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Original question:

TUSB8041: AC Coupling of SSTX/SSRX

TUSB8041: Question for TUSB8041 USB_SSRX_UP AC Coupling Issue

Jacky Wang(QD)
Genius 10920 points
Part Number: TUSB8041
Other Parts Discussed in Thread: TUSB8042, TUSB8042A

Hi Team,

Our customer is using TUSB8041 to work with Novatek SoC and added 100nF AC coupling capacitors on USB_SSRX_UP pins for TUSB8041, but they found that sometimes it will cause USB3.0 communication failure. But if we removed the AC coupling capacitors on USB_SSRX_UP pins, it will work fine.

As the Novatek side will ask customer to add these AC coupling capacitors on the USB_SSRX_UP pins, can you help to check if we can add them? And what's the recommended value if needed?

Thanks a lot!

  • 0
    TI__Guru 60870 points

    Hi Jacky,

    AC capacitors on the RX lines are optional, but the value must be between 297 nF and 363 nF.  We do not recommend two sets of 100 nF capacitors

    Optional but recommended for new designs. SSTX should be AC coupled with 220 nF and SSRX should be AC coupled with 330 nF.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Intellectual 430 points

    Hi, JMMN,

    Sorry to intercept this issue, but I'd like to make some questions to more clearly define the AC coupling capacitors.

    Q1) Is this capacitor value should be applied to upstream and downstream ports ?

    Q2) Is it common to TUSB8042, TUSB8042A and other hub devices ?

    Q3) really need coupling capacitor for SSRX ?

    Q4) could you recommend applicable parts for this application ?

    Best

    Sympson

  • 0 in reply to Sympson
    TI__Guru 60870 points

    Hi Sympson,

    The original USB 3.0 specifications recommended 100 nF  (75-200 nF) AC caps on the TX pairs, The optional AC caps on the RX pairs were added in an ECN to the USB 3.1 specification.  It can be applied on any USB 3.x port, it is not required.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Intellectual 430 points

    Hi, JMMN,

    Then, what's the meaning of your recommend on this topic ?

    Optional but recommended for new designs. SSTX should be AC coupled with 220 nF and SSRX should be AC coupled with 330 nF.

    Best

  • 0 in reply to Sympson
    TI__Guru 60870 points

    The RX AC caps are optional, but can help remove bias and provide some EOS protection.  They are not a functional requirement.

    Regards,

    JMMN