UC3842应用中文资料

ËóïððßAPPLICATIONNOTEËÝíèìîñíñìñëËÝíèìîñíñìñëøÐÉÓ÷×ÝËÝíèìîÜÝóÜÝÓÑÍÚÛÌËÝíèìîøËÝíèìîËÝíèìíËÝíèììËÝíèìë÷ËÝíèìîËÝíèìîËÝíèìîïFigure1.Two-LoopCurrent-ModeControlSystem3-53ïËóïððßFigure2.InductorLooksLikeaCurrentSourcetoSmallSignalsA)DirectDutyCycleControlB)CurrentModeControlFigure3.RequiredErrorAmplifierCompensationforContinuousInductorCurrentDesigns3-54îíí¿Ý·Î·¦Ý·Ý·Î·¦Ý·í¾Ý·îß÷Þ÷íËóïððßNote:1.A=DIL-8PinNumber.B=SO-16PinNumber.2.Toggleflipflopusedonlyin1844Aand1845A.Figure43-55ËÝíèìîñíñìñëÐÉÓ×ÝËÝïèìîñíñìñë×ÝÜÝñÜÝï³ßÐÉÓÒÓÑÍÚÛÌËÝïèìîËÝïèììïêÊïðÊËÊÔÑËÝïèìíËÝïèìëèòëÊéòçÊËÝïèìîËÝïèìíïððûËÝïèììËÝïèìëðûëðûÜÝóÜÝøï³ß÷ëððµØ¦Îы‹‹‹‹‹‹‹‹‹‹×ÝìËóïððßUNDER-VOLTAGELOCKOUTTheUVLOcircuitinsuresthatisadequatetomakethefullyoperationalbeforeenablingtheoutputstage.Figure5showsthattheUVLOturn-onandturn-offthresholdsarefixedinternallyat16Vand10Vre-spectively.The6Vhysteresispreventsoscillationsduringpowersequencing.Figure6showssupplycurrentrequirements.Start-upcurrentislessthan1mAforeffi-cientbootstrappingfromtherectifiedinputofanoff-lineconverter,asillustratedbyFigure6.Duringnormalcircuitoperation,isdevelopedfromauxiliarywindingwithandAtstart-up,however,mustbechargedto16VthroughWithastart-upcurrentof1mA,canbeaslargeas100andstillchargeGINwhen=90VRMS(lowline).Powerdissipationinwouldthenbelessthan350mWevenunderhighline=130VRMS)conditions.DuringUVLO;theoutputdriverisinalowstate.Whileitdoesn’texhibitthesamesaturationcharacteristicsasnor-maloperation,itcaneasilysink1milliamp,enoughtoin-suretheMOSFETisheldoff.Figure6.DuringUnder-VoltageLockout,theoutputdriverisbiasedtosinkminoramountsofcurrent.OSCILLATORTheUC3842oscillatorisprogrammedasshowninFigure8.TimingcapacitorCTischargedfrom(5V)throughthetimingresistoranddischargedbyaninternalcur-rentsource.Thefirststepinselectingtheoscillatorcomponentsistodeterminetherequiredcircuitdeadtime.Onceobtained,Figure9isusedtopinpointtheneareststandardvalueofCTforagivendeadtime.Next,theappropriatevalueisinterpolatedusingtheparametersforandoscillatorfrequency.Figure10illustratesthecombinationsversusoscillatorfrequency.Thetimingresistorcanbecal-culatedfromthefollowingformula.(kHz)=1.72/(k)×TheUC3844andUC3845haveaninternaldivide-by-twoflip-flopdrivenbytheoscillatorfora50%maximumdutycycle.Therefore,theiroscillatorsmustbesettorunattwicethedesiredpowersupplyswitchingfrequency.TheUC3842andUC3843oscillatorrunsATtheswitchingfre-quency.Eachoscillatorofthefamilycanbeusedtoamaximumof500kHz.