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LMV7239: Output changes improperly

franco carlo
Prodigy 30 points
Part Number: LMV7239
Other Parts Discussed in Thread: TLV3511

Tool/software:

Hi,

since few months I'm experiencing a problem with the component in question.

In the presence of a disturb generated by the single switching of a mosfet used as switch on the board, the comparator changes its output improperly (that is without its imput changing). Even by greatly reducing the disturbance, the problem remains.

Replacing the component with the equivalent st ts3021 there are not the same problems.

What could be the cause? how do the two ics differ?

thanks

  • 0
    TI__Guru 60476 points

    Hi Franco,

    Do you have any scope photos?

    The TS3021 is a little slower than the LMV7239, so the LMV7239 may respond to any 'glitches" faster. Neither have built-in hysteresis.

    Make sure the power supply is properly bypassed (0.1uF ceramic cap directly on the supply pins).

  • 0 in reply to Paul Grohe
    Prodigy 30 points

    Thank you. What parameters are you referring to when you say one is slower than the other?

    Below the schematic and some waveforms:

       

    Blue: Comparator Output; Purple: +Input; Green:-Input; Yellow: Noise Trigger (switch transition).

  • 0 in reply to franco carlo
    TI__Guru 60476 points

    Hi Franco,

    There is a lot of noise floating around. Which are the LMV and which are the ST?

    The LMV is actually slightly slower.

    The comparators cannot tell the difference between noise and signal - so they will react when the signals cross. You need to keep the inputs clean. But any filtering will cause a delay.

    You may wish to try the new TLV3511, which is faster, but has a few mV of internal hysteresis which can help with noise. It should drop-in replace the TLV or ST.