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LM393: response time / 2 step output

Yusuke Mukuno
Expert 7395 points
Part Number: LM393

Hi expert,

My customer found 6.6us response time and 2 steps output waveform.

Datasheet response time is 1.2us(typ), however waveform show 6.6us.

Could you please let me know the reason of this response time and 2 steps output waveform?

Thanks

Muk

  • 0
    TI__Mastermind 49896 points

    Hello Muk,

    I am assuming the LM393 supply is also 3.3V.

    At 3.3V supply, the maximum input voltage is VCC-1.5 at room, or 1.8V max.  (VCC-2V, or 1.3V max over temperature).

    The reference voltage is about 2.3V. The reference voltage is continuously violating the negative input 1.8V input limit.

    The input signal is going from 1V to about 3V. After 1.8V, the input range of the positive input is now violated.

    So, after the positive transition, BOTH inputs are being violated.

    When both inputs are above the input range, the input stage is cut-off (or nearly cut-off). This will cause increased propagation delay, and most likely the 2-step glitch.

    It will fail at cold temperatures.

    Please see section 2.2 to 2.5 of the LM393 family application note:

    Application Design Guidelines for LM339, LM393, TL331 Family Comparators (Rev. A)

    We recommend bringing both of the input signals to below 1.3V max for specified operation.

    The ideal device would be the new LM393LV, which is R-R input, faster, and would work as a drop-in replacement without circuit modification. However, the PW package release is still a few months away (now available in SOIC).