1Background ApplicationReport SPRAAR7–December2007 USB2.0BoardDesignandLayoutGuidelines DSPSApplications ABSTRACT Thisdocumentdiscussesschematicguidelineswhendesigningauniversalserialbus (USB)system. Contents 1Background 1 2USBPHYLayoutGuide 2 3ElectrostaticDischarge(ESD) 8 4References 10 ListofFigures 1SuggestedArrayCapacitorsandaFerriteBeadtoMinimizeEMI 2 2Four-LayerBoardStack-Up 3 3USBConnector 4 43WSpacingRule 4 5PowerSupplyandClockConnectiontotheUSBPHY 5 6USBPHYConnectorandCableConnector 6 7DoNotCrossPlaneBoundaries 7 8DoNotOverlapPlanes 7 9DoNotViolateImagePlanes 8 ClockfrequenciesgeneratethemainsourceofenergyinaUSBdesign.TheUSBdifferentialDP/DMpairs operateinhigh-speedmodeat480Mbps.Systemclockscanoperateat12MHz,48MHz,and60MHz. TheUSBcablecanbehaveasamonopoleantenna;takecaretopreventRFcurrentsfromcouplingonto thecable. WhendesigningaUSBboard,thesignalsofmostinterestare: •Deviceinterfacesignals:Clocksandothersignal/datalinesthatrunbetweendevicesonthePCB. •Powergoingintoandoutofthecable:TheUSBconnectorsocketpin1(VBUS)maybeheavily filteredandneedonlypasslowfrequencysignalsoflessthan~100KHz.TheUSBsocketpin4 (analogground)mustbeabletoreturnthecurrentduringdatatransmission,andmustbefiltered sparingly. •Differentialtwistedpairsignalsgoingoutoncable,DPandDM:Dependinguponthedatatransferrate, thesedeviceterminalscanhavesignalswithfundamentalfrequenciesof240MHz(highspeed),6 MHz(fullspeed),and750kHz(lowspeed). •Externalcrystalcircuit(deviceterminalsXIandX0):12MHz,19.2MHz,24MHz,and48MHz fundamental.Whenusinganexternalcrystalasareferenceclock,a24MHzandhighercrystalis highlyrecommended. Alltrademarksarethepropertyoftheirrespectiveowners. SPRAAR7–December2007USB2.0BoardDesignandLayoutGuidelines1 SubmitDocumentationFeedback www.ti.com 2USBPHYLayoutGuide 2.1GeneralRoutingandPlacement 2.2SpecificGuidelinesforUSBPHYLayout 2.2.1Analog,PLL,andDigitalPowerSupplyFiltering Analog PowerSupply SoCBoard FerriteBead Digital PowerSupply FerriteBead 0.1µF 0.01µF 0.001µF 10µF 0.1µF 0.01µF 0.001µF 10µF USBPHYLayoutGuide ThefollowingsectionsdescribeindetailthespecificguidelinesforUSBPHYLayout. UsethefollowingroutingandplacementguidelineswhenlayingoutanewdesignfortheUSBphysical layer(PHY).Theseguidelineshelpminimizesignalqualityandelectromagneticinterference(EMI) problemsonafour-or-morelayerevaluationmodule(EVM). •PlacetheUSBPHYandmajorcomponentsontheun-routedboardfirst.Formoredetails,see Section2.2.3. •Routethehigh-speedclockandhigh-speedUSBdifferentialsignalswithminimumtracelengths. •Routethehigh-speedUSBsignalsontheplaneclosesttothegroundplane,wheneverpossible. •Routethehigh-speedUSBsignalsusingaminimumofviasandcorners.Thisreducessignal reflectionsandimpedancechanges. •Whenitbecomesnecessarytoturn90°,usetwo45°turnsoranarcinsteadofmakingasingle90° turn.Thisreducesreflectionsonthesignaltracesbyminimizingimpedancediscontinuities. •DonotrouteUSBtracesunderornearcrystals,oscillators,clocksignalgenerators,switching regulators,mountingholes,magneticdevicesorIC’sthatuseorduplicateclocksignals. •Avoidstubsonthehigh-speedUSBsignalsbecausetheycausesignalreflections.Ifastubis unavoidable,thenthestubshouldbelessthan200mils. •Routeallhigh-speedUSBsignaltracesovercontinuousplanes(V CC orGND),withnointerruptions. Avoidcrossingoveranti-etch,commonlyfoundwithplanesplits. ThefollowingsectionsdescribeindetailthespecificguidelinesforUSBPHYLayout. TominimizeEMIemissions,adddecouplingcapacitorswithaferritebeadatpowersupplyterminalsfor theanalog,phase-lockedloop(PLL),anddigitalportionsofthechip.Placethisarrayasclosetothechip aspossibletominimizetheinductanceofthelineandnoisecontributionstothesystem.Ananalogand digitalsupplyexampleisshowninFigure1.Incaseofmultiplepowersupplypinswiththesamefunction, tiethemuptoasinglelow-impedancepointintheboardandthenaddthedecouplingcapacitors,in additiontotheferritebead.ThisarrayofcapsandferritebeadimproveEMIandjitterperformance.Take bothEMIandjitterintoaccountbeforealteringtheconfiguration. Figure1.SuggestedArrayCapacitorsandaFerriteBeadtoMinimizeEMI 2USB2.0BoardDesignandLayoutGuidelinesSPRAAR7–December2007 SubmitDocumentationFeedback www.ti.com 2.2.2Analog,Digital,andPLLPartitioning 2.2.3BoardStackup Signal1 PowerPlane GNDPlane Signal2 2.2.4CableConnectorSocket USBPHYLayoutGuide ConsidertherecommendationslistedbelowtoachieveproperESD/EMIperformance: •Usea0.01µFcaponeachcablepowerVBUSlinetochassisGNDclosetotheUSBconnectorpin. •Usea0.01µFcaponeachcablegroundlinetochassisGNDnexttotheUSBconnectorpin. •Ifvoltageregulatorsareused,placea0.01µFcaponbothinputandoutput.Thisistoincreasethe immunitytoESDandreduceEMI.Forotherrequirements,seethedevice-specificdatasheet. Ifseparatepowerplanesareused,theymustbetiedtogetheratonepointthroughalow-impedance bridgeorpreferablythroughaferritebead.Caremustbetakentocapacitivelydecoupleeachpowerrail closetothedevice.Theanalogground,digitalground,andPLLgroundmustbetiedtogethertothe low-impedancecircuitboardgroundplane. BecauseofthehighfrequenciesassociatedwiththeUSB,aprintedcircuitboardwithatleastfourlayers isrecommended;twosignallayersseparatedbyagroundandpowerlayerasshowninFigure2. Figure2.Four-LayerBoardStack-Up Themajorityofsignaltracesshouldrunonasinglelayer,preferablySIGNAL1.Immediatelynexttothis layershouldbetheGNDplane,whichissolidwithnocuts.Avoidrunningsignaltracesacrossasplitin thegroundorpowerplane.Whenrunningacrosssplitplanesisunavoidable,sufficientdecouplingmust beused.MinimizingthenumberofsignalviasreducesEMIbyreducinginductanceathighfrequencies. Shortthecableconnectorsocketsdirectlytoasmallchassisgroundplane(GNDstrap)thatexists immediatelyunderneaththeconnectorsockets.ThisshortsEMI(andESD)directlytothechassisground beforeitgetsontotheUSBcable.Thisetchplaneshouldbeaslargeaspossible,butalltheconductors comingoffconnectorpins1through6musthavetheboardsignalGNDplanerununder.Ifneeded,scoop