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TDP0604: Bias voltage is output to the TMDS line before powering up the custom board (before HPD is recognized)

Conor
Guru 12395 points
Part Number: TDP0604


Hi,

We use the TDP0604 on a custom board and implement it as an HDMI RX. The image receiving destination is FPGA. As a result of verification with a custom board, depending on the device to be connected, the bias voltage may be output to the TMDS line before the power of the custom board is turned on (before the HPD is recognized).

In this case, the power flows backward from the power supply pin (VCC) of the TDP0604 through the inside of the IC, causing some components to malfunction even though the custom board is powered off. Are there any countermeasures for this kind of situation? I understand that it is possible to add a capacitor to the IN side of the TDP0604 as an option, but I am concerned about performance problems when placing a capacitor when connecting HDMI (especially HDMI 2.0). Also, if you know, please let me know if the device that outputs the bias voltage to the TMDS line before recognizing the HPD conforms to the HDMI standard.

I need an urgent response, so it would be helpful if you could reply as soon as possible.

Thanks,

Eevee

  • 0
    TI__Guru**** 179886 points

    Eevee

    Are they also driving HPD high into TDP0604?

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Guru 12395 points

    Hi, David

    Please tell me the intention of your question. Pulldown processing is recommended for HPD, right?

    Thanks,

  • 0 in reply to Conor
    TI__Guru**** 179886 points

    Hi, 

    The source is a current source driver and expects to see the 50ohm termination to 3.3V on the sink (TDP0604) side. So if the TDP0604 is off and its termination voltage is then removed, then I do not see how the source can still drive the output. 

    On the HPD, when TDP0604 see HPD high, it will enable its 50ohm termination. When HPD is low, there is a 400k in the path. So if somehow the source is driving its TMDS output, you will see less leakage since it is 400k.

    Thanks

    David

  • 0 in reply to David (ASIC) Liu
    Guru 12395 points

    Hi, David

    ・Since the source is a current source driver, it cannot normally be driven when HPD is in the LOW state (50Ω termination disabled).
    ・Assuming that it can be driven, the TMDS input pin of the TPD0604 has an input impedance of 400kΩ, so there is no need to worry about IC damage.

    Is my understanding above correct?

    Thanks,

  • 0 in reply to Conor
    TI__Guru**** 179886 points

    Hi,

    Even with 400k, you will still have some leakage, but it will be less than the 50ohm termination. But I do not know the actual leakage since we do not expect the source to continue driving its output without the TDP0604 50ohm termination to 3.3V.

    Thanks

    David