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TPS3838E18 open-drain !RESET: allowable pull-up resistor values

Alex Pogostin65856
Prodigy 10 points
Other Parts Discussed in Thread: TPS3808, TPS3838

Will the TPS3838E18 open-drain !RESET pin allow for the use of a 3K ohm pullup resistor to 3.3Vdc? The Maximum low output current (IOL) is 5ma, which suggests this resistor value will work. But the documentation for another open-collector supervisor chip, the TPS3808 details a similar 5ma output on !RESET, but yet states on page 8 of its data sheet (top of column 2) that, "The pull-up resistor should be no smaller than 10k ohm as a result of the finite impedance of the RESET line."

  • 0
    TI__Guru**** 150770 points

    I do not recommend using a resistor that small with the TPS3838.  Using a 3k resistor would violate the recommended operating conditions in the TPS3838 datasheet which specifies a minimum value of pull up resistance.  This is only a recommended operating condition because it is not necessary under some conditions.

    If the 3.3V rail could be present before the TPS3838's Vdd is up, then the minimum pull-up value is required.  Using a pull-up resistor lower than this may put the IC in a special mode.  This restriction does not apply to the TPS3808.

    Figures 8 and 9 are the reason for recommending larger resistors on the RESET of the TPS3808.  Especially with lower Vdd's (which I believe you have), then the RESET FET cannot be driven into the linear region as well resulting in a higher RESET voltage.