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LT3071 Datasheet(PDF) 18 Page - Linear Technology

Part No. LT3071
Description  5A, Low Noise, Programmable Output, 85mV Dropout Linear Regulator with Analog Margining
Download  28 Pages
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT3071 Datasheet(HTML) 18 Page - Linear Technology

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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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