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LTC3526L Datasheet(PDF) 8 Page - Linear Technology |
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LTC3526L Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LTC3526L/LTC3526LB 3526lfc exceeds the input by 0.24V, the IC powers itself from VOUTinsteadofVIN.Atthispointtheinternalcircuitryhas nodependencyontheVINinputvoltage,eliminatingthe requirementforalargeinputcapacitor.Theinputvoltage candropaslowas0.5V.Thelimitingfactorfortheap- plicationbecomestheavailabilityofthepowersourceto supplysufficientenergytotheoutputatlowvoltages,and maximumdutycycle,whichisclampedat90%typical. Notethatatlowinputvoltages,smallvoltagedropsdue toseriesresistancebecomecritical,andgreatlylimitthe powerdeliverycapabilityoftheconverter. LOW NOISE FIXED FREQUENCY OPERATION Soft-Start The LTC3526L/LTC3526LB contain internal circuitry to providesoft-startoperation.Thesoft-startcircuitryslowly rampsthepeakinductorcurrentfromzerotoitspeakvalue of750mA(typical)inapproximately0.5ms,allowingstart- upintoheavyloads.Thesoft-startcircuitryisresetinthe eventofashutdowncommandorathermalshutdown. Oscillator An internal oscillator sets the switching frequency to 1MHz. Shutdown Shutdown is accomplished by pulling the SHDN pin below0.3VandenabledbypullingtheSHDNpinabove 0.8V.AlthoughSHDNcanbedrivenaboveVINorVOUT (uptotheabsolutemaximumrating)withoutdamage, theLTC3526L/LTC3526LBhaveaproprietarytestmode thatmaybeengagedifSHDNisheldintherangeof0.5V to1VhigherthanthegreaterofVINorVOUT.Ifthetest modeisengaged,normalPWMswitchingactionisinter- rupted,whichcancauseundesirableoperationinsome applications.Therefore,inapplicationswhereSHDNmay bedrivenaboveVIN,aresistordividerorothermeans mustbeemployedtokeeptheSHDNvoltagebelow(VIN +0.4V)topreventthepossibilityofthetestmodebeing engaged.PleaserefertoFigure1fortwopossibleimple- mentations. Error Amplifier Thepositiveinputofthetransconductanceerroramplifier isinternallyconnectedtothe1.195Vreferenceandthe negativeinputisconnectedtoFB.Clampslimitthemini- mumandmaximumerrorampoutputvoltageforimproved large-signaltransientresponse.Powerconvertercontrol loop compensation is provided internally. An external resistivevoltagedividerfromVOUTtogroundprograms theoutputvoltageviaFBfrom1.5Vto5.25V. V V R R OUT = + 1 195 1 2 1 . • Current Sensing Losslesscurrentsensingconvertsthepeakcurrentsignalof theN-channelMOSFETswitchintoavoltagethatissummed withtheinternalslopecompensation.Thesummedsignal iscomparedtotheerroramplifieroutputtoprovideapeak currentcontrolcommandforthePWM. Current Limit The current limit comparator shuts off the N-channel MOSFETswitchonceitsthresholdisreached.Thecur- rentlimitcomparatordelaytooutputistypically60ns. Peakswitchcurrentislimitedtoapproximately750mA, independentofinputoroutputvoltage,unlessVOUTfalls below0.7V,inwhichcasethecurrentlimitiscutinhalf. Zero Current Comparator Thezerocurrentcomparatormonitorstheinductorcur- renttotheoutputandshutsoffthesynchronousrectifier (Refer to Block Diagram) operAtion ZETEX ZC2811E 3526l F01 LTC3526L/LTC3526LB SHDN VIN VCNTRL 4M ±30% LTC3526L/LTC3526LB SHDN 4M ±30% 1M R VCNTRL 1M R > (VCNTRL/(VIN + 0.4) – 1)M Figure 1. Recommended Shutdown Circuits when Driving SHDN above VIN |
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