outthechassisGNDstrapetchtoallowforthesignalgroundtoextendundertheconnectorpins.Note thattheetchescomingfrompins1and4(VBUSpowerandGND)shouldbewideandvia-edtotheir respectiveplanesassoonaspossible,respectingthefilteringthatmaybeinplacebetweentheconnector pinandtheplane.SeeFigure3foraschematicexample. PlaceaferriteinserieswiththecableshieldpinsneartheUSBconnectorsockettokeepEMIfromgetting ontothecableshield.Theferritebeadbetweenthecableshieldandgroundmaybevaluedbetween10Ω and50Ωat100MHz;itshouldberesistivetoapproximately1GHz.TokeepEMIfromgettingontothe cablebuspowerwire(averylargeantenna)aferritemaybeplacedinserieswithcablebuspower, VBUS,neartheUSBconnectorpin1.Theferritebeadbetweenconnectorpin1andbuspowermaybe valuedbetween47Ωandapproximately1000Ωat100MHz.Itshouldcontinuebeingresistiveoutto approximately1GHz,asshowninFigure3. SPRAAR7–December2007USB2.0BoardDesignandLayoutGuidelines3 SubmitDocumentationFeedback www.ti.com U2 FerriteBead VBUS U1 USBSocket 5 4 3 2 1 6 SHIELD_GND GND DP DM +5V SHIELD_GND FerriteBead 2.2.5ClockRoutings Trace 3W 3W W USBPHYLayoutGuide Figure3.USBConnector Toaddressthesystemclockemissionsbetweendevices,placea~10to130Ωresistorinserieswiththe clocksignal.Useatrialanderrormethodoflookingattheshapeoftheclockwaveformonahigh-speed oscilloscopeandoftuningthevalueoftheresistancetominimizewaveformdistortion.Thevalueonthis resistorshouldbeassmallaspossibletogetthedesiredeffect.Placetheresistorclosetothedevice generatingtheclocksignal.Ifanexternalcrystalisused,followtheguidelinesdetailedintheSelection andSpecificationofCrystalsforTexasInstrumentsUSB2.0Devices(SLLA122). Whenroutingtheclocktracesfromonedevicetoanother,trytousethe3Wspacingrule.Thedistance fromthecenteroftheclocktracetothecenterofanyadjacentsignaltraceshouldbeatleastthreetimes thewidthoftheclocktrace.Manyclocks,includingslowfrequencyclocks,canhavefastriseandfall times.Usingthe3Wrulecutsdownoncrosstalkbetweentraces.Ingeneral,leavespacebetweeneachof thetracesrunningparallelbetweenthedevices.Avoidusingrightangleswhenroutingtracestominimize theroutingdistanceandimpedancediscontinuities.Forfurtherprotectionfromcrosstalk,runguardtraces besidetheclocksignals(GNDpintoGNDpin),ifpossible.Thislessensclocksignalcoupling,asshownin Figure4. Figure4.3WSpacingRule USB2.0BoardDesignandLayoutGuidelines 4SPRAAR7–December2007 SubmitDocumentationFeedback www.ti.com 2.2.6Crystals/Oscillator PowerPins USBPHY 0.1µF 0.001µF X1 X0 XTAL 2.2.7DP/DMTrace USBPHYLayoutGuide KeepthecrystalanditsloadcapacitorsclosetotheUSBPHYpins,XIandXO(seeFigure5).Notethat frequenciesfrompowersourcesorlargecapacitorscancausemodulationswithintheclockandshould notbeplacednearthecrystal.Intheseinstances,errorssuchasdroppedpacketsoccur.Aplaceholder foraresistor,inparallelwiththecrystal,canbeincorporatedinthedesigntoassistoscillatorstartup. Powerisproportionaltothecurrentsquared.ThecurrentisI=C*dv/dt,sincedv/dtisafunctionofthe PHY,currentisproportionaltothecapacitiveload.Cuttingtheloadtodecreasesthecurrentbyandthe powerto1/4theoriginalvalue.Formoredetailsoncrystalselection,seetheSelectionandSpecification ofCrystalsforTexasInstrumentsUSB2.0Devices(SLLA122). Figure5.PowerSupplyandClockConnectiontotheUSBPHY PlacetheUSBPHYascloseaspossibletotheUSB2.0connector.Thesignalswingduringhigh-speed operationontheDP/DMlinesisrelativelysmall(400mV±10%),soanydifferentialnoisepickedupon thetwistedpaircanaffectthereceivedsignal.WhentheDP/DMtracesdonothaveanyshielding,the tracestendtobehavelikeanantennaandpicksupnoisegeneratedbythesurroundingcomponentsin theenvironment.Tominimizetheeffectofthisbehavior: •DP/DMtracesshouldalwaysbematchedlengthsandmustbenomorethan4inchesinlength; otherwise,theeyeopeningmaybedegraded(seeFigure6). •RouteDP/DMtracesclosetogetherfornoiserejectionondifferentialsignals,paralleltoeachotherand withintwomilsinlengthofeachother(startthemeasurementatthechippackageboundary,nottothe ballsorpins). •Ahigh-speedUSBconnectionismadethroughashielded,twistedpaircablewithadifferential characteristicimpedanceof90Ω±15%.Inlayout,theimpedanceofDPandDMshouldeachbe45Ω ±10%. •DP/DMtracesshouldnothaveanyextracomponentstomaintainsignalintegrity.Forexample,traces cannotberoutedtotwoUSBconnectors. SPRAAR7–December2007USB2.0BoardDesignandLayoutGuidelines5 SubmitDocumentationFeedback www.ti.com Minimize ThisDistance USBPHY Connector D+ D- VBUS GND D- D+ Cable Connector 2.2.8DP/DMVias 2.2.9ImagePlanes USBPHYLayoutGuide Figure6.USBPHYConnectorandCableConnector Whenaviamustbeused,increasetheclearancesizearoundittominimizeitscapacitance.Eachvia introducesdiscontinuitiesinthesignal’stransmissionlineandincreasesthechanceofpickingup interferencefromtheotherlayersoftheboard.Becarefulwhendesigningtestpointsontwistedpairlines; through-holepinsarenotrecommended. Animageplaneisalayerofcopper(voltageplaneorgroundplane),physicallyadjacenttoasignalrouting plane.Useofimageplanesprovidesalowimpedance,shortestpossiblereturnpathforRFcurrents.Fora USBboard,thebestimageplaneisthegroundplanebecauseitcanbeusedforbothanaloganddigital circuits. •Donotroutetracessotheycrossfromoneplanetotheother.ThiscancauseabrokenRFreturnpath resultinginanEMIradiatingloopasshowninFigure7.Thisisimportantforhigherfrequencyor repetitivesignals.Therefore,onamulti-layerboard,itisbesttorunallclocksignalsonthesignal planeaboveasolidgroundplane. •Avoidcrossingtheimagepowerorgroundplaneboundarieswithhigh-speedclocksignaltraces immediatelyaboveorbelowtheseparatedplanes.Thisalsoholdstrueforthetwistedpairsignals(DP, DM).AnyunusedareaofthetopandbottomsignallayersofthePCBcanbefilledwithcopperthatis connectedtothegroundplanethroughvias. 