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TUSB321AI: About TUSB321AI OUT voltage limit issue, please help.

YenHui Wu
Prodigy 40 points
Part Number: TUSB321AI

Dear Sir:

In our system there are 2 power supply source, 1 is powered by USB, 2 is powered by POE.

In current system design, we use "CV_USB0_DETECT_VBUS” to detect USB insert or not.

If I powered by POE, the system could work. Theoretical "USB_5V(VDD Pin)” should be 0V, but there is slightly leakage due the system is powered on. Therefore, the value that I detect "USB_5V(VDD Pin)” is 0.376V, and the value "USB_C1(OUT2 Pin)” is 0.95V;

But according to Datasheet Page.4, OUT1 or OUT2 Max Voltage value is ”VDD + 0.3”.

In our case, 0.95V>0.376V+0.3V. Would it damage to ” TUSB321AI” ?

  • 0
    TI__Guru 64326 points

    Hi,

    There  is some potential for TUSB321AI to be damaged in this condition. TUSB321AI could also have some potential to be back-powered in this case which could cause internal state machine issues. Is there any voltage on USB_CD as well? I believe that TUSB321AI may be back powered in the POE case causing OUT2 to be in a bad state. Is there anything on USBC1 that could generate the 0.95V?

  • 0 in reply to Malik Barton57
    Prodigy 40 points

    Dear Sir:

    You say " Is there any voltage on USB_CD as well?", but I don't find the "USB_CD", Could you tell me where is the USB_CD??

    Because in our system there are 2 power supply source, 1 is powered by USB, 2 is powered by POE.(Alternative)

    So When powered by POE, at this time the system is normally powered. And  the voltage value on "USB_C1(OUT2 Pin)” is 0.95V, "USB_5V(VDD Pin)” is 0.376V

    So you think that in this situation, the  TUSB321AI will be damage, is it??

    Thank you.

  • 0 in reply to YenHui Wu
    TI__Guru 64326 points

    Hi,

    My apologies there was a typo. The message was refereeing to USB_C. There is a potential for damage but I cannot guarantee that TUSB321AI will be damaged in this case. I believe the reason you are seeing the voltage on USB_C1 is because TUSB321AI is being partially pack powered by POE. 

  • 0 in reply to Malik Barton57
    Prodigy 40 points

    Dear Sir:

    The voltage of USB_C0(OUT1 Pin) is the same to USB_C1(OUT2 Pin), about 0.95V.

    So you mean if our system powered by POE, and voltage of VDD Pin (USB_5V) is 0.376V, OUT1 or OUT2 voltage is 0.95V.

    It may cause TUSB321AI damage, is it??

    So you don't recommend this design, right??

    Thank you.

  • 0 in reply to YenHui Wu
    TI__Guru 64326 points

    Hi,

    For both questions, this is correct. I recommend using a stronger blocking diode or if possible use a power switch to disconnect USB_5V form POE12V.