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4.7PowerSupplies
4.7.1Power-SupplySequencing
4.7.2Power-SupplyDecoupling
TMS320C6727,TMS320C6726,TMS320C6722
Floating-PointDigitalSignalProcessors
SPRS268E–MAY2005–REVISEDJANUARY2007
FormoreinformationregardingTI’spowermanagementproductsandsuggesteddevicestopowerTI
DSPs,visitwww.ti.com/dsppower.
Thisdevicedoesnotrequirespecificpower-upsequencingbetweentheDV
DD
andCV
DD
voltagerails;
however,therearesomeconsiderationsthatthesystemdesignershouldtakeintoaccount:
1.Neithersupplyshouldbepoweredupforanextendedperiodoftime(>1second)whiletheother
supplyispowereddown.
2.TheI/ObufferspoweredfromtheDV
DD
railalsorequiretheCV
DD
railtobepoweredupinordertobe
controlled;therefore,anI/Opinthatissupposedtobe3-statedbydefaultmayactuallydrive
momentarilyuntiltheCV
DD
railhaspoweredup.Systemsshouldbeevaluatedtodetermineifthereis
apossibilityforcontentionthatneedstobeaddressed.InmostsystemswhereboththeDV
DD
and
CV
DD
suppliesramptogether,aslongasCV
DD
tracksDV
DD
closely,anycontentionisalsomitigatedby
thefactthattheCV
DD
railwouldreachitsspecifiedoperatingrangewellbeforetheDV
DD
railhasfully
ramped.
Inordertoproperlydecouplethesupplyplanesfromsystemnoise,placeasmanycapacitors(caps)as
possibleclosetotheDSP.Thecoresupplycapscanbeplacedintheinteriorspaceofthepackageand
theI/Osupplycapscanbeplacedaroundtheexteriorspaceofthepackage.FortheBGApackage,itis
recommendedthatboththecoreandI/OsupplycapsbeplacedontheundersideofthePCB.Forthe
TQFPpackage,itisrecommendedthatthecoresupplycapsbeplacedontheundersideofthePCBand
theI/OsupplycapsbeplacedonthetopsideofthePCB.
BothcoreandI/Odecouplingcanbeaccomplishedbyalternatingsmall(0.1µF)lowESRceramicbypass
capswithmedium(0.220µF)lowESRceramicbypasscapsclosetotheDSPpowerpinsandadding
largetantalumorceramiccaps(rangingfrom10µFto100µF)furtheraway.Assuming0603caps,itis
recommendedthatatleast6small,6medium,and4largecapsbeusedforthecoresupplyand12small,
12medium,and4largecapsbeusedfortheI/Osupply.
Anycapselectionneedstobeevaluatedfromanelectromagneticradiation(EMI)point-of-view;EMIvaries
fromonesystemdesigntoanothersoitisexpectedthatengineersalterthedecouplingcapacitorsto
minimizeradiation.RefertotheHigh-SpeedDSPSystemsDesignReferenceGuide(literaturenumber
SPRU889)formoredetaileddesigninformationondecouplingtechniques.
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