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TUSB8040A / detect VBUS present

K.Yaita
Guru 29710 points
Other Parts Discussed in Thread: TUSB8040A
Hi Team,
 
I'd like to ask about TUSB8040A.
There is following description about  USB_VBUS pin in datasheet page.9.
So, I have two following questions.
 
1)
 I calculated minimum voltage of USB_VBUS to detect VBUS present is about 0.393V .
Is my understanding correct?
 
2)
My customer observed 0.342V(<0.393V) instantaneously at USB_VBUS pin that is caused by the supply(VBUS) noise of his system.
The duration of this noise is about  <100nsec.
Is it possible case to become VBUS detect error for TUSB8040A?
 
Best Regards,
Yaita / Japan disty
  • 0
    TI__Guru 60865 points

    Hello Yaita-san,


    The VBUS input is debounced internally to avoid transitioning on noise.  This short spike of noise will not cause any issue with the system.

    Regards,

    JMMN

  • 0 in reply to JMMN
    Guru 29710 points
    Hello JMMN-san,
     
    Thank you for your answer.
    Could you tell me how much of duration(<0.393V) could cause issue as a guide?
     
    Best Regards,
    Yaita
  • 0 in reply to K.Yaita
    TI__Guru 60865 points

    Hi Yaita-san,


    VBUS is debounced for > 1ms, but only when going from low to high.  It is not debounced on a high to low transition.  This means that the customer apication could see a false VBUS low event.  Can they set VBUS higher in the system or increase capacitance to avoid the noise?

    Regards,

    JMMN

  • 0 in reply to JMMN
    Guru 29710 points
    Hi JMMN-san,
     
    Thank you for your support. I understood.
    I'd like to ask you one more.
     
    >Can they set VBUS higher in the system or increase capacitance to avoid the noise?
    My customer considers to change resisor value of USB_VBUS divider from 90.9kΩ to 75kΩ to raise VBUS detect (0.495V→0.588V).
    Do you think it is reasonable method to this issue?
     
    Best Regards,
    Yaita
  • 0 in reply to K.Yaita
    Guru 29710 points
    JMMN-san,
     
    Sorry for bothering you.
    However, I'd like to have your comment.
     
    Best Regards,
    Yaita
  • 0 in reply to K.Yaita
    TI__Guru 60865 points

    Hello Yaita-san,


    Changing the resistor value should address the issue.  Please be aware the USB specification requires 1 uF to 10 uF on the VBUS input to the hub as well, this capacitance may help address the noise spike as well.

    Regards,

    JMMN