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TUSB542: DC cut capacitor

Takashi Yamaguchi
Genius 13290 points
Part Number: TUSB542

Hi team,

I received following question from my customer. Could you please support this?

[Background]

In Figure 16 on page 18 of the TUSB542 datasheet, a DC cut capacitor is placed between the USB3andPMIC and the TUSB542 for both TX and RX. The USB3andPMIC equivalent ICs that we use are specified as not requiring DC cut capacitors. Also, the DC cut capacitor seems to be in the way in the compliance test.

[Question]

For the TUSB542, is a DC cut capacitor required for RX? If yes, under what conditions is it required?

Regards,

Yamaguchi

  • 0
    TI__Guru*** 144735 points

    When TUSB542 is directly connector to USB host or device, AC cap 65nf to 265nf on RX s required.

    If connected to USB connector, AC cap on RX is not required since there is AC cap on the other side of the connector

  • 0 in reply to Brian Zhou
    TI__Genius 13290 points

    Hi Brian-san,

    Thank you for the answer. I received additinal questions as follows. Could you please support the questons below?

    1. Please let me check my understanding just in case. The AC cap is required for the RX_AP, RX_CON1, and RX_CON2 pins when directly connected to USB host or device, and not connected to a connector. Is that correct?

    2. Why does it need AC cap? What kind of problems can occur if the AC cap is not attached?

    3. When this AC cap (0.1uF) is not connected in the customer's prototype, the bER improves and compliance testing can be performed. What are the possible causes of bER deterioration with the AC cap?

    Regards,

    Yamaguchi

  • 0 in reply to Takashi Yamaguchi
    TI__Guru*** 144735 points

    1: right, but stll need AC cap in the link, AC cap is just on the other side of the connector.

    2: USB3 or PCIe is differential AC coupled link. it mainly remove common mode voltage error

    3: you mean 0.uf removed on RX? or TX?

  • 0 in reply to Brian Zhou
    TI__Genius 13290 points

    Hi Brian-san,

    Thank you for the asnwer. Could you please advise to the questions below?

    For #2 : In other words, if there is no common mode voltage, there is no problem without the capacitor? Could you please provide the characteristics of the DC output?

    For #3 :  I mean 0.1uF removed on RX. With the same set of boards, same test setup, with 0.1uF AC caps and without AC caps, the test results were improved without AC caps.
    　The same question, what could be the effect of the AC cap?

    Regards,

    Yamaguchi

  • 0 in reply to Takashi Yamaguchi
    TI__Guru*** 144735 points

    In the USB3.2 Specification (Sept. 22, 2017) Table 6-18 and Table 6-22 give some characteristics for the TX and RX lines.

    Basically the TX lines have AC-Coupling capacitors to block DC and let just the AC signals pass.  Min/max is 75nF to 265nF

    The spec also mentions that having AC-Coupling on the RX lines is optional (because the device-side already has series capacitors on its Tx lines, which connect to the Host's Rx lines, so it isn't needed for DC blocking).

    It is  recommend 330nF capacitors in order to meet the minimum capacitance requirement (75nF to 265nF) for RX detection..

    So 0.1uf on RX is too big on RX. either on Cap or 330nf.

  • +1 in reply to Brian Zhou
    TI__Guru*** 144735 points

    either no cap or cap with 330nf