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LTC3541EDD-1-TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3541EDD-1-TRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LTC3541-1 35411fa APPLICATIO S I FOR ATIO ThebasicLTC3541-1applicationcircuitisshownonthe firstpageofthisdatasheet.Externalcomponentselection isdrivenbytheloadrequirementandrequirestheselection ofL,followedbyCIN,COUT,andfeedbackresistorvalues forthebuckandtheselectionoftheoutputcapacitorand feedbackvaluesfortheVLDOandlinearregulator. BUCK REGULATOR Inductor Selection Formostapplications,theappropriateinductorvaluewill beintherangeof1.5 µHto3.3µHwith2.2µHthemost commonly used. The exact inductor value is chosen largely based on the desired ripple current and burst rippleperformance.Generally,largevalueinductorsre- duceripplecurrent,andconversely,smallvalueinductors producehigherripplecurrent.HigherVINorVOUTmay alsoincreasetheripplecurrentasshowninEquation1. Areasonablestartingpointforsettingripplecurrentis ΔIL=200mA(40%of500mA). Δ = ( )( ) − I f L V V V L OUT OUT IN 1 1 (1) TheDCcurrentratingoftheinductorshouldbeatleast equaltothemaximumloadcurrentplushalftheripple currenttopreventcoresaturation.Thus,a600mArated inductorshouldbeenoughformostapplications(500mA +100mA).Forbetterefficiency,choosealowDCresis- tanceinductor. Inductor Core Selection Different core materials and shapes will change the size/currentandprice/currentrelationshipofaninduc- tor.Toroidorshieldedpotcoresinferriteorpermalloy materialsaresmallanddon’tradiatemuchenergy,but generallycostmorethanpowderedironcoreinductors withsimilarelectricalcharacteristics.Thechoiceofwhich styleinductortouseoftendependsmoreonthepricevs sizerequirementandanyradiatedfield/EMIrequirements ratherthanwhattheLTC3541-1requirestooperate.Table2 showssometypicalsurfacemountinductorsthatwork wellinLTC3541-1applications. CIN and COUT Selection Incontinuousmode,thesourcecurrentofthetopMOSFET isasquarewaveofdutycycleVOUT/VIN.Topreventlarge voltagetransients,alowESRinputcapacitorsizedforthe maximumRMScurrentmustbeused.ThemaximumRMS capacitorcurrentisgivenby: c I V V V IN OMAX OUT IN OUT required IRMS ≅ − ( ) 1/22 VIN This formula has a maximum at VIN = 2VOUT, where IRMS=IOUT/2.Thissimpleworst-caseconditioniscommon- lyusedfordesign.Notethatthecapacitormanufacturer’s ripplecurrentratingsareoftenbasedon2000hoursof life.Thismakesitadvisabletofurtherderatethecapaci- tororchooseacapacitorratedatahighertemperature thanrequired.Alwaysconsultthemanufacturerwithany questionregardingpropercapacitorchoice. TheselectionofCOUTforthebuckregulatorisdrivenby thedesiredbucklooptransientresponse,requiredeffective seriesresistance(ESR)andburstrippleperformance. TheLTC3541-1minimizestherequirednumberofexternal components by providing internal loop compensation forthebuckregulatorloop.Loopstability,transientre- sponseandburstperformancecanbetailoredbychoice ofoutputcapacitance.Formanyapplications,desirable stability,transientresponseandrippleperformancecan Table 2. Representative Surface Mount Inductors PART NUMBER VALUE (µH) DCR ( Ω MAX) MAX DC CURRENT (A) SIZE W × L × H (mm3) Sumida CDRH3D23 1.0 1.5 2.2 3.3 0.025 0.029 0.038 0.048 2.0 1.5 1.3 1.1 3.9 ×3.9×2.4 Sumida CMD4D06 2.2 3.3 0.116 0.174 0.950 0.770 3.5 ×4.3×0.8 Coilcraft ME3220 1.0 1.5 2.2 3.3 0.058 0.068 0.104 0.138 2.7 2.2 1.0 1.3 2.5 ×3.2×2.0 Murata LQH3C 1.0 2.2 0.060 0.097 1.00 0.79 2.5 ×3.2×2.0 Sumida CDRH2D11/HP 1.5 2.2 0.06 0.10 1.00 0.72 3.2 ×3.2×1.2 |
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