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LTC3858EGN-1 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3858EGN-1 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 38 page  LTC3858-1 38581fb operaTion (Refer to the Functional Diagram) INTVCC/EXTVCC Power PowerforthetopandbottomMOSFETdriversandmost otherinternalcircuitryisderivedfromtheINTVCCpin.When theEXTVCCpinisleftopenortiedtoavoltagelessthan 4.7V,theVINLDO(lowdropoutlinearregulator)supplies 5.1VfromVINtoINTVCC.IfEXTVCCistakenabove4.7V, theVINLDOisturnedoffandtheEXTVCCLDOisturnedon. Onceenabled,theEXTVCCLDOsupplies5.1VfromEXTVCC toINTVCC.UsingtheEXTVCCpinallowstheINTVCCpower tobederivedfromahighefficiencyexternalsourcesuch asoneoftheLTC3858-1switchingregulatoroutputs. EachtopMOSFETdriverisbiasedfromthefloatingboot- strapcapacitor,CB,whichnormallyrechargesduringeach switchingcyclethroughanexternaldiodewhenthetop MOSFETturnsoff.IftheinputvoltageVINdecreasesto avoltageclosetoVOUT,theloopmayenterdropoutand attempt to turn on the top MOSFET continuously. The dropoutdetectordetectsthisandforcesthetopMOSFET offforaboutone-twelfthoftheclockperiodeverytenth cycletoallowCBtorecharge. Shutdown and Start-Up (RUN1, RUN2 and SS1, SS2 Pins) ThetwochannelsoftheLTC3858-1canbeindependently shutdownusingtheRUN1andRUN2pins.Pullingeitherof thesepinsbelow1.26Vshutsdownthemaincontrolloop forthatcontroller.Pullingbothpinsbelow0.7Vdisables bothcontrollersandmostinternalcircuits,includingthe INTVCCLDOs.Inthisstate,theLTC3858-1drawsonly8µA ofquiescentcurrent. TheRUNpinmaybeexternallypulledupordrivendirectly bylogic.WhendrivingtheRUNpinwithalowimpedance source,donotexceedtheabsolutemaximumratingof 8Vonthispin.TheRUNpinhasaninternal11Vvoltage clampthatallowstheRUNpintobeconnectedthrougha resistortoahighervoltage(forexample,VIN),solongas themaximumcurrentintotheRUNpindoesnotexceed 100µA. Thestart-upofeachcontroller’soutputvoltageVOUTis controlledbythevoltageontheSSpinforthatchannel. When the voltage on the SS pin is less than the 0.8V internalreference,theLTC3858-1regulatestheVFBvolt- agetotheSSpinvoltageinsteadofthe0.8Vreference. ThisallowstheSSpintobeusedtoprogramasoft-start byconnectinganexternalcapacitorfromtheSSpinto SGND.Aninternal1µApull-upcurrentchargesthisca- pacitorcreatingavoltagerampontheSSpin.AstheSS voltageriseslinearlyfrom0Vto0.8V(andbeyondupto theabsolutemaximumratingof6V),theoutputvoltage VOUTrisessmoothlyfromzerotoitsfinalvalue. Short-Circuit Latch-Off After the controller has been started and been given adequate time to ramp up the output voltage, the SS capacitorisusedinashort-circuittime-outcircuit.Spe- cifically,oncethevoltageontheSSpinrisesabove2V (the armingthreshold),theshort-circuittimeoutcircuitis enabled(seeFigure1).Iftheoutputvoltagefallsbelow 70%ofitsnominalregulatedvoltage,theSScapacitor beginsdischargingwithanet9µApull-downcurrenton theassumptionthattheoutputisinanovercurrentand/or short-circuitcondition.Iftheconditionlastslongenough toallowtheSSpinvoltagetofallbelow1.5V(the latchoff threshold),thecontrollerwillshutdown(latchoff)until theRUNpinvoltageortheVINvoltageisrecycled. Thedelaytimefromwhenanshort-circuitoccursuntil thecontrollerlatchesoffcanbecalculatedusingthefol- lowingequation: t C V V µA LATCH SS SS ≈ – . 1 5 9 whereVSSistheinitialvoltage(mustbegreaterthan2V) ontheSSpinatthetimetheshort-circuitoccurs.Normally theSSpinvoltagewillhavebeenpulleduptotheINTVCC voltage(5.1V)bytheinternal1µApull-upcurrent. NotethatthetwocontrollersontheLTC3858-1havesepa- rate,independentshort-circuitlatchoffcircuits.Latchoff canbeoverridden/defeatedbyconnectingaresistor150k orlessfromtheSSpintoINTVCC.Thisresistorprovides enoughpull-upcurrenttoovercomethe9µApull-down currentpresentduringashort-circuit.Notethatthisresis- toralsoshortensthesoft-startperiod. |
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