• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 33 subscribers
  • Views 305 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TUSB4020BI: What if VDD can't be 10 us stable before VDD33?

firas ghani
Prodigy 50 points
Part Number: TUSB4020BI

We have a board that uses TUSB4020BIPHP(R)

I want to check with you if this design can work.

- We have GRSTz connected to GND through capacitor.

- Since our VDD is generated from VDD33 we can’t make the VDD stable before VDD33.

The question if this design would still work?

Note: the board is already sent to manufacture and we want to make sure earlier that we can make a rework or must make a new revision.

  • +1
    TI__Guru 60870 points

    Hello Firas,

    I was not able to access the schematic, but yes, it is acceptable to have VDD sourced from VDD33.

    Since the GRSTz pin has an internal pullup to 3.3V the concern with VDD vs. VDD33 ramp timing, is that if VDD33 is up well before VDD and only a capacitor is used on GRSTz, then the reset could end before VDD is even powered on.  In most applications as long as VDD33 and VDD ramp close together this is not a concern.  You can also increase the size of the capacitor on GRSTz to 2 uF or 4.7 uF to further increase the reset timing.

    Regards,

    JMMN

  • +1 in reply to JMMN
    Prodigy 50 points

    Hi, 
    Thanks for your answer, 
    can you share the concept of the inside circuit that is GRSTz connected to?

    is it Open-Drain that has high impedance when the reset is asserted and then pulled-up by a resistor to get "1"? 

    Best Regards, 

    Firas Abdel Ghani

  • 0 in reply to firas ghani
    TI__Guru 60870 points

    Hi Firas,

    It is a buffer input with a weak internal pullup.  For a passive reset circuit, only an external capacitor is needed.

    Regards,

    JMMN