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Maximum driver output resistor for signal at WDI of UCC2946

Amir Arab
Prodigy 20 points
Other Parts Discussed in Thread: UCC2946

Hi,

I am presently using UCC2946 with a capacitor at pin WP of 680pF. I observe that the WDO remains high while WDI is statically driven to 0V through a 10K pull-down resistor. This indicates that the watchdog is disabled. I suspect that 10K is too high, but the datasheet of the UCC2946 does not indicate the maximum value of the output resistance of the circuit driving the WDI pin. Is it possible to indicate what would be that maximum value?

Thanks

Amir 

  • 0
    TI__Guru 55940 points

    Hi Amir,

    The strength of a pull-up resistor would be determined by the processor. WDI is an input pin to the device and is simply looking for transitions.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 20 points

    Hi Ryan,

    Thanks but, I will re-phrase my question. In my circuit, the UCC2946 has no transitions on WDI input, only 0V DC, yet WDO remains high. This means that the watchdog function is disabled. I am trying to understand how can that happen? In other watchdog chips currently available from TI, if the output resistance of the circuit driving WDI is too high, that effectvely disables the function. In application note slva145.pdf it is clearly explained how the watchdog function (TPS36xx, and ohers) can be disabled, and it is by making sure that the bias current supplied to WDI is below 10uA. Other wtahdog chips have similar behavior and impose a limt to the output resistance of the ciruit driving the WDI pin.  

    In my case,  use the UCC2946, there is nothing attached to the WDI pin but a 10K pull-down resistor to 0V. Is 10K too high? What is the value of pull-down resistor, below which it is guaranteed that the watchdog function of the UCC2946 will always work?

    Thanks

  • 0 in reply to Amir Arab
    TI__Guru 55940 points

    Hi Amir,

    The app note is not applicable to every supervisor.  Could you provide a scope shot with VDD, /WDO, RTH and /RES?

    Very Respectfully,

    Ryan