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LTC3526-2 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3526-2 Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page LTC3526-2/LTC3526B-2 35262b2fc 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 TheLTC3526-2/LTC3526B-2containinternalcircuitryto providesoft-startoperation.Thesoft-startcircuitryslowly rampsthepeakinductorcurrentfromzerotoitspeakvalue of700mA(typical)inapproximately0.5ms,allowingstart- upintoheavyloads.Thesoft-startcircuitryisresetinthe eventofashutdowncommandorathermalshutdown. Oscillator An internal oscillator sets the switching frequency to 2MHz. Shutdown Shutdown is accomplished by pulling the SHDN pin below0.3VandenabledbypullingtheSHDNpinabove 0.8Vtypical.AlthoughSHDNcanbedrivenaboveVINor VOUT(uptotheabsolutemaximumrating)withoutdam- age,theLTC3526-2/LTC3526B-2haveaproprietarytest modethatmaybeengagedifSHDNisheldintherange of0.5Vto1VhigherthanthegreaterofVINorVOUT.If thetestmodeisengaged,normalPWMswitchingaction is interrupted, which can cause undesirable operation in some applications. Therefore, in applications where SHDNmaybedrivenaboveVIN,aresistordividerorother meansmustbeemployedtokeeptheSHDNvoltagebelow (VIN+0.4V)topreventthepossibilityofthetestmode beingengaged.PleaserefertoFigure1fortwopossible implementations. Error Amplifier Thepositiveinputofthetransconductanceerroramplifier isinternallyconnectedtothe1.195Vreferenceandthe negativeinputisconnectedtoFB.Clampslimitthemini- mumandmaximumerrorampoutputvoltageforimproved large-signaltransientresponse.Powerconvertercontrol loop compensation is provided internally. An external resistivevoltagedividerfromVOUTtogroundprograms theoutputvoltageviaFBfrom1.6Vto5.25V. 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. Peakswitchcurrentislimitedtoapproximately700mA, independentofinputoroutputvoltage,unlessVOUTfalls below0.7V,inwhichcasethecurrentlimitiscutinhalf. Zero Current Comparator Thezerocurrentcomparatormonitorstheinductorcur- renttotheoutputandshutsoffthesynchronousrectifier (Refer to Block Diagram) operAtion ZETEX ZC2811E 35262b2 F01 LTC3526-2/LTC3526B-2 SHDN VIN VCNTRL 4M ±30% LTC3526-2/LTC3526B-2 SHDN 4M ±30% 1M R R > (VCNTRL/(VIN + 0.4) – 1)M VCNTRL 1M Figure 1. Recommended Shutdown Circuits when Driving SHDN above VIN |
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