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SP7661 数据表(PDF) 6 Page - Sipex Corporation |
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SP7661 数据表(HTML) 6 Page - Sipex Corporation |
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6 / 20 page  6 Mar -07 Rev N SP766 Wide Input Voltage Range 3A, 600kHz Buck Regulator © 2007 Sipex Corporation THEORY OF OPERATION General Overview The SP7661 is a fixed frequency, volt- age mode, synchronous PWM regulator optimizedforhighefficiency.Theparthas beenspecificallydesignedforsinglesupply operationfroma5Vto22Vinput. TheheartoftheSP7661isawidebandwidth transconductanceamplifierdesignedtoac- commodateTypeIIandTypeIIIcompensa- tionschemes.Aprecision0.8Vreference, presentonthepositiveterminaloftheerror amplifier, permits the programming of the output voltage down to 0.8V via the VFB pin.Theoutputoftheerroramplifier,COMP, iscomparedtoa1.1Vpeak-to-peakramp, whichisresponsiblefortrailingedgePWM control.ThisvoltagerampandPWMcontrol logicaregovernedbytheinternaloscillator thataccuratelysetsthePWMfrequencyto 600kHz. The SP7661 contains two unique control featuresthatareverypowerfulindistributed applications.First,nonsynchronousdriver controlisenabledduringstartup,toprohibit thelowsideswitchfrompullingdownthe output until the high side switch has at- temptedtoturnon.Second,a100%duty cycletimeoutensuresthatthelowsideswitch is periodically enhanced during extended periodsat100%dutycycle.Thisguaran- tees the synchronized refreshing of the BSTcapacitorduringverylargedutyratios. TheSP7661alsocontainsanumberofvalu- ableprotectionfeatures.ProgrammableVIN UVLOallowstheusertosettheexactvalue atwhichtheconversionvoltagecansafely begindown-conversion,andaninternalVCC UVLOwhichensuresthatthecontrolleritself has enough voltage to properly operate. Other protection features include thermal shutdownandshort-circuitdetection.Inthe eventthateitherathermal,short-circuit,or UVLOfaultisdetected,theSP7661isforced intoanidlestatewheretheoutputdrivers areheldoffforafiniteperiodbeforearestart isattempted. Soft Start “SoftStart”isachievedwhenapowercon- verter ramps up the output voltage while controllingthemagnitudeoftheinputsup- plysourcecurrent.Inamodernstepdown converter,rampingupthepositiveterminal oftheerroramplifiercontrolssoftstart.Asa result,excesssourcecurrentcanbedefined asthecurrentrequiredtochargetheoutput capacitor. IViN = COut•(∆VOut/∆TSOft-StArt) TheSP7661providestheuserwiththeop- tiontoprogramthesoftstartratebytying acapacitorfromtheSSpintoGND.The selectionofthiscapacitorisbasedonthe 10µApullupcurrentpresentattheSSpin andthe0.8Vreferencevoltage.Therefore, theexcesssourcecanberedefinedas: IViN = COut • [∆VOut•10µA/(CSS•0.8V)] Under Voltage Lock Out (UVLO) TheSP7661hastwoseparateUVLOcom- paratorstomonitorthebias(Vcc)andInput (ViN)voltagesindependently.TheVccUVLO isinternallysetto4.25V.TheViNUVLOis programmable through UViN pin. When UVINpinisgreaterthan2.5VtheSP7661 ispermittedtostartuppendingtheremoval ofallotherfaults.Apairofinternalresistors isconnectedtoUVINasshowninthefigure below. SP7661 R6 R7 + - 140KΩ 50KΩ 2.5V ON 2.2V OFF GND UVIN VIN Internal and external bias of UVIN |
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