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Using UCC2946 watchdog without its voltage supervisor

Other Parts Discussed in Thread: UCC2946, TPL5000

Hello,
I am planning to use the UCC2946 for a purpose slightly different than its intended use. I intend to use it as a fail-safe to disable a laser if the WDI pin has not been toggled for a certain amount of time. The watchdog function is perfect for our needs. However, according to the datasheet the watchdog timer is also reset if RES is asserted, independent of the watchdog timer, therefore enabling the laser device during power on and power fluctuations. Is there any way to keep RES from ever being asserted? I have no need for the voltage supervisor function. (There is brown-out detection elsewhere in the circuit)

Alternatively, am I correct to assume that if I AND together WDO and RES and set the reset period to be longer than the watchdog timeout period the output would never be high unless WDI is being toggled?

  • Hello Thorin,

    The hard part is keeping WDO from asserting during power-on. I would advise using something from TI's timer portfolio with a watchdog function.

    I am not too familiar with this device, but the TPL5000 is a timer with a watchdog function that may work.

    Regards,

    David

  • The issue with most other watchdog ICs I have found is that they only cause a reset pulse on timeout, instead of keeping the pin asserted until the timer reset pin is toggled again. In addition, most of them react to only rising or falling edges, while I require it to react to either one.


    Actually, WDO asserting during power-on does not matter as during power on the laser is being kept off by another part of the circuit.

    I did draw up a schematic where I set the reset pulse duration accordingly and "AND" WDO and RESET from the UCC2946 together, but it drastically increases part count to the point where I might be better off building the watchdog circuitry from basic logic gates and discrete components...

    So my question still is: Is there a way to keep the voltage supervisor from ever asserting, except on power-on? Alternatively: Are you aware of a device that has the same watchdog functionality (edge-detection on the watchdog reset input; keep WDO low unti watchdog timer is reset) but without the voltage supervisor functionality? I have checked the TPL5000, it is not suitable.

    Regards,

    Thorin

    On a completely unrelated note: Your name is the same as my fathers... That really confused me for a second. :)

  • Thorin,

    You could try to prevent it from asserting by putting an RC circuit on the RTH in to keep transients from coupling through.

    Basically, connect RTH to Vdd with a resistor, than connect a capacitor to GND from RTH. It will slow turn-on but buy you a large amount of time until RESET asserts - you can also use a higher Vdd in even further increase this time. For example, if R = 100k, C=10uF, tau=RC=1 second ... then set Vdd = 5V and for RTH to fall below Vdd it will take around 1.3 seconds.

    Regards,

    David

    Oh and good ole Welsh/English last names - its amazing how many Hopkins's there are - it was weird when I met somebody a few years back who shared my full name.