6USB2.0BoardDesignandLayoutGuidelinesSPRAAR7–December2007 SubmitDocumentationFeedback www.ti.com Don't Do AnalogPowerPlane UnwantedCapacitance DigitalPowerPlane USBPHYLayoutGuide Figure7.DoNotCrossPlaneBoundaries •Donotoverlapplanesthatdonotreferenceeachother.Forexample,donotoverlapadigitalpower planewithananalogpowerplaneasthisproducesacapacitancebetweentheoverlappingareasthat couldpassRFemissionsfromoneplanetotheother,asshowninFigure8. Figure8.DoNotOverlapPlanes SPRAAR7–December2007USB2.0BoardDesignandLayoutGuidelines7 SubmitDocumentationFeedback www.ti.com SlotinImagePlane RFReturn Current Bad SlotinImagePlane RFReturn Current Better 2.2.10JTAGInterface 2.2.11PowerRegulators 3ElectrostaticDischarge(ESD) 3.1IECESDStressingTest ElectrostaticDischarge(ESD) •Avoidimageplaneviolations.TracesthatrouteoveraslotinanimageplaneresultsinapossibleRF returnloop,asshowninFigure9. Figure9.DoNotViolateImagePlanes FortestanddebugoftheUSBPHYonly,anIEEEStandard1149.1-1990,IEEEStandardTestAccess PortandBoundary-ScanArchitecture(JTAG)andSerialTestandConfigurationInterface(STCI)maybe availableontheSystem-on-Chip(SoC).Ifavailable,keeptheUSBPHYJTAGinterfacelessthansix inches;keepingthisdistanceshortreducesnoisecouplingfromotherdevicesandsignallossdueto resistance. Switchingpowerregulatorsareasourceofnoiseandcancausenoisecouplingifplacedclosetosensitive areasonacircuitboard.Therefore,theswitchingpowerregulatorshouldbekeptawayfromtheDP/DM signals,theexternalclockcrystal(orclockoscillator),andtheUSBPHY. InternationalElectronicCommission(IEC)61000-4-xxisasetofabout25testingspecificationsfromthe IEC.IECESDStressingisdonebothun-poweredandwithpowerapplied,andwiththedevicefunctioning. Theremustbenophysicaldamage,andthedevicemustkeepworkingnormallyaftertheconclusionof thestressing.Typically,equipmenthastopassIECstressingat8kVcontactand15kVairdischarge,or higher.Tomarketproducts/systemsintheEuropeancommunity,allproducts/systemsmustbeCE compliantandhavetheCEMark.ToobtaintheCEMark,allproducts/systemsneedtogothroughand passIECstandardrequirements;forESD,itis61000-4-2.61000-4-2requiresthattheproducts/systems passcontactdischargeat8kVandairdischargeat15kV.WhenperforminganIECESDStressing,only pinsaccessibletotheoutsideworldneedtopassthetest.Thesystemintowhichtheintegratedcircuit(IC) isplacedmakesadifferenceinhowwelltheICdoes.Forexample: •CablebetweenthezappointandtheICattenuatethehighfrequenciesinthewaveform. •SeriesinductanceonthePCBboardattenuatesthehighfrequencies. •Unlessthecapacitor’sgroundconnectionisinductive,capacitancetogroundshuntsawayhigh frequencies. ThefollowingsectionsdescribeindetailtheIECESDStressingTestmodesandtesttypes. 8USB2.0BoardDesignandLayoutGuidelinesSPRAAR7–December2007 SubmitDocumentationFeedback www.ti.com 3.1.1TestMode 3.1.2AirDischargeMode 3.1.3TestType 3.2TIComponentLevelIECESDTest 3.3ConstructionofaCustomUSBConnector ElectrostaticDischarge(ESD) TheIECESDStressingtestisdonethroughtwomodes:contactdischargemodeandairdischargemode. Forthecontactdischargetestmode,thepreferredwayisdirectcontactappliedtotheconductivesurfaces oftheequipmentundertest(EUT).InthecaseoftheUSBsystem,theconductivesurfaceistheouter casingoftheUSBconnector.TheelectrodeoftheESDgeneratorisheldincontactwiththeEUTora couplingplanepriortodischarge.Thearcformationiscreatedundercontrolledconditions,insidearelay, resultinginrepeatablewaveforms;however,thisarcdoesnotaccuratelyrecreatethecharacteristicunique tothearcofanactualESDevent. Theairdischargeusuallyappliestoanon-conductivesurfaceoftheEUT.Insteadofadirectcontactwith theEUT,thechargedelectrodeoftheESDgeneratorisbroughtclosetotheEUT,andasparkintheairto theEUTactuatesthedischarge.Comparedtothecontactdischargemode,theairdischargeismore realistictotheactualESDoccurrence.However,duetothevariationsofthearclength,itmaynotbeable toproducerepeatablewaveform. TheIECESDStressingtesthastwotesttypes:directdischargeandindirectdischarge.Directdischarge isappliesdirectlytothesurfaceorthestructureoftheEUT.Itincludesbothcontactdischargeandair dischargemodes.IndirectdischargeappliestoacouplingplaneinthevicinityoftheEUT.Theindirect dischargeisusedtosimulatepersonaldischargetoobjectswhichareadjacenttotheEUT.Itincludes contactdischargemodeonly. TIComponentLevelIECESDTesttestsonlytheICterminalsthatareexposedinsystemlevel applications.Itcanbeusedtodeterminetherobustnessofon-chipprotectionandthelatch-upimmunity. TheICcanonlypasstheTIComponentLevelIECESDtestwhenthereisnolatch-upandICisfully functionalafterthetest. AstandardUSBconnector,eithertypeAortypeB,providesgoodESDprotection.However,ifacustom USBconnectorisdesired,thefollowingguidelinesshouldbeobservedtoensuregoodESDprotection. •Thereshouldbeaneasilyaccessibleshieldplatenexttotheconnectorforair-dischargemode purpose. •TietheoutershieldoftheconnectortoGND.Whenacableisinsertedintotheconnector,theshieldof thecableshouldfirstmakecontactwiththeoutershield. •IftheconnectorincludespowerandGND,theleadofpowerandGNDneedtobelongerthanthe leadsofsignal. •Theconnectorneedstohaveakeytoensureproperinsertionofthecable. •SeethestandardUSBconnectorforreference. SPRAAR7–December2007USB2.0BoardDesignandLayoutGuidelines9 SubmitDocumentationFeedback www.ti.com 3.4ESDProtectionSystemDesignConsideration 4References References ESDprotectionsystemdesignconsiderationiscoveredinSection2ofthisdocument.Thefollowingare additionalconsiderationsforESDprotectioninasystem. •MetallicshieldingforbothESDandEMI •ChassisGNDisolationfromtheboardGND •AirgapdesignedonboardtoabsorbESDenergy •ClampingdiodestoabsorbESDenergy •CapacitorstodivertESDenergy •TheuseofexternalESDcomponentsontheDP/DMlinesmayaffectsignalqualityandarenot recommended. •USB2.0Specification,Intel,2000,http://www.usb.org/developers/docs/ •HighSpeedUSBPlatformDesignGuidelines,Intel,2000, http://www.intel.com/technology/usb/download/usb2dg_R1_0.pdf •SelectionandSpecificationofCrystalsforTexasInstrumentsUSB2.0Devices(SLLA122) 10USB2.0BoardDesignandLayoutGuidelinesSPRAAR7–December2007 SubmitDocumentationFeedback [...]