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LT3070

3070fa

ApplicAtions inFormAtion

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 LT3070 excels,

suchasFPGA,ASICprocessororDSPsupplies,typically

requireahighfrequencydecouplingcapacitornetworkfor

thedevicebeingpowered.Thisnetworkgenerallyconsists

ofmanylowvalueceramiccapacitorsinparallel.Insome

applications,thistotalvalueofcapacitancemaybeclose

totheLT3070’sminimum15µFcapacitancerequirement.

Thismayreducetherequiredvalueofcapacitancedirectly

attheLT3070’soutput.Multiplelowvaluecapacitorsin

parallelpresentafavorablefrequencycharacteristicthat

pushesmanyoftheparasiticpoles/zeroesbeyondthe

LT3070’sunity-gaincrossoverfrequency.Thistechnique

illustrates the method that extracts the full bandwidth

performanceoftheLT3070.

Giveadditionalconsiderationtotheuseofceramiccapaci-

tors.Ceramiccapacitorsaremanufacturedwithavarietyof

dielectrics,eachwithdifferentbehavioracrosstemperature

andappliedvoltage.Themostcommondielectricsused

arespecifiedwithEIAtemperaturecharacteristiccodesof

Z5U,Y5V,X5RandX7R.TheZ5UandY5Vdielectricsare

goodforprovidinghighcapacitancesinasmallpackage,

buttheytendtohavestrongvoltageandtemperature

coefficientsasshowninFigures4and5.Whenusedwith

a5Vregulator,a16V10µFY5Vcapacitorcanexhibitan

effectivevalueaslowas1µFto2µFfortheDCbiasvoltage

appliedandovertheoperatingtemperaturerange.TheX5R

andX7Rdielectricsresultinmorestablecharacteristics

andaremoresuitableforuseastheoutputcapacitor.

The X7R type has better stability across temperature,

whiletheX5Rislessexpensiveandisavailableinhigher

values.CarestillmustbeexercisedwhenusingX5Rand

X7R capacitors; the X5R and X7R codes only specify

operatingtemperaturerangeandmaximumcapacitance

change over temperature. Capacitance change due to

DCbiaswithX5RandX7RcapacitorsisbetterthanY5V

andZ5Ucapacitors,butcanstillbesignificantenoughto

dropcapacitorvaluesbelowappropriatelevels.Capacitor

DCbiascharacteristicstendtoimproveascomponent

casesizeincreases,butexpectedcapacitanceatoperat-

ingvoltageshouldbeverified.Voltageandtemperature

coefficientsarenottheonlysourcesofproblems.Some

ceramiccapacitorshaveapiezoelectricresponse.Apiezo-

electricdevicegeneratesvoltageacrossitsterminalsdue

tomechanicalstress,similartothewayapiezoelectric

microphoneworks.Foraceramiccapacitorthestress

canbeinducedbyvibrationsinthesystemorthermal

transients.

2.2µF

4.7µF

10µF

47µF

3070 F03

Lo-Z

INPUT

LOAD PLANE

LT3070

SENSE

IN OUT

GND

Figure 3. Example PCB Layout

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