Sending Power Over Coax in DS90UB913A Designs

1 ApplicationReportSNLA224 June2014 SendingPowerOverCoaxin DS90UB913 ADesignsABSTRACTTheDS90UB913 Awasdesignedas a serializerto oftenlocatedin remotepositionssuchas bumpersor trunklids, and a majorcomponentofthe systemcost is the this reasonit is desirableto minimizethe wiringto the videodata,alongwith a bidirectionalcontrolchannel,and powerto all be applicationreportdiscussesthe constraintsinvolvedin the powerdesignportionof the Operationof the Operationfor PowerOverCoax(POC)..23A PowerFeedCircuitwith Use of Figures1 Conceptfor Powerfromthe SignalPathwith Inductorand a 100 H Inductorto CircuitUsingTwo the Operationof the datathat the DS90UB913 Areceivesfromthe imageris encodedand encodedword(payload)is 14 bits long,so for eachPCLK,thereare 14 bit cellsoutputfromthe with a 100 MHzPCLK,the serialrate out of the serializeris Gbps,whichcorrespondsto abandwidthrequirementof 700 MHz,or a unit intervaltime of backchannelis separatedfromthe forwardchannelin the backchannelfrequencyis fixedand is MHz,but it can changewith processvariation,powersupplyand backchannelfrequencyis kept low to minimizeany potentialinterferencewith the June2014 SendingPowerOverCoaxin DS90UB913 ADesignsSubmitDocumentationFeedbackCopyr ight 2014,TexasInstrumentsIncorporatedACDCDC & ACPowerSourceTxRxBraidedShieldCoaxial CableL2L1 FPD-Link IIIPOWER50 47nF100nF50 47nFPower Regulator100nFTheoryof Operationfor PowerOverCoax(POC) Operationfor PowerOverCoax(POC)

2 Boththe forwardand the backchannelare the caseof the backchannel,the bandofinterestis from1 MHzup to about4 MHz,and for the forwardchannel,we are interestedin frequenciesfromabout70 MHzto 700 the forwardchannelwereoperatingat a lowerPCLK rate,thentheforwardchannelfrequencybando f interestwouldbe scaleddownto a POCwe wanttodesigna circuitwhichwill split the inputsignalinto two branchswith one branchcarryingthe DC powerforthe POCcircuit,and the secondbranchcarryingthe signalswithoutthe DC do this, we put anelementin the signalpathbranchwhichwill passboththe backchanneland the forwardchannel,but blockthe DC. A simplecapacitorwill workfor this sincea uF capacitorhas very low impedancefromthestartof the 1 MHzbackchannelbandthroughthe 700 MHzupperlimit of the a uF 0603capacitoris on the orderif 1 nH, so it doesnot reallycomeinto playwithinthe bandof secondcircuit one whichpassesthe DC, but doesn t interferewith the AC signalis a bit Conceptfor PowerOverCoaxSincethe datachannelsare beingpassedovera controlledimpedancetransmissionline, we wanttomakecertainthat the impedanceof the low passcircuitis largeoverthe bandof the forwardchannel.

3 Ifwe wantthe powercircuitto not interferewith the datapath,thenwe wantto makesurethat theimpedanceof this circuitis > ~20 Xour characteristicimpedance,so for a 50 coaxline, we wanttheimpedanceto be greaterthan1 K from1 MHzup to 700 MHzJust like the capacitorworkedfor thedatachannel,and idealinductorwouldworkfor this ,idealinductorsare muchharderto find have>1 K impedanceat the 1 MHzlowerbandof thebackchannel,we wouldneedabouta 100 H inductor,but a typical100 H inductorhas a parasiticcapacitancewhichdropsits impedancebelow1 K at frequenciesabove70 MHz,so it wouldinterferewith the Separatingthe Powerfromthe SignalPathwithInductorand Capacitor2 SendingPowerOverCoaxin DS90UB913 ADesignsSNLA224 June2014 SubmitDocumentationFeedbackCopyright 2014,TexasInstrumentsIncorporatedFrequen cy (Hz)Impedance (:)1E+31E+41E+51E+61E+71E+81E+ RealWorldInductor3A RealWorldInductorA goodmodelfor an inductoris to consideran idealinductor,whichhas in parallelwith it a low frequencies,the capacitorrepresentsa very high impedance,and the inductorbehavesasan very high frequencies,the capacitorlookslike a shortcircuit,at whichpointtheimpedanceis equalto RFigure3.

4 Modelfor a RealWorldInductorAt a frequencyof 1/ ( LC) the inductorand capacitorresonate,and the impedancebecomesvery net resultis that if you measurethe impedanceof a real worldinductor,you will get a curvewith ashapesuchas that seenin Figure4 wherethe blue line showsthe impedanceof an idealinductor,the redline in Figure4 showsthe impedanceof the parasiticcapacitance,and the purpleline showsthe totalimpedance this graphignoresthe effectof the self resonance,whichactuallycausesthe real curvetopeakmoreat the top, self resonance,thereis anotherintrusionof the real worldon convertingthe electricalenergyin the circuitinto an electricfield whichis voltageacrossthe cap is linearlyrelatedto the field strengthup until the pointwhenthedielectricbreaksdown,and thenthe capacitorceasesto behaveas a capacitor this voltageis knownasthe breakdownvoltage,and everycapacitorthat you buy has a ratedbreakdownvoltage so long as thisvoltageis higherthanthe voltageat whichthe cap will be operating.