... safety and regulatory ramifications of their applications, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be provided by TI Further, Buyers must fully indemnify TI and its... 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Whenroutingtheclocktracesfromonedevicetoanother,trytousethe3Wspacingrule.Thedistance fromthecenteroftheclocktracetothecenterofanyadjacentsignaltraceshouldbeatleastthreetimes thewidthoftheclocktrace.Manyclocks,includingslowfrequencyclocks,canhavefastriseandfall times.Usingthe3Wrulecutsdownoncrosstalkbetweentraces.Ingeneral,leavespacebetweeneachof thetracesrunningparallelbetweenthedevices.Avoidusingrightangleswhenroutingtracestominimize theroutingdistanceandimpedancediscontinuities.Forfurtherprotectionfromcrosstalk,runguardtraces besidetheclocksignals(GNDpintoGNDpin),ifpossible.Thislessensclocksignalcoupling,asshownin Figure4. Figure4.3WSpacingRule USB2 .0BoardDesignandLayoutGuidelines 4SPRAAR7–December 200 7 SubmitDocumentationFeedback www.ti.com 2. 2.6Crystals/Oscillator PowerPins USB PHY 0. 1µF 0. 001 µF X1 X0 XTAL 2. 2.7DP/DMTrace USBPHYLayoutGuide KeepthecrystalanditsloadcapacitorsclosetotheUSBPHYpins,XIandXO(seeFigure5).Notethat frequenciesfrompowersourcesorlargecapacitorscancausemodulationswithintheclockandshould notbeplacednearthecrystal.Intheseinstances,errorssuchasdroppedpacketsoccur.Aplaceholder foraresistor,inparallelwiththecrystal,canbeincorporatedinthedesigntoassistoscillatorstartup. Powerisproportionaltothecurrentsquared.ThecurrentisI=C*dv/dt,sincedv/dtisafunctionofthe PHY,currentisproportionaltothecapacitiveload.Cuttingtheloadtodecreasesthecurrentbyandthe powerto1/4theoriginalvalue.Formoredetailsoncrystalselection,seetheSelectionandSpecification ofCrystalsforTexasInstrumentsUSB2.0Devices(SLLA 122 ). Figure5.PowerSupplyandClockConnectiontotheUSBPHY PlacetheUSBPHYascloseaspossibletotheUSB2.0connector.Thesignalswingduringhigh-speed operationontheDP/DMlinesisrelativelysmall( 400 mV± 10% ),soanydifferentialnoisepickedupon thetwistedpaircanaffectthereceivedsignal.WhentheDP/DMtracesdonothaveanyshielding,the tracestendtobehavelikeanantennaandpicksupnoisegeneratedbythesurroundingcomponentsin theenvironment.Tominimizetheeffectofthisbehavior: •DP/DMtracesshouldalwaysbematchedlengthsandmustbenomorethan4inchesinlength; otherwise,theeyeopeningmaybedegraded(seeFigure6). •RouteDP/DMtracesclosetogetherfornoiserejectionondifferentialsignals,paralleltoeachotherand withintwomilsinlengthofeachother(startthemeasurementatthechippackageboundary,nottothe ballsorpins). •Ahigh-speedUSBconnectionismadethroughashielded,twistedpaircablewithadifferential characteristicimpedanceof 90 ±15%.Inlayout,theimpedanceofDPandDMshouldeachbe45Ω ± 10% . •DP/DMtracesshouldnothaveanyextracomponentstomaintainsignalintegrity.Forexample,traces cannotberoutedtotwoUSBconnectors. SPRAAR7–December 200 7USB2 .0BoardDesignandLayoutGuidelines5 SubmitDocumentationFeedback www.ti.com Minimize ThisDistance USB PHY Connector D+ D- VBUS GND D- D+ Cable Connector 2. 2.8DP/DMVias 2. 2.9ImagePlanes USBPHYLayoutGuide Figure6.USBPHYConnectorandCableConnector Whenaviamustbeused,increasetheclearancesizearoundittominimizeitscapacitance.Eachvia introducesdiscontinuitiesinthesignal’stransmissionlineandincreasesthechanceofpickingup interferencefromtheotherlayersoftheboard.Becarefulwhendesigningtestpointsontwistedpairlines; through-holepinsarenotrecommended. Animageplaneisalayerofcopper(voltageplaneorgroundplane),physicallyadjacenttoasignalrouting plane.Useofimageplanesprovidesalowimpedance,shortestpossiblereturnpathforRFcurrents.Fora USBboard,thebestimageplaneisthegroundplanebecauseitcanbeusedforbothanaloganddigital circuits. •Donotroutetracessotheycrossfromoneplanetotheother.ThiscancauseabrokenRFreturnpath resultinginanEMIradiatingloopasshowninFigure7.Thisisimportantforhigherfrequencyor repetitivesignals.Therefore,onamulti-layerboard,itisbesttorunallclocksignalsonthesignal planeaboveasolidgroundplane. •Avoidcrossingtheimagepowerorgroundplaneboundarieswithhigh-speedclocksignaltraces immediatelyaboveorbelowtheseparatedplanes.Thisalsoholdstrueforthetwistedpairsignals(DP, DM).AnyunusedareaofthetopandbottomsignallayersofthePCBcanbefilledwithcopperthatis connectedtothegroundplanethroughvias. 6USB2 .0BoardDesignandLayoutGuidelinesSPRAAR7–December 200 7 SubmitDocumentationFeedback www.ti.com Don't Do AnalogPowerPlane UnwantedCapacitance DigitalPowerPlane USBPHYLayoutGuide Figure7.DoNotCrossPlaneBoundaries •Donotoverlapplanesthatdonotreferenceeachother.Forexample,donotoverlapadigitalpower planewithananalogpowerplaneasthisproducesacapacitancebetweentheoverlappingareasthat couldpassRFemissionsfromoneplanetotheother,asshowninFigure8. Figure8.DoNotOverlapPlanes SPRAAR7–December 200 7USB2 .0BoardDesignandLayoutGuidelines7 SubmitDocumentationFeedback www.ti.com SlotinImagePlane RFReturn Current Bad SlotinImagePlane RFReturn Current Better 2. 2.10JTAGInterface 2. 2.11PowerRegulators 3ElectrostaticDischarge(ESD) 3.1IECESDStressingTest ElectrostaticDischarge(ESD) •Avoidimageplaneviolations.TracesthatrouteoveraslotinanimageplaneresultsinapossibleRF returnloop,asshowninFigure9. Figure9.DoNotViolateImagePlanes FortestanddebugoftheUSBPHYonly,anIEEEStandard1149.1-19 90, IEEEStandardTestAccess PortandBoundary-ScanArchitecture(JTAG)andSerialTestandConfigurationInterface(STCI)maybe availableontheSystem-on-Chip(SoC).Ifavailable,keeptheUSBPHYJTAGinterfacelessthansix inches;keepingthisdistanceshortreducesnoisecouplingfromotherdevicesandsignallossdueto resistance. Switchingpowerregulatorsareasourceofnoiseandcancausenoisecouplingifplacedclosetosensitive areasonacircuitboard.Therefore,theswitchingpowerregulatorshouldbekeptawayfromtheDP/DM signals,theexternalclockcrystal(orclockoscillator),andtheUSBPHY. InternationalElectronicCommission(IEC)6 100 0-4-xxisasetofabout25testingspecificationsfromthe IEC.IECESDStressingisdonebothun-poweredandwithpowerapplied,andwiththedevicefunctioning. Theremustbenophysicaldamage,andthedevicemustkeepworkingnormallyaftertheconclusionof thestressing.Typically,equipmenthastopassIECstressingat8kVcontactand15kVairdischarge,or higher.Tomarketproducts/systemsintheEuropeancommunity,allproducts/systemsmustbeCE compliantandhavetheCEMark.ToobtaintheCEMark,allproducts/systemsneedtogothroughand passIECstandardrequirements;forESD,itis6 100 0-4 -2. 