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Original question:

TPS3828-33 Remaining Continuously in Reset (Reset# Does not go High)

TPS3828: TPS3828 Decoupling WDI During Reset Event.

user4034761
Prodigy 40 points
Part Number: TPS3828

Hello.

I am using the TPS3828 IC for microcontroller supervision. It is, however, proving to be a pain to program the microcontroller with the TPS3828 soldered on the board.

To help with that (to atleast make sure that the reset output doesn't latch when connecting an external circuit to WDI pin), I would like to decouple the WDI pin from the reset line, however the schematic prodived uses a pass transistor.

The output of a pass transistor circuit is always below the supply voltage minus gate threshold voltage, meaning if I have a transistor with gate threshold voltage from 1V to 2V and a supply voltage of 3V, then worst case scenario the WDI signal at the input is around 1V peak to peak, well below the datasheet specified 0.7*Vdd minimum at the input.

Is it possible to elaborate further on choosing the mosfet and the conditions under which I should be able to expect the circuit to work properly? Or should I just reduce the entire issue to "Use a mosfet with lowest threshold voltage possible"?

Thank you.

  • 0
    TI__Genius 12630 points
    User,

    Yes, if decoupling the WDI pin using a pass transistor, ideally you want to operate the MOSFET in the saturation region so the solution will not work when the gate threshold (1V to 2V) is more than 33% to 66% of the available supply voltage (3V) when the input signal to WDI requires at least 70% of VDD. This means that you only have 30% of VDD available for the gate threshold. This means that for a 3V supply, you ideally want a pass transistor with 0.9V or less otherwise there is not enough headroom to power the pass transistor and meet the input voltage requirements.

    Here are some TI n-channel MOSFETs that could meet the requirement:

    www.ti.com/.../products.html

    Please be sure to test the pass transistor as the Vgs threshold could vary from typical due to device accuracy and operating conditions. Let me know if you have any additional questions or need any more support. Thank you!

    -Michael