0
\$\begingroup\$

I am trying to find the error in my schematic. I want to use the TPS3840DL supervisor to reset (turn off and on again) the TPS54202 step-down converter either when the voltage drops below the threshold or a manual reset is present (either button or GPIO pulled low).

The converter without the supervisor works flawlessly. As soon as I connect the supervisor, even without any manual reset signal (MR) present, the converter stops.

circuit

The reset output is open drain, the enable input has an internal pullup as well as the manual reset input.

I tried using different values for the reset time, this did not change anything.

Thank you very much for your help in advance, I think I am missing something very stupid.

All the best pitchbent

\$\endgroup\$

1 Answer 1

Reset to default
3
\$\begingroup\$

The reset pin may well be open drain but, there will be parasitic diodes on that pin to its local 3.3 volt rail (at 0 volts from a cold start). So, before the reset can enable the converter, you need 3.3 volts on the reset controller chip but, the reset chip is powered by the power converter you are inevitably resetting.

So, you have a stalemate situation at start up and nothing happens.

Voltage threshold required on the TPS54202 to enable it: -

enter image description here

On a good day, with the wind in the right direction, it might work at 1.15 volts but, as I said earlier, the TPS3840 won't be able to muster enough voltage when its supply is not active due to the clamping effect of an internal diode to Vcc on the reset line. This image shows what I mean and, take note: although the image might imply the upper MOSFET isn't present, it will be there but inactive. Note the red body diode (my addition) of that inactive MOSFET: -

enter image description here

\$\endgroup\$
4
  • \$\begingroup\$ Thank you very much for the good explanation! Thats a pity. I do have another rail available (5V), which shares the same ground. Do you think a pull up on the enable to >1.3V (made via voltage divider from 5V) would work? \$\endgroup\$
    – pitchbent
    Commented Jun 28, 2023 at 12:54
  • \$\begingroup\$ @pitchbent you should increase R4 to maybe a 1 kohm then try a pull-up on the buck converter enable line of maybe 10 kohm. It wouldn't surprise me if there are a few iterations to get this going. If we are done here, please take note of this: What should I do when someone answers my question. If you are still confused about something then leave a comment to request further clarification. \$\endgroup\$
    – Andy aka
    Commented Jun 28, 2023 at 13:01
  • \$\begingroup\$ Maybe also a 100 nF capacitor from EN to Vin might also help. \$\endgroup\$
    – Andy aka
    Commented Jun 28, 2023 at 13:02
  • 1
    \$\begingroup\$ Alright, thank you! I will try that :) \$\endgroup\$
    – pitchbent
    Commented Jun 29, 2023 at 8:30

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.