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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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LT3071EUFD датащи(PDF) 18 Page - Linear Technology |
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LT3071EUFD датащи(HTML) 18 Page - Linear Technology |
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18 / 28 page  LT3071 3071f ApplicAtions inFormAtion Typical0603or0805case-sizecapacitorshaveanESLof ~800pHandPCBmountingcancontributeupto~200pH. Thus, it becomes necessary to reduce the parasitic inductancebyusingaparallelcapacitorcombination. Asuitablemethodologymustcontrolthisparallelingas capacitorswiththesameself-resonantfrequency,fR,will formatankcircuitthatcaninduceringingoftheirown accord.SmallamountsofESR(5mΩto20mΩ)havesome benefitindampeningtheresonantloop,buthigherESRs degradethecapacitorresponsetotransientloadsteps withrise/falltimeslessthan1µs.Themostareaefficient parallelcapacitorcombinationisagraduated4/2/1scale offRofthesamecasesize.Undertheseconditions,the individualESLsarerelativelyuniform,andtheresonance peaksaredeconstructivelyspreadbeyondtheregulator bandwidth.Therecommendedparallelcombinationthat approximates15µFis10µF+4.7µF+2.2µF.Capacitors withcasesizeslargerthan0805havehigherESLand lower ESR (<5mΩ). Therefore, more capacitors with smallervalues(<10µF)mustbechosen.Usersshould considernewgeneration,lowinductancecapacitorsto pushoutfRandmaximizestability.Refertothesurface mountceramiccapacitormanufacturer’sdatasheetsfor capacitorspecifications.Figure3illustratesanoptimum PCBlayoutfortheparalleloutputcapacitorcombination, butalsoillustratestheGNDconnectionbetweentheIN capacitorandtheOUTcapacitorstominimizetheAC GNDloopforfastloadtransients.Thistightbypassing connectionminimizesEMIandoptimizesbypassing. Many of the applications in which the LT3071 excels, suchasFPGA,ASICprocessororDSPsupplies,typically requireahighfrequencydecouplingcapacitornetworkfor thedevicebeingpowered.Thisnetworkgenerallyconsists ofmanylowvalueceramiccapacitorsinparallel.Insome applications,thistotalvalueofcapacitancemaybeclose totheLT3071’sminimum15µFcapacitancerequirement. Thismayreducetherequiredvalueofcapacitancedirectly attheLT3071’soutput.Multiplelowvaluecapacitorsin parallelpresentafavorablefrequencycharacteristicthat pushesmanyoftheparasiticpoles/zeroesbeyondthe LT3071’sunity-gaincrossoverfrequency.Thistechnique illustrates the method that extracts the full bandwidth performanceoftheLT3071. Giveadditionalconsiderationtotheuseofceramiccapaci- tors.Ceramiccapacitorsaremanufacturedwithavarietyof dielectrics,eachwithdifferentbehavioracrosstemperature andappliedvoltage.Themostcommondielectricsused arespecifiedwithEIAtemperaturecharacteristiccodesof Z5U,Y5V,X5RandX7R.TheZ5UandY5Vdielectricsare goodforprovidinghighcapacitancesinasmallpackage, buttheytendtohavestrongvoltageandtemperature coefficientsasshowninFigures4and5.Whenusedwith a5Vregulator,a16V10µFY5Vcapacitorcanexhibitan effectivevalueaslowas1µFto2µFfortheDCbiasvoltage appliedandovertheoperatingtemperaturerange.TheX5R andX7Rdielectricsresultinmorestablecharacteristics andaremoresuitableforuseastheoutputcapacitor. Figure 3. Example PCB Layout 2.2µF 4.7µF 10µF 47µF 3071 F03 Lo-Z INPUT LOAD PLANE LT3071 SENSE IN OUT GND |
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