6 100 0-4-2requiresthattheproducts/systems passcontactdischargeat8kVandairdischargeat15kV.WhenperforminganIECESDStressing,only pinsaccessibletotheoutsideworldneedtopassthetest.Thesystemintowhichtheintegratedcircuit(IC) isplacedmakesadifferenceinhowwelltheICdoes.Forexample: •CablebetweenthezappointandtheICattenuatethehighfrequenciesinthewaveform. •SeriesinductanceonthePCBboardattenuatesthehighfrequencies. •Unlessthecapacitor’sgroundconnectionisinductive,capacitancetogroundshuntsawayhigh frequencies. ThefollowingsectionsdescribeindetailtheIECESDStressingTestmodesandtesttypes. 8USB2 .0BoardDesignandLayoutGuidelinesSPRAAR7–December 200 7 SubmitDocumentationFeedback www.ti.com 3.1.1TestMode 3.1.2AirDischargeMode 3.1.3TestType 3.2TIComponentLevelIECESDTest 3.3ConstructionofaCustomUSBConnector ElectrostaticDischarge(ESD) TheIECESDStressingtestisdonethroughtwomodes:contactdischargemodeandairdischargemode. Forthecontactdischargetestmode,thepreferredwayisdirectcontactappliedtotheconductivesurfaces oftheequipmentundertest(EUT).InthecaseoftheUSBsystem,theconductivesurfaceistheouter casingoftheUSBconnector.TheelectrodeoftheESDgeneratorisheldincontactwiththeEUTora couplingplanepriortodischarge.Thearcformationiscreatedundercontrolledconditions,insidearelay, resultinginrepeatablewaveforms;however,thisarcdoesnotaccuratelyrecreatethecharacteristicunique tothearcofanactualESDevent. Theairdischargeusuallyappliestoanon-conductivesurfaceoftheEUT.Insteadofadirectcontactwith theEUT,thechargedelectrodeoftheESDgeneratorisbroughtclosetotheEUT,andasparkintheairto theEUTactuatesthedischarge.Comparedtothecontactdischargemode,theairdischargeismore realistictotheactualESDoccurrence.However,duetothevariationsofthearclength,itmaynotbeable toproducerepeatablewaveform. TheIECESDStressingtesthastwotesttypes:directdischargeandindirectdischarge.Directdischarge isappliesdirectlytothesurfaceorthestructureoftheEUT.Itincludesbothcontactdischargeandair dischargemodes.IndirectdischargeappliestoacouplingplaneinthevicinityoftheEUT.Theindirect dischargeisusedtosimulatepersonaldischargetoobjectswhichareadjacenttotheEUT.Itincludes contactdischargemodeonly. TIComponentLevelIECESDTesttestsonlytheICterminalsthatareexposedinsystemlevel applications.Itcanbeusedtodeterminetherobustnessofon-chipprotectionandthelatch-upimmunity. TheICcanonlypasstheTIComponentLevelIECESDtestwhenthereisnolatch-upandICisfully functionalafterthetest. AstandardUSBconnector,eithertypeAortypeB,providesgoodESDprotection.However,ifacustom USBconnectorisdesired,thefollowingguidelinesshouldbeobservedtoensuregoodESDprotection. •Thereshouldbeaneasilyaccessibleshieldplatenexttotheconnectorforair-dischargemode purpose. •TietheoutershieldoftheconnectortoGND.Whenacableisinsertedintotheconnector,theshieldof thecableshouldfirstmakecontactwiththeoutershield. •IftheconnectorincludespowerandGND,theleadofpowerandGNDneedtobelongerthanthe leadsofsignal. •Theconnectorneedstohaveakeytoensureproperinsertionofthecable. •SeethestandardUSBconnectorforreference. SPRAAR7–December 200 7USB2 .0BoardDesignandLayoutGuidelines9 SubmitDocumentationFeedback www.ti.com 3.4ESDProtectionSystemDesignConsideration 4References References ESDprotectionsystemdesignconsiderationiscoveredinSection2ofthisdocument.Thefollowingare additionalconsiderationsforESDprotectioninasystem. •MetallicshieldingforbothESDandEMI •ChassisGNDisolationfromtheboardGND •AirgapdesignedonboardtoabsorbESDenergy •ClampingdiodestoabsorbESDenergy •CapacitorstodivertESDenergy •TheuseofexternalESDcomponentsontheDP/DMlinesmayaffectsignalqualityandarenot recommended. USB2 .0Specification,Intel , 20 00, http://www .usb. org/developers/docs/ •HighSpeedUSBPlatformDesignGuidelines,Intel , 20 00, http://www.intel.com/technology /usb/ download /usb2 dg_R1 _0. pdf •SelectionandSpecificationofCrystalsforTexasInstrumentsUSB2.0Devices(SLLA 122 ) 1 0USB2 .0BoardDesignandLayoutGuidelinesSPRAAR7–December 200 7 SubmitDocumentationFeedback IMPORTANTNOTICE TexasInstrumentsIncorporatedanditssubsidiaries(TI)reservetherighttomakecorrections,modifications,enhancements, improvements,andotherchangestoitsproductsandservicesatanytimeandtodiscontinueanyproductorservicewithoutnotice. Customersshouldobtainthelatestrelevantinformationbeforeplacingordersandshouldverifythatsuchinformationiscurrentand complete.AllproductsaresoldsubjecttoTI’stermsandconditionsofsalesuppliedatthetimeoforderacknowledgment. TIwarrantsperformanceofitshardwareproductstothespecificationsapplicableatthetimeofsaleinaccordancewithTI’s standardwarranty.TestingandotherqualitycontroltechniquesareusedtotheextentTIdeemsnecessarytosupportthis warranty.Exceptwheremandatedbygovernmentrequirements,testingofallparametersofeachproductisnotnecessarily performed. TIassumesnoliabilityforapplicationsassistanceorcustomerproductdesign.Customersareresponsiblefortheirproductsand applicationsusingTIcomponents.Tominimizetherisksassociatedwithcustomerproductsandapplications,customersshould provideadequatedesignandoperatingsafeguards. 