.
5M.
)AN555-2CustomSiliconSolutions,Inc.
20093Ver.
1.
2–July2009CSS555(C)MicropowerTimer(SPICEModel)CSS555SPICEModel(continued)InputComparatorsTheinputcomparatorsaremodeledwithidealgainelements(EC1andEC2).
Theyare"E"componentswithavoltagegainof1000.
(Therealcomparatorsuseafolded-cascodetopologyandareoperatedinasub-thresholdregion.
Level1modelsarenotveryaccurateforthisregion,soidealgainblockswereusedinstead.
Thegainelementsalsoprovideashortersimulationruntime.
)Eachgainelementisfollowedbya"Limiter"circuit.
AnRCfilter(RN,CDN)limitsthemaximumslewrateandaddsasmalldelay.
Diodes(DLNN&DLPN)clampthecomparatoroutputvoltageifitgoesbelow(VSS–0.
5V)orabove(VDD+0.
5V).
RSFlip-FlopTheRSflip-flopismodeledbytwocross-coupledCMOSNORgates,justliketheactualcircuit.
Itssimulatedspeed,inputlevelsandswitchingcurrentareaccuratelymodeledoversupplyvoltageandtemperature.
Thefeedbackpathsincludea10nsRCdelaytoimproveconvergence.
PropagationDelayTimeThepropagationdelayfromthetimerinput(TRIGGERorTHRESHOLD)tothetimerOUTPUTisdominatedbythespeedofthecomparators.
ItislumpedintoasingleRCdelaybetweentheRSflip-flopandoutputdriver.
AmoreaccuratelocationwouldplaceanRCdelaycircuitaftereachlimiter.
However,aslowsignaldrivingaflip-flopinputcancauseconvergenceproblemsduetothefeedbackwithintheflip-flop.
Forthatreason,thedelayislocatedaftertheflip-flop.
Thedownsidetothislocationistheincreasedsensitivitytoanarrowglitchattheinputs.
Therealcircuitwillfilteroutglitchesthatthemodelwillrespondto.
OutputDriversThetwooutputs,"OUTPUT"and"DISCHARGE",aremodeledwithlevel1MOSFET's.
Theirdrivestrengthsareautomaticallyadjustedforbothsupplyvoltageandtemperature.
ThemodelshavebeencenteredforoperationatVDD=3.
0Vand25°C.
ThedrivestrengthoftheOUTPUTpinisbalanced.
(Itssinkandsourcecurrentsareapproximatelyequal.
)Tocheckthemodel,forcetheoutputcurrentto±4mAwithVDD=3.
0Vandtemperature=25°C.
Theoutputvoltageshouldbe:VOL~152mVandVOH~(VDD-145mV).
At85°C,theoutputvoltageshouldbe:VOL~185mVandVOH~(VDD-175mV).
(~20%increaseinRON.
)IntherealCSS555IC,theoutputdriveremploysa"Break-Before-Make"pre-driver.
ThispreventsthelargecurrentspikesthatcanoccurifbothoutputFET'sareonduringtheswitchingtime.
Thepre-driverdelaystheturn-onsignaluntiltheFETthatwasonturnscompletelyoff.
Forsimplicity,thepre-driverismodeledwiththreecascadedinverters.
Itdoesnotincludethe"Break-Before-Make"logic.
Thereforethemodelwilltendtooverestimatetheswitchingcurrent.
Tomakeitmorerealistic,thefirststageofthepre-driverispoweredby"BufferedVDD".
ItsswitchingcurrentisnotincludedwithVDD.
Theinputsignaltothepre-drivercomesdirectlyfromtheRCdelaycircuitandisveryslow,whichisnotthecaseintherealcircuit.
Theslowriseandfalltimesgeneratehigherswitchingcurrentsthanwhatwouldbeseenwiththerealcircuit.
InputProtectionDiodesandPadCapacitanceVirtuallyallCMOSIC'shaveclampdiodesontheirI/OpinstopreventdamageduetoESD.
Clampdiodeshavebeenincludedinthismodel.
IfanI/OpinisforcedaboveVDDorbelowVSS,theclampdiodewillconductheavilyandlimitthevoltageatthepin.
Thediode'sseriesresistanceisabout5ohms.
Eachpadhasapproximately5pFcapacitancetoVSS.
ConvergenceErrorsThefeedbackpathintheRSflip-flopwilloccasionallycauseconvergenceproblems.
