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LTC3210-2 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3210-2 Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page LTC3210-2/LTC3210-3 0 321023fb operaTion Figure 2. Charge Pump Thevenin Equivalent Open-Loop Circuit TEMPERATURE (°C) –40 3.4 3.6 3.8 60 3.2 3.0 –15 10 35 85 321023 F03 2.8 2.6 2.4 VBAT = 3V VCPO = 4.2V C2 = C3 = C4 = 2.2µF Figure 3. Typical 1.5x ROL vs Temperature +– ROL CPO 1.5VBAT OR 2VBAT + – 321023 F02 Soft-Start Initially,whenthepartisinshutdown,aweakswitchcon- nectsVBATtoCPO.ThisallowsVBATtoslowlychargethe CPOoutputcapacitortopreventlargechargingcurrents. TheLTC3210-2/LTC3210-3alsoemployasoft-startfeature onitschargepumptopreventexcessiveinrushcurrent andsupplydroopwhenswitchingintothestep-upmodes. ThecurrentavailabletotheCPOpinisincreasedlinearly overatypicalperiodof150µs.Soft-startoccursatthe startofboth1.5xand2xmodechanges. Charge Pump Strength and Regulation RegulationisachievedbysensingthevoltageattheCPO pin and modulating the charge pump strength based ontheerrorsignal.TheCPOregulationvoltagesareset internally,andaredependentonthechargepumpmodes asshowninTable1. Table 1. Charge Pump Output Regulation Voltages CHARGE PUMP MODE REGULATED VCPO 1.5x 4.55V 2x 5.05V WhentheLTC3210-2/LTC3210-3operateineither1.5x modeor2xmode,thechargepumpcanbemodeledas aThevenin-equivalentcircuittodeterminetheamountof currentavailablefromtheeffectiveinputvoltageandef- fectiveopen-loopoutputresistance,ROL(Figure2). ROLisdependentonanumberoffactorsincludingthe switching term, 1/(2fOSC•CFLY),internalswitchresis- tancesandthenonoverlapperiodoftheswitchingcircuit. However,foragivenROL,theamountofcurrentavailable willbedirectlyproportionaltotheadvantagevoltageof 1.5VBAT–CPOfor1.5xmodeand2VBAT–CPOfor2x mode.ConsidertheexampleofdrivingwhiteLEDsfrom a3.1Vsupply.IftheLEDforwardvoltageis3.8Vandthe currentsourcesrequire100mV,theadvantagevoltagefor 1.5xmodeis3.1V•1.5–3.8V–0.1Vor750mV.Notice thatiftheinputvoltageisraisedto3.2V,theadvantage voltagejumpsto900mV—a20%improvementinavail- ablestrength. From Figure 2, for 1.5x mode the available current is givenby: I V V R OUT BAT CPO OL = ( . – ) 1 5 For2xmode,theavailablecurrentisgivenby: I V V R OUT BAT CPO OL = ( – ) 2 Noticethattheadvantagevoltageinthiscaseis3.1V•2 –3.8V–0.1V=2.3V.ROLishigherin2xmodebutasig- nificantoverallincreaseinavailablecurrentisachieved. TypicalvaluesofROLasafunctionoftemperatureare showninFigure3andFigure4. Shutdown Current Inshutdownmodeallthecircuitryisturnedoffandthe LTC3210-2/LTC3210-3drawaverylowcurrentfromtheVBAT supply.Furthermore,CPOisweaklyconnectedtoVBAT.The LTC3210-2/LTC3210-3entershutdownmodewhenboththe ENMandENCpinsarebroughtlowat150µs(typ).ENM |
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