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FollowingareURLswhereyoucanobtaininformationonotherTexasInstrumentsproductsandapplicationsolutions: ProductsApplications Amplifiersamplifier.ti.comAudiowww.ti.com/audio DataConvertersdataconverter.ti.comAutomotivewww.ti.com/automotive DSPdsp.ti.comBroadbandwww.ti.com/broadband Interfaceinterface.ti.comDigitalControlwww.ti.com/digitalcontrol Logiclogic.ti.comMilitarywww.ti.com/military PowerMgmtpower.ti.comOpticalNetworkingwww.ti.com/opticalnetwork Microcontrollersmicrocontroller.ti.comSecuritywww.ti.com/security RFIDwww.ti-rfid.comTelephonywww.ti.com/telephony LowPowerwww.ti.com/lpwVideo&Imagingwww.ti.com/video Wireless Wirelesswww.ti.com/wireless MailingAddress:TexasInstruments,PostOfficeBox655 303 ,Dallas,Texas7 526 5 Copyright 20 07,TexasInstrumentsIncorporated . 8 ClockfrequenciesgeneratethemainsourceofenergyinaUSBdesign.TheUSBdifferentialDP/DMpairs operateinhigh-speedmodeat480Mbps.Systemclockscanoperateat12MHz,48MHz ,and6 0MHz. TheUSBcablecanbehaveasamonopoleantenna;takecaretopreventRFcurrentsfromcouplingonto thecable. WhendesigningaUSBboard,thesignalsofmostinterestare: •Deviceinterfacesignals:Clocksandothersignal/datalinesthatrunbetweendevicesonthePCB. •Powergoingintoandoutofthecable:TheUSBconnectorsocketpin1(VBUS)maybeheavily filteredandneedonlypasslowfrequencysignalsoflessthan~ 100 KHz.TheUSBsocketpin4 (analogground)mustbeabletoreturnthecurrentduringdatatransmission,andmustbefiltered sparingly. •Differentialtwistedpairsignalsgoingoutoncable,DPandDM:Dependinguponthedatatransferrate, thesedeviceterminalscanhavesignalswithfundamentalfrequenciesof240MHz(highspeed),6 MHz(fullspeed) ,and7 50kHz(lowspeed). •Externalcrystalcircuit(deviceterminalsXIandX0):12MHz,19.2MHz ,24 MHz ,and4 8MHz fundamental.Whenusinganexternalcrystalasareferenceclock,a24MHzandhighercrystalis highlyrecommended. Alltrademarksarethepropertyoftheirrespectiveowners. SPRAAR7–December 200 7USB2 .0BoardDesignandLayoutGuidelines1 SubmitDocumentationFeedback www.ti.com 2USBPHYLayoutGuide 2. 1GeneralRoutingandPlacement 2. 2SpecificGuidelinesforUSBPHYLayout 2. 2.1Analog,PLL,andDigitalPowerSupplyFiltering Analog PowerSupply SoC Board FerriteBead Digital PowerSupply FerriteBead 0. 1µF 0. 01µF 0. 001 µF 10 µF 0. 1µF 0. 01µF 0. 001 µF 10 µF USBPHYLayoutGuide ThefollowingsectionsdescribeindetailthespecificguidelinesforUSBPHYLayout. UsethefollowingroutingandplacementguidelineswhenlayingoutanewdesignfortheUSBphysical layer(PHY).Theseguidelineshelpminimizesignalqualityandelectromagneticinterference(EMI) problemsonafour-or-morelayerevaluationmodule(EVM). •PlacetheUSBPHYandmajorcomponentsontheun-routedboardfirst.Formoredetails,see Section2 .2. 3. •Routethehigh-speedclockandhigh-speedUSBdifferentialsignalswithminimumtracelengths. •Routethehigh-speedUSBsignalsontheplaneclosesttothegroundplane,wheneverpossible. •Routethehigh-speedUSBsignalsusingaminimumofviasandcorners.Thisreducessignal reflectionsandimpedancechanges. •Whenitbecomesnecessarytoturn 90 ,usetwo45°turnsoranarcinsteadofmakingasingle 90 turn.Thisreducesreflectionsonthesignaltracesbyminimizingimpedancediscontinuities. •DonotrouteUSBtracesunderornearcrystals,oscillators,clocksignalgenerators,switching regulators,mountingholes,magneticdevicesorIC’sthatuseorduplicateclocksignals. •Avoidstubsonthehigh-speedUSBsignalsbecausetheycausesignalreflections.Ifastubis unavoidable,thenthestubshouldbelessthan 200 mils. •Routeallhigh-speedUSBsignaltracesovercontinuousplanes(V CC orGND),withnointerruptions. Avoidcrossingoveranti-etch,commonlyfoundwithplanesplits. ThefollowingsectionsdescribeindetailthespecificguidelinesforUSBPHYLayout. TominimizeEMIemissions,adddecouplingcapacitorswithaferritebeadatpowersupplyterminalsfor theanalog,phase-lockedloop(PLL),anddigitalportionsofthechip.Placethisarrayasclosetothechip aspossibletominimizetheinductanceofthelineandnoisecontributionstothesystem.Ananalogand digitalsupplyexampleisshowninFigure1.Incaseofmultiplepowersupplypinswiththesamefunction, tiethemuptoasinglelow-impedancepointintheboardandthenaddthedecouplingcapacitors,in additiontotheferritebead.ThisarrayofcapsandferritebeadimproveEMIandjitterperformance.Take bothEMIandjitterintoaccountbeforealteringtheconfiguration. Figure1.SuggestedArrayCapacitorsandaFerriteBeadtoMinimizeEMI 2USB2 .0BoardDesignandLayoutGuidelinesSPRAAR7–December 200 7 SubmitDocumentationFeedback www.ti.com 2. 2.2Analog,Digital,andPLLPartitioning 2. 2.3BoardStackup Signal1 PowerPlane GNDPlane Signal 2 2 .2. 4CableConnectorSocket USBPHYLayoutGuide ConsidertherecommendationslistedbelowtoachieveproperESD/EMIperformance: •Usea0 .01 µFcaponeachcablepowerVBUSlinetochassisGNDclosetotheUSBconnectorpin. •Usea0 .01 µFcaponeachcablegroundlinetochassisGNDnexttotheUSBconnectorpin. •Ifvoltageregulatorsareused,placea0 .01 µFcaponbothinputandoutput.Thisistoincreasethe immunitytoESDandreduceEMI.Forotherrequirements,seethedevice-specificdatasheet. Ifseparatepowerplanesareused,theymustbetiedtogetheratonepointthroughalow-impedance bridgeorpreferablythroughaferritebead.Caremustbetakentocapacitivelydecoupleeachpowerrail closetothedevice.Theanalogground,digitalground,andPLLgroundmustbetiedtogethertothe low-impedancecircuitboardgroundplane. BecauseofthehighfrequenciesassociatedwiththeUSB,aprintedcircuitboardwithatleastfourlayers isrecommended;twosignallayersseparatedbyagroundandpowerlayerasshowninFigure2. Figure2.Four-LayerBoardStack-Up Themajorityofsignaltracesshouldrunonasinglelayer,preferablySIGNAL1.Immediatelynexttothis layershouldbetheGNDplane,whichissolidwithnocuts.Avoidrunningsignaltracesacrossasplitin thegroundorpowerplane.Whenrunningacrosssplitplanesisunavoidable,sufficientdecouplingmust beused.MinimizingthenumberofsignalviasreducesEMIbyreducinginductanceathighfrequencies. Shortthecableconnectorsocketsdirectlytoasmallchassisgroundplane(GNDstrap)thatexists immediatelyunderneaththeconnectorsockets.ThisshortsEMI(andESD)directlytothechassisground beforeitgetsontotheUSBcable.Thisetchplaneshouldbeaslargeaspossible,butalltheconductors