5 The capacitorwill behaveas youwouldexpectfroma a similarfashion,exceptratherthanstoringen ergyinthe formof an electricfield,they storeenergyas a magneticfield withinthe coreof the likea capacitorhas a maximumvoltagethat it can support,an inductorhas a maximummagneticfieldstrengththat it can support,whichis relatedto the currentflowingthroughthe long as thecurrentis belowthat limit,the inductorbehaveslike an inductor,but if you exceedthe maximumcurrent(knownas the saturationcurrent)thenyourinductorceases to behavelike an RealInductorImpedance- 100 H Inductorto BlockBackchannel4 Designingthe PowerFeedCircuitwithRealComponentsThe circuitrequiredmusthavevery low impedanceat DC, and the impedancemustbe >1 K in boththebackchannelband(1-4 MHz),and the forwardchannelband(PCLK* PCLK*7)Ideally,theimpedanceis also above1 K in the regionbetweenthe backchannelbandand the forwardchannelband,but this is not June2014 SendingPowerOverCoaxin DS90UB913 ADesignsSubmitDocumentationFeedbackCopyr ight 2014,TexasInstrumentsIncorporatedFrequen cy (Hz)Impedance (:)1E+31E+41E+51E+61E+71E+81E+9050010001 500200025003000D002100 2H 2H InductorSeries CombinationHowto Use 100 H inductorwill havean impedanceof 1 K at 1 MHz,so this will coverthe backchannel,and if theparasiticcapacitancedoesn t comeinto play at frequenciesbelow700 MHz,thenthis is all we shapeof the impedancecurveis symmetricalaboutthe self resonancefrequencyon a logscale,so if we selecta 100 H inductorlike the MSS7341T-104 MLBfromcoilcraft,and look at thedatasheetwe will find that the Self ResonantFrequency(SRF)is MHz,whichmeansthat theimpedancewill stay above1 K until about50 MHz.

6 Unfortunately50 MHzis well belowthe saturationcurrentof the MSS7341T-104 MLBwe see that it is A, sowe needto makecertainthat our serializerdesigndoesnot drawmorethanthis amountof current,and ifit does,thenwe needto find boostthe impedanceof the circuitin the frequencybandof the forwardchannel,we will needa loweredgeof the forwardchannelbandis 70 MHzat the maximumPCLK frequency,but ifwe wantto allowfor lowerPCLK frequencies,we may wantto selecta lowerfrequencyband perhaps35 MHz. To get 1 K of impedancefroman inductorat 35 MHzwe needa valueof uH, so selectingthecloseststandardvaluegivesus uH. The Coilcraftpn 1008PS-472 KLBis a H inductor,with asaturationcurrentof A, and a SRFof 160 MHz this SRFmeansthat the uH inductorshouldbegoodup to greaterthan700 can be seenin Figure3 whereyou can see the individualimpedancesof the 100 H and H inductors,alongwith the compositeof the two in serieswith theseriescombination,the impedanceis very low at low frequencies,allowingthe DC powerto passeasilythroughthe circuit,but from1 MHzto over1 GHz,the impedanceis greaterthan1 K preventinginterferencewith eitherthe forwardor the keepthe impedancefromspikingtoo high,1K resistorscan be placedin parallelwith eachinductor,allowingthe total impedanceof that portionofthe circuitto vary between0 for DC and 1 K for high Impedanceof CircuitUsingTwoSeriesInductors5 Howto Use SmallerInductorsThe physicsof the requirementsdictatethe valuesof the inductorsthat we use (100 H and H)

7 , butthe physicalsize is dominatedby the abilityof the coreto sustainthe way to use a lowersaturationcurrentinductoris to reducethe currentrequirementof the circuitwhichcan be doneby increasingthe voltagethat is beingcarriedbythe our W, and we are sendingthe poweroverthe coaxat a voltageof5 V, thenwe will need300 mA and the 100 uH inductorthat we selectedis probablyaboutthe smallestphysicalsize that we couldtolerate(it is 7mmx 7mmx 4mmin size).However,if we wentto a 12 Vsupply,thenonly 125 mA is coilcraft1812LZ-104has a SRFof 12 MHz,a saturationcurrentof 150 mA, and a footprintwhichtakesup the spaceof the DS90UB913 ADesignsSNLA224 June2014 SubmitDocumentationFeedbackCopyright 2014, ExampleCameraDesignwithPOC6 VoltageRegulatorsOncethe voltageis transferredacrossthe coaxcableto the camera,therewill be a needfor LMR22007simpleswitchernanoregulatoris oftena goodchoiceforvoltageregulatorwith it s combinationof programmableoutputvoltage,very high efficiency(>90%)andphysicallysmall( )

8 Lowervoltageswith minimalcurrent,a miniatureLDOsuchas the LP3990is a goodoption7 SummaryPowersufficientto powera smallcameracan be sent overthe samecoaxcablethat is beingusedfordatatransmissionso long as the designmakescertainthat boththe backchanneland forwardchannelbandsare not meansa circuitwith two seriesinductors one whichis~100 H for the backchanneland one whichis ~ H for the physicalsize of theinductorsis dictatedby coresaturation,so if smallersize componentsare required,the voltagebeingsentoverthe cablemustbe increased,to decreasethe be usedtoprovideregulatedvoltagesuppliesto the imagesensorand serializerwith very little loss of *June2014 FirstRelease5 SNLA224 June2014 SendingPowerOverCoaxin DS90UB913 ADesignsSubmitDocumentationFeedbackCopyr ight 2014,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedand its subsidiaries(TI) reservethe rightto makecorrections,enhancements,improvement sand otherchangesto its semiconductorproductsand servicesper JESD46,latestissue,and to discontinueany productor serviceper JESD48, latestrelevantinformationbeforeplacingor dersand shouldverifythat suchinformationis semiconductorproducts(alsoreferredto hereinas components )

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