êËÊÔÑÊÝÝËÝíèìîñíñìñëëËÊÔÑïêÊïðÊêÊÊÝÝêï³ßêÊÝÝÜïÝ×ÒÉßËÈÝ×ÒÎ×ÒïêÊï³ßÎ×ÒïððµÊßÝãçðÊÎÓÍÙ×ÒÎ×Òíëð³ÉøÊßÝãïíðÊÎÓÍ÷ËÊÔÑï³ßÓÑÍÚÛÌËÝíèìîèÝÌÎÌÊÎÛÚøëÊ÷çÝÌÝÌÎÌïðÎÌñÝÌÚ±­½øµØ¦÷ãïòéîñøÎÌøµ÷ÝÌøº÷÷ËÝíèììËÝíèìëëðûËÝíèìîËÝíèìíËÝíèìîñíñìñëëððµØ¦3-56éËÝíèìîñíñìñëËóïððß0019-9Figure8Deadtimevs5k)Figure9TimingResistancevsFrequency0019-10FREOUENCY-(Hz)0019-11Figure100019-13Figure11.Transformer-CoupledCurrentSensingËÝíèìîËÝíèìíïððûËÝíèììËÝíèìëëðû×ÝïëûÜÓßÈÜÓßÈãïŠø¬ÜÛßÜñ¬ÐÛÎ×ÑÜ÷ËÝíèìîñíÜÓßÈãïŠø¬ÜÛßÜñÐÛÎ×ÑÜ÷ËÝíèììñë¬ÜÛßÜãïñÚÝÌøÎÌëµ÷èçïðËÝíèìîïîÎÍÎÍÛñßøÊ݊ïòìÊ÷øíÎÍ÷ÊÝããÛñßïïÎÍÎÍÊÝÊÎÍø°µ÷ÒÎÍíÎÍÒããï·ø°µ÷ÒÊÝíÎÍïïñÎÝÎݲ­ËÝíèìîïÊïîíÒ¨ïÊÎÍ×Ðã·ø°µ÷ãÒøÊ݊ïòìÊ÷ø÷ãã·³¿¨ã3-57ËóïððßððïçóïîFigure12.CurrentSensingERRORAMPLIFIERTheerroramplifier(E/A)configurationisshowninFigure13.Thenon-invertinginputisnotbroughtouttoapin,butisinternallybiasedto2.5V±2%.TheE/Aoutputisavailableatpin1forexternalcompensation,allowingtheusertocontroltheconverter’sclosed-loopfrequencyre-sponse.Figure14showsanE/Acompensationcircuitsuitableforstabilizinganycurrent-modecontrolledtopologyexceptforflybackandboostconvertersoperatingwithinductorcur-rent.Thefeedbackcomponentsaddapoletothelooptransferfunctionat=andarecho-sensothatthispolecancelsthezerooftheoutputfiltercapacitorESRinthepowercircuit.andfixthelow-frequencygain.Theyarechosentoprovideasmuchgainaspossiblewhilestillallowingthepoleformedbytheout-putfiltercapacitorandloadtorollofftheloopgaintouni-ty(0dB)atfThistechniqueinsuresconverterstabilitywhileprovidinggooddynamicresponse.Figure14.CompensationTheE/Aoutputwillsource0.5mAamdsink2mA.Alow-erlimitforisgivenby:ððïçóïìFigure13.E/AConfiguration3-58ïîøÛñß÷ïíîòëÊîûÛñßïïìÛñߺÐãïñîÎÚôÝÚÎÚÝÚÛÍÎÎ×ÎÚººÍÉ×ÌÝØ×ÒÙñìøð¼Þ÷Ûñßðòë³ßî³ßÎÚïíÛñßïìËóïððß101001K10K100K1M10MFREQUENCY-(Hz)0019-16Figure15.ErrorAmplifierOpen-LoopFrequencyResponse0019-17Figure16.E/ACompensationCircuitforContinuousBoostandFlybackTopologies3-59ÛñßîßÎ×øÊÑ÷ÜÝÊÑøÓßÈ÷ãøîß÷Î×Î×ïëËÝíèìîÛñßÛñßïðÓئïÓئÎØÐïêÎÐÝÐËÝíèìîÐÉÓïßÓÑÍÚÛÌõîðð³ßÊ×ÒãíðÊèð³ÉîððµØ¦×ÝÓÑÍÚÛÌÊÝ×ݼÊñ¼ÌÚÛÌÐÉÓ×Ýîðð³ßðòíÊïíßÐÉÓÐÉÓïëïêÛñßËóïððßOUTPUTCURRENTSOURCEORSINK-(A)0019-18Figure17.OutputSaturationCharacteristics20TO30VFigure19.IsolatedMOSFETDrive10TO20VFigure18.DirectMOSFETDrive12TO20VFigure20.BipolarDrivewithNegativeTurn-OffBiasÚÛÌïèïçîðËÝíèìîÓÑÍÚÛÌ×ÝÓÑÍÚÛÌïßïèÚÛÌÜï×ÝïçÓÑÍÚÛÌîðÎïÎîÝïËÝíèìîËÝíéðê×ÝËÝíéðëñêñé×ÝÚÛÌïéïèÓÑÍÚÛÌïçÓÑÍÚÛÌîð3-61ËóïððßFigure22.SyncCircuitImplementation0019-32Figure21.(a.)NoiseonPin4cancauseoscillatortopre-trigger.(b.)Withexternalsync.,noisedoesnotapproachthresholdlevel.3-61ÐÝ×ÝËÝíèìîÊÝÝÊÎÛÚøðòïÚ÷îïÎÌñÝÌÎÌñÝÌîòéÊÝÌÝÌÝÌïððð°ÚêÓÑÍÚÛÌêíïîïÞÎÌñÝÌøÝÌ÷ÝÌÝÌÝÌÐÉÓÎÌÝÌø´±½¿