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TUSB544: AC-coupling capacitors at DRX2P/DRX2N/DRX1P/DRX1N

K.Yaita
Guru 29710 points
Part Number: TUSB544

Hi Team,

The datasheet shows the following and there are no AC-coupling capacitors at DRX2P/DRX2N/DRX1P/DRX1N (red part).



On the other hand, EVM has these capacitors.
(page 6)
http://www.ti.com/lit/ug/sllu260/sllu260.pdf

Could you tell me why there are AC-coupling capacitors in EVM?
Are  thes AC-coupling capacitors required?

Best Regards,
Yaita / Japan disty

  • 0
    TI__Mastermind 36085 points

    Hello Yaita-san,

       This is because there is an Engineering Change Notice (ECN) regarding optional Rx AC Coupling Capacitors.  This capacitor is useful for ESD/EOS protection and remove unwanted bias level from Rx side. Because RX side of different UBS-SS system have variable common mode voltage (use different termination scheme), this option can remove unmatched bias level for interconnection logic. After adding this capacitor leakage current through RX pin can be ideally 0mA and it is helpful to prevent water corrosion. (Same as TX side)

    Here you go the original document which can be found within The USB specification.

    USB 3.1 ECN Rx AC Coupling Capacitor Option.pdf

    Best regards,

    Diego.

  • 0 in reply to Diego Cortes
    Guru 29710 points

    Hi Diego-san,

    I appreciate for your support.
    I would like to ask additional question as the following.

    1)
    Is there recommended value of voltage rating for these Rx AC coupling capacitors?
    User's guide for TUSB544EVM doesn't show the information.

    2)
    Do you recommend to set resistor between Rx and GND in addition to AC coupling capacitor?
    I heard competitor device recommended it.

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Expert 7395 points

    Yaita-san,

    As specified on the ECN document Diego attached (Table 6-2), the value for AC coupling capacitors should be between 297nF and 363nF
     

    PD resistor is optional. We would recommend to add the placeholder on your PCB (Not populated; with the pad over the RX trace to avoid having a stub), and populate only if you later detect it is needed.
     
    Regards,
    Jorge

  • 0 in reply to Jorge Llamas (SWAT)
    Guru 29710 points

    Hi Jorge-san,

    Thank you for your kind support.

    How about the following inquiry?
    It isn't capacitance but voltage rating for Rx AC coupling capacitors.
    -----------------------------------------------
    1)
    Is there recommended value of voltage rating for these Rx AC coupling capacitors?
    -----------------------------------------------

    Best Regards,
    Yaita

  • 0 in reply to K.Yaita
    TI__Mastermind 36085 points
    Yaita-san,

    We don't have a recommended rating, the customer can use the same rating as for TX pairs. Actually I will give more importance to the packaging of these capacitors, please use 0201 if possible, I recommend 0402 as maximum size.

    Regards,
    Diego.