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LM193: Overfrequency usage

Colin Chen
Expert 1105 points

Part Number: LM193
Other Parts Discussed in Thread: LM139,

Dear Experts,

We are evaluating LM139, and some of the input frequencies may be at 5MHz or 10MHz. We would like to know what state this input frequency will cause the output to become and what impact it will have on the output. Thank you

  • 0
    TI__Guru 67956 points

    5MHz and 10MHz is way too fast.

    Max toggle rate is ftoggle = 1/( TPHL + TPLH +tr + tf)

    For the LM193 family, conservatively:

    Ftoggle = 1 / (1.3u + 1.3u + 100n + 100n) = 357kHz. 

    The output would either not even respond, or it may "stick" high. No way to predict.

    For 10MHz, 1/10MHz = 100ns period. There are two transitions in a period, so the comparator needs to be twice as fast.

    A comparator would have to have *minimum* of 50ns prop delay. And that is not assuming a "nice" output (ignoring rise/fall times).

    If you want a clean, symmetrical output, you should add at least a 20-30% margin. I would stay below 30ns.

    Do you need the low temperature (-55) or high voltage? (30V)

    Most of the "fast" comparators (sub 50ns) will be 5V max supply.

  • 0 in reply to Paul Grohe
    Expert 1105 points

    Hi Paul,

    Thank you for your response.
    May I ask if this stick high state is related to the reference input being on the positive or negative input pin?

  • 0 in reply to Colin Chen
    TI__Genius 13845 points

    Hi Colin,

    There's no way to predict the output behavior. The input switching frequency is simply too high (whether it be on the positive or negative pin) to reliably predict how the output of the comparator is going to respond.