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HD3SS3220: AC coupling caps and resistors when using mux as UFP to TUSB8043A - Design review

DHaas59
Prodigy 140 points
Part Number: HD3SS3220

Can you please review the attached schematic for correct implementation of the AC coupling caps and associated resistors?

If the resistors always ensure CM voltage in this application why would they not be integrated into the mux die?  This is an application specific mux so would it ever be a bad ideal to implement the resistors?  It would sure make the routing more mistake proof by implement in die and not having to glom on resistor pads.

Thanks,
Dan

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    TI__Genius 13250 points

    Hi Dan,

    DHaas59 said:
    Can you please review the attached schematic for correct implementation of the AC coupling caps and associated resistors?

    The AC capacitors on your RX lines need to be 0.33uF. The TX line capacitors are correct. For the resistors, I recommend removing them since the hub will bias the HD3SS3220. 

    DHaas59 said:
    If the resistors always ensure CM voltage in this application why would they not be integrated into the mux die?

    I believe the reasoning is that we don't assume every connected system will be biased towards 0V, so we leave that up to the designer to implement.

    For example, if you have a USB host/device that has a CMV of 1V and our device had internal pulldowns to GND, there would be unnecessary current leakage through the MUX.

    Best,

    Shane