555 Timing principle

《555 Timing principle》由会员上传分享，免费在线阅读，更多相关内容在学术论文-天天文库。

1、555TimingPrinciplesForFlipFlop,thetruthtableisSet0110Rst1010Qn+101NotAllowedQnNote:Qn,oldstate;Qn+1,newstate.Takeasimplecircuitasanexampletodemonstratethetimingprinciplesof555.Therearetwovoltagecomparatorsnameduppercomparatorandlowercomparatorrespectively.Thevoltageonthenegativepo

2、intoflowercomparatoris1/3VCC,whilethevoltageonthepositivepointofuppercomparatoris2/3VCC,whichisrealizedovervoltagedivision.Step1PoweronThecapacitorstartstochargeoncepoweron.Wecantakeitasshorttoground.Thelogiclevelsareshowedabove.Thestatewillretainuntilthecapacitorischargedover1/3V

3、CC.Step2Voltageofcapacitorisbetween1/3VCCand2/3VCCAsshownabove,theoutputretainstheoldstate.Thatmeanstheoutputishigh.Step3Voltageofcapacitorisover2/3VCCIfthevoltageofcapacitorislargerthan2/3VCC,itwilldischargewithlowstateoutput.Thenthevoltagewillgetbacktotherangeoflargerthan1/3VCCa

4、ndsmallerthan2/3VCC,whichmakestheoutputretainthelaststate.Sotheoutputstayslowstate.Step4Voltageofcapacitorissmallerthan1/3VCCTheanalysisisthesamewithstep1.Summary:Withoutconsiderationaboutthepoweronstate,thevoltageofcapacitorisvaryingbetween1/3VCCand2/3VCC.PulsetimecalculationAsde

5、scribedbefore,theoutputishighwhenthecapacitorischargingform1/3VCCto2/3VCC.Theoutputstayslowwhenthecapacitorisdischargingfrom2/3VCCto1/3VCC.ThehighstatedependsontheR1+R2andthecapacitorC,andthelowstatedependsontheR2andtheC.PS:ForRCcircuitshownasbelowu(t)=u(t=0)+[u(t=∞)‐u(t=0)][1‐exp

6、(‐t/(R*C))]u(t):voltageofcapacitoranytime;u(t=0):initialvoltage;u(t=∞):thefinalvoltageifchargeforever.Ifu(t=0)=1/3VCCu(t=∞)=VCCThenLetu(t)=2/3VCCt=ln2*R*CSoforthe555applicationcircuitshownabove,IthinkthatThighshouldbe0.693*(R1+R2)*CTlowshouldbe0.693*R2*C.ButsomeonethoughtThigh=0.6

7、7*(R1+R2)*C,Tlow=0.67*R2*C.Hereisanotherschematicincluding555circuit.Theanalysismethodisthesameasdepictedabove.Butthepinforcharginganddischargingturnstopin3.Whenthevoltageofpin2isbetween1/3VCCand2/3VCC,theoutputishighwhichcanjustchargethecapacitor.