comingoffconnectorpins1through6musthavetheboardsignalGNDplanerununder.Ifneeded,scoop outthechassisGNDstrapetchtoallowforthesignalgroundtoextendundertheconnectorpins.Note thattheetchescomingfrompins 1and4 (VBUSpowerandGND)shouldbewideandvia-edtotheir respectiveplanesassoonaspossible,respectingthefilteringthatmaybeinplacebetweentheconnector pinandtheplane.SeeFigure3foraschematicexample. PlaceaferriteinserieswiththecableshieldpinsneartheUSBconnectorsockettokeepEMIfromgetting ontothecableshield.Theferritebeadbetweenthecableshieldandgroundmaybevaluedbetween 10 and 50 at 100 MHz;itshouldberesistivetoapproximately1GHz.TokeepEMIfromgettingontothe cablebuspowerwire(averylargeantenna)aferritemaybeplacedinserieswithcablebuspower, VBUS,neartheUSBconnectorpin1.Theferritebeadbetweenconnectorpin1andbuspowermaybe valuedbetween47Ωandapproximately 100 0Ωat 100 MHz.Itshouldcontinuebeingresistiveoutto approximately1GHz,asshowninFigure3. SPRAAR7–December 200 7USB2 .0BoardDesignandLayoutGuidelines3 SubmitDocumentationFeedback www.ti.com U2 FerriteBead VBUS U1 USB Socket 5 4 3 2 1 6 SHIELD_GND GND DP DM +5V SHIELD_GND FerriteBead 2. 2.5ClockRoutings Trace 3W. 4SPRAAR7–December 200 7 SubmitDocumentationFeedback www.ti.com 2. 2.6Crystals/Oscillator PowerPins USB PHY 0. 1µF 0. 001 µF X1 X0 XTAL 2. 2.7DP/DMTrace USBPHYLayoutGuide KeepthecrystalanditsloadcapacitorsclosetotheUSBPHYpins,XIandXO(seeFigure5).Notethat frequenciesfrompowersourcesorlargecapacitorscancausemodulationswithintheclockandshould notbeplacednearthecrystal.Intheseinstances,errorssuchasdroppedpacketsoccur.Aplaceholder foraresistor,inparallelwiththecrystal,canbeincorporatedinthedesigntoassistoscillatorstartup. Powerisproportionaltothecurrentsquared.ThecurrentisI=C*dv/dt,sincedv/dtisafunctionofthe PHY,currentisproportionaltothecapacitiveload.Cuttingtheloadtodecreasesthecurrentbyandthe powerto1/4theoriginalvalue.Formoredetailsoncrystalselection,seetheSelectionandSpecification ofCrystalsforTexasInstrumentsUSB2.0Devices(SLLA 122 ). Figure5.PowerSupplyandClockConnectiontotheUSBPHY PlacetheUSBPHYascloseaspossibletotheUSB2.0connector.Thesignalswingduringhigh-speed operationontheDP/DMlinesisrelativelysmall( 400 mV± 10% ),soanydifferentialnoisepickedupon thetwistedpaircanaffectthereceivedsignal.WhentheDP/DMtracesdonothaveanyshielding,the tracestendtobehavelikeanantennaandpicksupnoisegeneratedbythesurroundingcomponentsin theenvironment.Tominimizetheeffectofthisbehavior: •DP/DMtracesshouldalwaysbematchedlengthsandmustbenomorethan4inchesinlength; otherwise,theeyeopeningmaybedegraded(seeFigure6). •RouteDP/DMtracesclosetogetherfornoiserejectionondifferentialsignals,paralleltoeachotherand withintwomilsinlengthofeachother(startthemeasurementatthechippackageboundary,nottothe ballsorpins). •Ahigh-speedUSBconnectionismadethroughashielded,twistedpaircablewithadifferential characteristicimpedanceof 90 ±15%.Inlayout,theimpedanceofDPandDMshouldeachbe45Ω ± 10% . •DP/DMtracesshouldnothaveanyextracomponentstomaintainsignalintegrity.Forexample,traces cannotberoutedtotwoUSBconnectors. SPRAAR7–December 200 7USB2 .0BoardDesignandLayoutGuidelines5 SubmitDocumentationFeedback www.ti.com Minimize ThisDistance USB PHY Connector D+ D- VBUS GND D- D+ Cable Connector 2. 2.8DP/DMVias 2. 2.9ImagePlanes USBPHYLayoutGuide Figure6.USBPHYConnectorandCableConnector Whenaviamustbeused,increasetheclearancesizearoundittominimizeitscapacitance.Eachvia introducesdiscontinuitiesinthesignal’stransmissionlineandincreasesthechanceofpickingup interferencefromtheotherlayersoftheboard.Becarefulwhendesigningtestpointsontwistedpairlines; through-holepinsarenotrecommended. Animageplaneisalayerofcopper(voltageplaneorgroundplane),physicallyadjacenttoasignalrouting plane.Useofimageplanesprovidesalowimpedance,shortestpossiblereturnpathforRFcurrents.Fora USBboard,thebestimageplaneisthegroundplanebecauseitcanbeusedforbothanaloganddigital circuits. •Donotroutetracessotheycrossfromoneplanetotheother.ThiscancauseabrokenRFreturnpath resultinginanEMIradiatingloopasshowninFigure7.Thisisimportantforhigherfrequencyor repetitivesignals.Therefore,onamulti-layerboard,itisbesttorunallclocksignalsonthesignal planeaboveasolidgroundplane. •Avoidcrossingtheimagepowerorgroundplaneboundarieswithhigh-speedclocksignaltraces immediatelyaboveorbelowtheseparatedplanes.Thisalsoholdstrueforthetwistedpairsignals(DP, DM).AnyunusedareaofthetopandbottomsignallayersofthePCBcanbefilledwithcopperthatis connectedtothegroundplanethroughvias. 6USB2 .0BoardDesignandLayoutGuidelinesSPRAAR7–December 200 7 SubmitDocumentationFeedback www.ti.com Don't Do AnalogPowerPlane UnwantedCapacitance DigitalPowerPlane USBPHYLayoutGuide Figure7.DoNotCrossPlaneBoundaries •Donotoverlapplanesthatdonotreferenceeachother.Forexample,donotoverlapadigitalpower planewithananalogpowerplaneasthisproducesacapacitancebetweentheoverlappingareasthat couldpassRFemissionsfromoneplanetotheother,asshowninFigure8. Figure8.DoNotOverlapPlanes SPRAAR7–December 200 7USB2 .0BoardDesignandLayoutGuidelines7 SubmitDocumentationFeedback www.ti.com SlotinImagePlane RFReturn Current Bad SlotinImagePlane RFReturn Current Better 2. 2.10JTAGInterface 2. 