Ferrite Beads Selection.pdf

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1、AN583:DesigningPowerIsolationFilterswithFerriteBeadsforAlteraFPGAs©July2009AN-583-1.0ThisapplicationnoteprovidesguidelinesfordesigningferritebeadfilternetworkstoisolatesharedpowerrailsofStratix®IVFPGAs.AdvancementsinFPGAtechnologyhaveincreaseddataratesbeyond10Gbps.Toachieveth

2、esedatarates,FPGAmanufacturerstypicallyrequiremultipleisolateddigitalandanalogsupplyrailstoseparatelypowerthecoreandI/Osaswellasthesensitivephase-lockedloops(PLLs)andgigabittransceiverblockswithintheFPGA.Asaresult,thecomplexityofthepowerdistributionsystemontheboardisgreatlyin

3、creased.Withlimitedboardspace,layercount,andcostbudgets,boarddesignersarefindingitincreasinglychallengingtodesigntheirFPGAboardswithintheconstraintsoftheirsystemrequirements.OnewaytosimplifythispowerdesignfortheStratixIVGXandGTfamilyofmulti-gigabittransceiver-basedFPGAsistobe

4、abletocleanlysharesimilarvoltagerailswhilemaintainingadequatehigh-frequencyseparationbetweenthesharedrails.Acommonstrategyistouseferritebeads.Thisapplicationnotedescribesproperbeadselectionanddesignconsiderationssuchasanti-resonance,inductor-capacitor(LC)tankoscillationavoida

5、nce,transferimpedanceanalysis,andDCvoltage(IR)dropminimization,whilemeetingtargetimpedancerequirementsfordecoupling.AlsodescribedisanalternativemethodofusingaPCBlayoutstructuretoreplacetheferritebeadastheinductivefiltercomponentinsomeinstances.Simulationresultscomparingtheper

6、formanceofthePCBstructuretotheferritebeadareevaluatedtovalidatetheeffectivenessofthePCBstructurefilternetworkandtounderstanditslimitations.FerriteBeadSelectionGenerally,ferritebeadsfallintotwocategories(1):■High-QBeads—Typicallyusedasresonatorsandmustnotbeusedinpowerisolation

7、circuits.■Low-QBeads—Alsoknownasabsorptivebeads,aremorelossyandmakegoodpowerfilternetworksbecausetheyaredesignedtoabsorbhigh-frequencynoisecurrentsanddissipateitasheat.Thesebeadshavehighimpedanceoverwidehigh-frequencybands,makingthemidealaslow-passnoisefilters.Manufacturersty

8、picallyspecifythecharacteristicperformanceoftheirferritebeadsinterms