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LM293: How to estimate minimum input pulse width

Shinya Sawamoto
Expert 5260 points
Part Number: LM293
Other Parts Discussed in Thread: LM239

Hello,

I'd like to know how to estimate minimum input pulse width for LM293.
I know it is a very simple comparator without any pulse limitation feature like deglitch filter.
I wonder how long LM293 can ignore(tolerant) against an unexpected spike noise.

Thanks in advance.

  • 0
    TI__Mastermind 49896 points
    Hi Shinya,

    Generally 2x the prop delay - but that is not leaving much margin. You can push it to 1x the prop delay - but I guarantee you you will not like the results (output distortions, possible missed pulses).

    You really do not want to push the limits. The AC characteristics are typical and not guaranteed on the LM239 - so any change in the devices could cause problems later. I would recommend at least 10x the prop delay to ensure a nice, clean transition.

    So if your minimum pulse width is 1us, then you should be using a 100ns or faster device.

    Open collector devices, such as the LM293, are a bit more difficult to estimate because the risetime is dependent on the time constant of the pull-up resistor and load current. This stretches out the overall pulse width.

    What is your minimum pulse width and supply voltage?
  • 0 in reply to Paul Grohe
    TI__Expert 5260 points
    Hi Paul,
    Thanks a lot for your clear explanation.
    The background is to know if 10ns of noise spike can toggle the output unexpectedly and there is no worry for that now.

    Regards,
    Shinya Sawamoto
  • 0 in reply to Shinya Sawamoto
    TI__Mastermind 49896 points
    Hello Shinya,

    10ns should not be a problem.

    But as you approach the prop delay number, it is possible to get partial "blips" on the output, which is not a full L-H transition, but a partial transition. The output will start to rise, but then immediately falls back.