2.11PowerRegulators 3ElectrostaticDischarge(ESD) 3.1IECESDStressingTest ElectrostaticDischarge(ESD) •Avoidimageplaneviolations.TracesthatrouteoveraslotinanimageplaneresultsinapossibleRF returnloop,asshowninFigure9. Figure9.DoNotViolateImagePlanes FortestanddebugoftheUSBPHYonly,anIEEEStandard1149.1-19 90, IEEEStandardTestAccess PortandBoundary-ScanArchitecture(JTAG)andSerialTestandConfigurationInterface(STCI)maybe availableontheSystem-on-Chip(SoC).Ifavailable,keeptheUSBPHYJTAGinterfacelessthansix inches;keepingthisdistanceshortreducesnoisecouplingfromotherdevicesandsignallossdueto resistance. Switchingpowerregulatorsareasourceofnoiseandcancausenoisecouplingifplacedclosetosensitive areasonacircuitboard.Therefore,theswitchingpowerregulatorshouldbekeptawayfromtheDP/DM signals,theexternalclockcrystal(orclockoscillator),andtheUSBPHY. InternationalElectronicCommission(IEC)6 100 0-4-xxisasetofabout25testingspecificationsfromthe IEC.IECESDStressingisdonebothun-poweredandwithpowerapplied,andwiththedevicefunctioning. Theremustbenophysicaldamage,andthedevicemustkeepworkingnormallyaftertheconclusionof thestressing.Typically,equipmenthastopassIECstressingat8kVcontactand15kVairdischarge,or higher.Tomarketproducts/systemsintheEuropeancommunity,allproducts/systemsmustbeCE compliantandhavetheCEMark.ToobtaintheCEMark,allproducts/systemsneedtogothroughand passIECstandardrequirements;forESD,itis6 100 0-4 -2. 6 100 0-4-2requiresthattheproducts/systems passcontactdischargeat8kVandairdischargeat15kV.WhenperforminganIECESDStressing,only pinsaccessibletotheoutsideworldneedtopassthetest.Thesystemintowhichtheintegratedcircuit(IC) isplacedmakesadifferenceinhowwelltheICdoes.Forexample: •CablebetweenthezappointandtheICattenuatethehighfrequenciesinthewaveform. •SeriesinductanceonthePCBboardattenuatesthehighfrequencies. •Unlessthecapacitor’sgroundconnectionisinductive,capacitancetogroundshuntsawayhigh frequencies. ThefollowingsectionsdescribeindetailtheIECESDStressingTestmodesandtesttypes. 8USB2 .0BoardDesignandLayoutGuidelinesSPRAAR7–December 200 7 SubmitDocumentationFeedback www.ti.com 3.1.1TestMode 3.1.2AirDischargeMode 3.1.3TestType 3.2TIComponentLevelIECESDTest 3.3ConstructionofaCustomUSBConnector ElectrostaticDischarge(ESD) TheIECESDStressingtestisdonethroughtwomodes:contactdischargemodeandairdischargemode. Forthecontactdischargetestmode,thepreferredwayisdirectcontactappliedtotheconductivesurfaces oftheequipmentundertest(EUT).InthecaseoftheUSBsystem,theconductivesurfaceistheouter casingoftheUSBconnector.TheelectrodeoftheESDgeneratorisheldincontactwiththeEUTora couplingplanepriortodischarge.Thearcformationiscreatedundercontrolledconditions,insidearelay, resultinginrepeatablewaveforms;however,thisarcdoesnotaccuratelyrecreatethecharacteristicunique tothearcofanactualESDevent. Theairdischargeusuallyappliestoanon-conductivesurfaceoftheEUT.Insteadofadirectcontactwith theEUT,thechargedelectrodeoftheESDgeneratorisbroughtclosetotheEUT,andasparkintheairto theEUTactuatesthedischarge.Comparedtothecontactdischargemode,theairdischargeismore realistictotheactualESDoccurrence.However,duetothevariationsofthearclength,itmaynotbeable toproducerepeatablewaveform. TheIECESDStressingtesthastwotesttypes:directdischargeandindirectdischarge.Directdischarge isappliesdirectlytothesurfaceorthestructureoftheEUT.Itincludesbothcontactdischargeandair dischargemodes.IndirectdischargeappliestoacouplingplaneinthevicinityoftheEUT.Theindirect dischargeisusedtosimulatepersonaldischargetoobjectswhichareadjacenttotheEUT.Itincludes contactdischargemodeonly. TIComponentLevelIECESDTesttestsonlytheICterminalsthatareexposedinsystemlevel applications.Itcanbeusedtodeterminetherobustnessofon-chipprotectionandthelatch-upimmunity. TheICcanonlypasstheTIComponentLevelIECESDtestwhenthereisnolatch-upandICisfully functionalafterthetest. AstandardUSBconnector,eithertypeAortypeB,providesgoodESDprotection.However,ifacustom USBconnectorisdesired,thefollowingguidelinesshouldbeobservedtoensuregoodESDprotection. •Thereshouldbeaneasilyaccessibleshieldplatenexttotheconnectorforair-dischargemode purpose. •TietheoutershieldoftheconnectortoGND.Whenacableisinsertedintotheconnector,theshieldof thecableshouldfirstmakecontactwiththeoutershield. •IftheconnectorincludespowerandGND,theleadofpowerandGNDneedtobelongerthanthe leadsofsignal. •Theconnectorneedstohaveakeytoensureproperinsertionofthecable. •SeethestandardUSBconnectorforreference. SPRAAR7–December 200 7USB2 .0BoardDesignandLayoutGuidelines9 SubmitDocumentationFeedback www.ti.com 3.4ESDProtectionSystemDesignConsideration 4References References ESDprotectionsystemdesignconsiderationiscoveredinSection2ofthisdocument.Thefollowingare additionalconsiderationsforESDprotectioninasystem. •MetallicshieldingforbothESDandEMI •ChassisGNDisolationfromtheboardGND •AirgapdesignedonboardtoabsorbESDenergy •ClampingdiodestoabsorbESDenergy •CapacitorstodivertESDenergy •TheuseofexternalESDcomponentsontheDP/DMlinesmayaffectsignalqualityandarenot recommended. USB2 .0Specification,Intel , 20 00, http://www .usb. org/developers/docs/ •HighSpeedUSBPlatformDesignGuidelines,Intel , 20 00, http://www.intel.com/technology /usb/ download /usb2 dg_R1 _0. pdf •SelectionandSpecificationofCrystalsforTexasInstrumentsUSB2.0Devices(SLLA 122 ) 1 0USB2 .0BoardDesignandLayoutGuidelinesSPRAAR7–December 200 7 SubmitDocumentationFeedback IMPORTANTNOTICE TexasInstrumentsIncorporatedanditssubsidiaries(TI)reservetherighttomakecorrections,modifications,enhancements, improvements,andotherchangestoitsproductsandservicesatanytimeandtodiscontinueanyproductorservicewithoutnotice. Customersshouldobtainthelatestrelevantinformationbeforeplacingordersandshouldverifythatsuchinformationiscurrentand complete.AllproductsaresoldsubjecttoTI’stermsandconditionsofsalesuppliedatthetimeoforderacknowledgment. TIwarrantsperformanceofitshardwareproductstothespecificationsapplicableatthetimeofsaleinaccordancewithTI’s standardwarranty.TestingandotherqualitycontroltechniquesareusedtotheextentTIdeemsnecessarytosupportthis warranty.Exceptwheremandatedbygovernmentrequirements,testingofallparametersofeachproductisnotnecessarily performed. TIassumesnoliabilityforapplicationsassistanceorcustomerproductdesign.Customersareresponsiblefortheirproductsand applicationsusingTIcomponents.Tominimizetherisksassociatedwithcustomerproductsandapplications,customersshould provideadequatedesignandoperatingsafeguards. 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