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TUSB1044: Pull down register on RX pin

Kazuki Kuramochi
Expert 8330 points
Part Number: TUSB1044
Other Parts Discussed in Thread: TUSB1002A

Hello Professional,

I have question for TUSB1044.

In TUSB1002A's Datasheet, . Host Implementation Schematic said 220k resister is optional on AC coupling RX line. However TUSB1044's datasheet don't explain about this optional register.
Would you explain about why TUSB1044's datasheet don't has this optional resister?
I think this resistor is for following 250ms Tdischarge specification on USB3.1 compliance. Is this right? And is there any hardware concern when we exceed this 250ms discharge time?


Also, customer will use this device for USB signal and Alt mode DP signal. Is there any impact regarding this usage for this pull down register?


Best regards,
Kazuki Kuramochi

  • 0
    TI__Guru 64326 points

    Hi Kazuki-san,

    Typically this spec is satisfied with TUSB1044 without the addition pull down resistors.  The additional resistors help to further reduce the discharge time to be well within the spec limit of 250 ms. As stated in the TUSB1002A these resistors are not required assuming leakage current from external components is enough to discharge capacitor. There will only be signal integrity impact to USB or DP performance if the resistors are added. The stub created from placing the optional (not necessary) pull down resistor should be minimized to avoid negative affects. 

  • 0 in reply to Malik Barton57
    TI__Expert 8330 points

    Hi expert,

    Thank you for your prompt reply.
    I understood about this optional resister for reducing discharge time and it has trade off between signal integrity and discharge time.
     
    I'd like to confirm about the risk when exceed the discharge time regulation.
    Could you tell me about why is there this 250ms regulation and what risk is there when this discharge time is stretched?

    Thank you and best regards,
    Kazuki Kuramochi

  • 0 in reply to Kazuki Kuramochi
    TI__Expert 8330 points

    Hi expert,

    Sorry for rushing you but would you answer for my qustion?

    Best Regards,
    Kazuki Kuramochi

  • 0 in reply to Kazuki Kuramochi
    TI__Guru 64326 points

    The Tdischarge spec is there to ensure that the USB receiver, at the connector, does not hold a previous common mode voltage and prevent a newly plugged device form seeing this common mode. In terms of risk is hard to say but the charged AC coupling cap at the receiver could have the potential to back drive a plugged in device.