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Controlling TMDS141 outputs in DC-coupled environment

Anthony Woodward
Prodigy 210 points

Other Parts Discussed in Thread: TMDS141

I have a scenario where the TMDS141 is driving DC-coupled into a chip that wants to see common mode voltage at 0.7V, running at 2.25Gbps. There are 50 ohm pullups to 3.3V on the outputs.

If these external resistors and rail are changed to produce the desired common mode voltage, and RVSADJ is used to control swing, can this be made to work?

I think we have the freedom to make these changes, since the bus is entirely isolated on the board, so it just needs to be made to work.

Using weaker pullups is a no-brainer change, but the pull-up *rail* would be more difficult to change.

Thanks!

  • 0
    TI__Mastermind 23815 points

    Hi Anthony,

    I have some concerns regarding your implementation.

    Is it for HDMI/DVI? If it is for this purpose this is out of specification. If you plan to use it a general purpose redriver, you should be careful with input termination resistors included in TMDS141.

    Since this is not an standard application we have no information if it would work. If voltage in output pins drops too much it may polarize the transistors incorreclty or even shut them down. You can try lowering the voltage little by little we don;t know if it will reach the CMV desired.

    Regards

  • 0 in reply to Moises Garcia
    Prodigy 210 points
    Thanks, Moises.

    The more I look at it, the more I think it's not going to work. Yes, it's for HDMI.

    The TMDS141 datasheet says the minimum single-ended output voltage is AVCC – 0.6V, and the minimum AVCC voltage is spec’ed at 3V, so it shouldn't even be expected to work below 2.4V. It’s pleasantly surprising, then, to see that we have seen it work all the way down to 1.6V. Yet, that's still a long way from 0.7V.

    I guess I was hoping for some clever miracle, such as a similar part, or an unpublished app note, or something....

    For the record, this isn't our implementation, and in fact goes against our recommended reference design. It's complicated.