施密特触发器

SCEA046–September2011

UnderstandingSchmittTriggers

ChrisCockrill

MostCMOS,BiCMOSandTTLdevicesrequirefairlyfastedgesonthehighandlowtransitionsontheirinputs.Iftheedgesaretooslowtheycancauseexcessivecurrent,oscillationandevendamagethedevice.Slowedgesaresometimeshardtoavoidatpoweruporwhenusingpushbuttonormanualtypeswitcheswiththelargecapacitorsneededforfiltering.Heavilyloadedoutputscanalsocauseinputriseandfalltimetobeoutofspecforthenextpartdowntheline.

Onanormal(non-Schmitttrigger)inputthepartwillswitchatthesamepointontherisingedgeandfallingedge.Withaslowrisingedgethepartwillswitchatthethreshold.WhentheswitchoccursitwillrequirecurrentfromVcc.WhencurrentisforcedfromVCC,theVCClevelcandropcausingthethresholdtoshift.Whenthethresholdshiftsitwillcrosstheinputagaincausingtheparttoswitchagain.Thiscangoonandoncausingoscillationwhichcancauseexcessivecurrent.Thesamethingcanhappenifthereisnoiseontheinput.Thenoisecancrossthethresholdmultipletimesandcauseoscillationormultipleclocking.ThesolutiontotheseproblemsistouseaSchmitttriggertypedevicetotranslatetheslowornoisyedgesintosomethingfasterthatwillmeettheinputriseandfallspecsofthefollowingdevice.AtrueSchmitttriggerinputwillnothaveriseandfalltimelimitations.

TherearepartsthathaveSchmitttriggeraction.Thismeansthattheywillhaveasmallamountof

hysteresisthatwillhelpwithnoiserejectionbutwillstillhaveaninputriseandfalltimelimit.ThesewillusuallynothaveanyVTspecsinthedatasheetandwillhaveriseandfalltimelimitationsspecifiedfortheinputsintherecommendedoperatingconditions.

ThetrueSchmitttriggerinputhastheswitchingthresholdadjustedwherethepartwillswitchatahigherpoint(Vt+)ontherisingedgeandatalowerpoint(Vt–)onthefallingedge.ThedifferenceintheseswitchingpointsiscalledHysteresis(^Vt).HereisanexampleofSchmitttriggerspecs.

PARAMETER

TESTCONDITIONS

VCC1.65V

VT+

(Positive-goinginputthresholdvoltage)

2.3V3V4.5V5.5V1.65V

VT–

(Negative-goinginputthresholdvoltage)

2.3V3V4.5V5.5V1.65V2.3V

ΔVT

Hysteresis(VT+–VT–)

3V4.5V5.5V

(1)

25°C

MIN

TYP

(1)

MAX

MIN0.761.081.482.192.650.350.560.1.511.880.360.450.510.580.69

MAX1.131.561.922.743.330.590.881.21.972.40.0.780.830.931.04

UNIT

V

V

V

NOTEREFERENCEDINTABLE,NONOTESUBMITTED

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Theimportantthingtorememberhereisthat(Vt+max)=Vihand(VT–min)=Vil.

InthespecsyouwillseemultiplelimitsrelatedtotheSchmitttriggerinputs.Allofthelimitsareimportantfordifferentreasons.Ontheinputrisingedgethepartwillbeguaranteedtoswitchbetween(Vt+min)and(Vt+max).Onthefallingedgethepartwillbeguaranteedtoswitchbetween(Vt–max)and(Vt–min).Thepartisguaranteednottoswitchbetween(Vt–max)and(Vt+min).Thisisimportantfornoiserejection.Thehysteresisisthedeltabetweenwherethepartswitchesontherisingedgeandwhereitswitchesonthefallingedge.Wespecifythishysteresiswillbeatleasttheminandnomorethanthemax(^Vt)spec.

VT + maxVT + minVT - maxVT - minInput SignalSwitches High HereVihSwitches Low HereVilOutput SignalInthefigureabove,theinputlevelsVihandVilmustbegreaterthan(VT+max)andlessthan(VT–min)toinsurethepartwillswitch.Theswitchingpointsontheplotaboveareseparatedtogiveaclearervisualpicture.Inrealitythe(VT+min)and(VT–max)mayoverlap.

OnecommonmisconceptionisthatthecurrentconsumptionwillbelesswhenswitchingaslowsignalintoaSchmitttrigger.ThisispartlytruebecausetheSchmitttriggerpreventsoscillationwhichcandrawalotofcurrent;howeveryouwillstillseehigherIcccurrentduetotheamountoftimetheinputisnotattherail.ThisisDeltaIcc.DeltaIcciswheretheinputsarenotattherailsandupperorlowerdrivetransistorsarepartiallyon.TheplotbelowshowsIccacrosstheinputvoltagesweep.

2.5

2

ICC- Supply Current - mA1.5

1

0.5

0

01

23VI- Input Voltage - V

45

Figure1.SupplyCurrentasaFunctionofInputVoltage

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Schmitttriggersshouldbeusedanytimeyouneedtotranslateasignwaveintoasquarewaveasshowninthisoscillatorapplication.Ortheyshouldbeusedwhereaslowornoisyinputneedstobespeduporcleanedupasintheswitchde-bouncercircuit.

RF~2.2 MWSN74LVC1GU04RS~1 kWC50 pFC1~32 pF

CL16 pFC2~32 pFFigure2.OscillatorApplicationUsingSchmittTriggerInverter

5 V10 kW1/6 74C1410 nCircuit Symbol for aSchmittTriggerFigure3.SwitchDe-bouncerUsingSchmittTriggerInverter

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Conclusion

Schmitttriggersshouldbeusedanytimeyouneedto1.Changeasignwaveintoasquarewave.

IN

(a)OUT

2.Havenoisysignalsthatneedtobecleanedup

IN(a)OUT3.Haveslowedgesthatneedtobeconvertedtofastedges.

IN

(a)

OUT

Wespecifythepartwillswitchontherisingedgebetween(VT+min)and(VT+max).Wespecifythepartwillswitchonthefallingedgebetween(VT–max)and(VT–min).

Between(VT+min)and(VT–max)weassurethepartwillnotswitch.Thiscanbeusedfornoiserejection.These2limitscanoverlap.

Weassuretherewillbeaminimumamountofhysteresis.ThisisspecifiedasdeltaVTmin.Vih=(VT+max)Vil=(VT–min)

TexasInstrumentSchmitttriggerfunctionsareavailableinmostalltechnologyfamiliesfromthe30yearold74XXfamilytothelatestAUP1Tfamily.ThesetwoSchmittTriggerfunctionsareavailableinmostallfamilies.

14forinvertingSchmitttrigger

17fornon-invertingSchmitttriggerTexasInstrumentalsohasacompletelineoflittlelogicproductswithSchmitttriggerinputs.Configurations

SN74LVC1G57,SN74LVC1G58,SN74LVC1G97,SN74LVC1G98,SN74LVC1G99SN74AUP1G57,SN74AUP1G58,SN74AUP1G97,SN74AUP1G98,SN74AUP1G99

LowtoHighTranslators

SN74AUP1T02,SN74AUP1T04,SN74AUP1T08,SN74AUP1T14,SN74AUP1T157,SN74AUP1T158,SN74AUP1T17,SN74AUP1T32,SN74AUP1T86

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