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TUSB8041: TUSB8041 AC couple cap value range

user4979899
Expert 3080 points
Part Number: TUSB8041


Hi Sir,

TUSB8041 will connect with AMD PCH and the AC coupled cap value define is different between AMD & TUSB8041,
May I know which value is correct for the design?
below is the design guide from AMD document, and TUSB8041 datasheet.
Can we follow the AMD spec for AC-capacitor?? Any risk on TUSB8041?
Thanks

  • 0
    TI__Guru**** 180041 points
    Anne

    Under what condition is the cap value being defined in the AMD spec?

    The spec for AC coupling cap is 75nF min and 265nF max. If AMD PCH and TUSB8041 are designed on the same board, then 100nF on both TX and RX would be sufficient. But it would be good to understand how AMD defined the value first.

    Thanks
    David
  • 0 in reply to David (ASIC) Liu
    TI__Expert 3080 points

    Hi David,

     

    Please see the AMD comment. Please advise.

    =======================================================

    As checked USB3.1 AC coupling caps option note document below, it seems does not conflict with our mortarboard design guide.

    Since AC coupling caps recommended by SP3 MBDG is follow up USB-IF (USB3.1) spec, and these values have passed through most customer motherboards during SI validation.

    Therefore, we suggest the designer follow our MBDG to design.

  • 0 in reply to user4979899
    TI__Guru**** 180041 points
    Anne

    Base on AMD response, I believe AMD's AC coupling capacitors requirement is based on the assumption that there is no hub between AMD and the USB connector, so AMD PCH is connected directly to the connector. In this case, 330nF cap on the RX and 100nF cap on the corresponding TX side would result in 76nF effective capacitance.

    When there is a hub between AMD and the USB connector, then the ECN applies to the DFP port of the hub as shown in Figure 6-5 of the ECN. For UFP port of the hub, it will be 100nF on TX and 100nF on RX.

    Thanks
    David