Ifthecircuitfailstoconverge,trythefollowing:1)Increasethe"TransientTimePointIterationLimit"(suggestatleast25)2)Decreasethe"RelativeAccuracy"3)Changethe"MaximumStepSize"(suggest1nsto100ns)4)Enable"GMINStepping"(ifavailable)5)IncreasethesupplyvoltageAN555-2CustomSiliconSolutions,Inc.
20094Ver.
1.
2–July2009CSS555(C)MicropowerTimer(SPICEModel)CSS555SPICEModel(continued)WorstCaseModelsThemodelssuppliedfortheCSS555deviceare"Typical"models.
Themodelparametersarecenteredforanaveragedevice.
Itisfairlystraightforwardtomodifythemodelforworst-casespeed,power,delayandoutputdriveconditions.
Theseparametersaregenerallydominatedbyoneortwocomponentsorprocessparameters.
Thefollowingtablesshowthechangestothemodeltocoverworst-caseconditions.
(Theworst-casemodelparametersarebasedonstatisticaldatafortheCSS555deviceandthewaferprocess.
Theyareapproximations.
Theyrepresentathree-sigmavariationbasedonavailabledata.
)Micro-power,StandardTripLevelsParameterSymbolComponentsSlowSpeedLowPowerTypicalFastSpeedHighPowerSupplyCurrentIDD0RBIAS,IBIAS2.
90M,1.
0uA2.
25M,1.
4uA1.
70M,1.
9uATripLevelsVTRIG,VCTRLR3,R4,R53.
0,3.
03,3.
0M2.
5,2.
5,2.
5M2.
0,1.
97,2.
0MDelayTimetDLYRDLY,CDLY2.
4M,1pF2.
0M,1pF1.
6M,1pFTimerOutputDriverVOL,VOHMP3,MN3(KPinmodel)2.
1E-5,6.
0E-52.
6E-5,7.
0E-53.
1E-5,8.
0E-5DischargeOutputVDISMN7(KPinmodel)6.
0E-57.
0E-58.
0E-5Table3ALow-power,StandardTripLevelsParameterSymbolComponentsSlowSpeedLowPowerTypicalFastSpeedHighPowerSupplyCurrentIDD0RBIAS,IBIAS520K,6.
9uA400K,9.
3uA300K,12.
5uATripLevelsVTRIG,VCTRLR3,R4,R53.
0,3.
03,3.
0M2.
5,2.
5,2.
5M2.
0,1.
97,2.
0MDelayTimetDLYRDLY,CDLY600K,1pF500K,1pF400K,1pFTimerOutputDriverVOL,VOHMP3,MN3(KPinmodel)2.
1E-5,6.
0E-52.
6E-5,7.
0E-53.
1E-5,8.
0E-5DischargeOutputVDISMN7(KPinmodel)6.
0E-57.
0E-58.
0E-5Table3BMicro-power,LowVoltageTripLevelsParameterSymbolComponentsSlowSpeedLowPowerTypicalFastSpeedHighPowerSupplyCurrentIDD0RBIAS,IBIAS2.
90M,1.
0uA2.
25M,1.
4uA1.
70M,1.
9uATripLevelsVTRIG,VCTRLR3,R4,R50.
60,4.
85,0.
60M0.
75,6.
0,0.
75M0.
90,7.
13,0.
90MDelayTimetDLYRDLY,CDLY2.
4M,1pF2.
0M,1pF1.
6M,1pFTimerOutputDriverVOL,VOHMP3,MN3(KPinmodel)2.
1E-5,6.
0E-52.
6E-5,7.
0E-53.
1E-5,8.
0E-5DischargeOutputVDISMN7(KPinmodel)6.
0E-57.
0E-58.
0E-5Table3CLow-power,LowVoltageTripLevelsParameterSymbolComponentsSlowSpeedLowPowerTypicalFastSpeedHighPowerSupplyCurrentIDD0RBIAS,IBIAS520K,6.
9uA400K,9.
3uA300K,12.
5uATripLevelsVTRIG,VCTRLR3,R4,R50.
60,4.
85,0.
60M0.
75,6.
0,0.
75M0.
90,7.
13,0.
90MDelayTimetDLYRDLY,CDLY600K,1pF500K,1pF400K,1pFTimerOutputDriverVOL,VOHMP3,MN3(KPinmodel)2.
1E-5,6.
0E-52.
6E-5,7.
0E-53.
1E-5,8.
0E-5DischargeOutputVDISMN7(KPinmodel)6.
0E-57.
0E-58.
0E-5Table3DAN555-2CustomSiliconSolutions,Inc.
20095Ver.
1.
2–July2009CSS555(C)MicropowerTimer(SPICEModel)CSS555SPICEModelSchematicAN555-2