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Original question:

TPS3851: Could be Watchdog timing capacitor of Ccwd bigger than 1uf?

TPS3851: What will happen some risk about conneting RESET's resistor is 110k ?

Lian Wu
Genius 5250 points
Part Number: TPS3851

Dears:

From the below schematic we design the R248 is 10K, however, the RESET pin's discretion from the datasheet is  we should connect RESET using a 1k to 100k resistor to the correct pullup voltage rail(Vpu).

You know the internal resistor of RESET Pin is 100K, and we design a 10k resistor, then connecting RESET's resistor is 110k (100k add 10k).

What will happen some risk about the design?

Pls. give some advice about it.

Thanks a lot!

Best regards

Luck Wu

  • 0
    Guru 140200 points
    Hi Lian,

    what voltage is present PMIC_ON_REQ?

    What is R253 doing?

    C837 can become dangerous for the /RESET output: If the /RESET output toggles low the C837 can immediately discharge into the /RESET output and destroy it. Output current must be limited to 20mA, accroding to datasheet.

    Kai
  • 0
    TI__Genius 12630 points

    Luck,

    Kai brings up good points. I recommend to remove C837 since this capacitor will change the delay, slew rate, and is not necessary. The voltage at PMIC_ON_REQ is important to know to make sure the RESET current is within spec. And /RESET does not have internal resistor, only /MR has internal resistor. The impedance of the /RESET pin is that of the internal FET RDSon but this is a small value and can be ignored.

    -Michael

  • 0 in reply to Michael_DeSando
    Genius 5250 points
    Hi Michael:
    Sorry, I took a mistake about the issue.
    RESET Pin has no 100K internal resistor, and the signal of PMIC_ON_REQ(Connecting CPU) has 100K internal resistor.

    And I want to know if connecting RESET's resistor is 110k what will happen some risk about the design?
    Pls. give some advice.

    Btw, we will remove C837, it is very useful.
    Many thaks!

    Best regards!
    Luck Wu
  • 0 in reply to Lian Wu
    Guru 140200 points
    Hi Lian,

    what is connected to the PMIC_ON_REQ? A reset input of a microcontroller? And this input already contains a pull resistor? Or is it a pull down resistor? How is this microcontroller powered?

    Kai
  • 0 in reply to Lian Wu
    TI__Genius 12630 points
    Luck Wu,

    You have to check the current through the RESET resistor by using the pull-up voltage and the RESET resistor value. The current through RESET resistor should be less than 10mA. The recommended values for the pull-up resistor are between 1kohm and 100kohm.

    -Michael