• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 63 subscribers
  • Views 159 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.

Original question:

TPS3850: CRST limitation when latching WDO

TPS3850: Latching WDO with open drain buffer

Shota Mago
Expert 8110 points
Part Number: TPS3850
Other Parts Discussed in Thread: SN74LVC1G07, TLV7041

Tool/software:

Hi team,

My customer tried to implement WDO latch referring below thread.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1298948/tps3850-q1-latching-wdo-watchdog-output 

However, when he placed SN74LVC1G07DKCR(open drain buffer) between WDO and CRST, WDO  latch didn't work. Meanwhile, when they added 2pcs of N-ch FET on output to emulate external open drain buffer, WDO latch worked as expected.

Do you have any idea why adding only open drain buffer doesn't work?

Of course, he confirmed SN74LVC1G07DKCR with standalone operation works fine.

He used 1.8V for power supply of the buffer and TPS3850.

Best regards,

Shota Mago

  • 0
    TI__Mastermind 18730 points

    Hi Shota,

    Can you verify if the output of the SN74LVC1G07 is able to pull CRST all the way to 0V? For this application, the latch is implemented by preventing the internal capacitor from charging. Also, what is the pull-up resistor value for the WDO?

    Jesse 

  • 0 in reply to Jesse Villanueva
    TI__Expert 8110 points

    Hi Jesse,

    Maybe I found cause of this issue. Leakage current of the logic buffer is much larger than CRST pin charge current of TPS3850. This prevents correct operation.

    I ran some simulation using PSPICE for TI. When open drain comparator(TLV7041) which has low output leakage current, WDO latch worked as expected.

    TPS3850_WDO_Latch_Sim.pdf

    Best regards,

    Shota Mago

  • 0 in reply to Jesse Villanueva
    TI__Expert 8110 points

    Hi Jesse,

    I have update for this inquiry. In addition to low leakage comparator/buffer, a capacitor is needed on CRST pin for latching WDO operation since NC and pull-up to VDD of CRST pin seem don't activate CRST charge current as these mode use factory programmed reset delay.

    The customer confirmed latching WDO works as expected when a capacitor on CRST pin and TLV7041 are used.

    Best regards,

    Shota Mago