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LM393: Problem with LM393D and LMV393

fabricepeden
Intellectual 495 points
Part Number: LM393
Other Parts Discussed in Thread: LMV393

Hi everybody,

I was using a LM393D as window detector in one of my design. When the voltage is between 1 and 2.8V, my output is 3.3V and when the input voltage is outside of these values, I got 0V. The output is wired to my microcontroller.

Sometimes, at startup, the output voltage is at 0V while my input voltage is between the limits. I just wait a few minutes, I reset the alarm, and the output voltage is right. I was looking for explanations about this problem, and I read on the TI website, that for 3V3 power supply, the LMV393 is better. So I change my LM393 with LMV393. But, on some cards, this one is working perfectly, and some other cards, I got a wrong output voltage.

If you have any advices to help me to understand this problem.

Please find below my design.

Best regards,

Fabrice Péden

  • 0
    Intellectual 495 points
    Hi,

    I just forgot an import point. Between terminals SOUNDER + and SOUNDER-, a 2k2 end of liner resistor is connected. So that means, that there is a bridge divider with resistor 22k and 2k2, so the voltage on inputs of amplifier is 2.1V.

    Best regards,

    Fabrice Péden
  • 0 in reply to fabricepeden
    TI__Guru*** 139956 points

    Fabrice,

    The issue issue is input common mode range.

    The upper comparator can exceed the valid range on both devices.

    A little known fact on these two devices the "input common mode" = the lesser of either input. It's not the average.

    LM393 data sheet section 8.2.2.1 note 4 says output goes high low when both inputs exceed common mode range.

    LMV393 data sheet does not declare output state in this case. However the section 8.2 schematic gives away the fact that output may be low or high. 

    You may lower the input voltage or raise the supply voltage to fix the issue

  • 0 in reply to Ron Michallick
    Intellectual 495 points
    Hi Ronald,

    Thank you for these informations.

    For LM393, common mode range is from 0 to VCC-1.5 (in my case that means 1.8V). So, in my case for IC6A, IN+ is 2.8V and IN- is 2.1V, so both inputs are higher than common mode range, so the output goes low (and not high as you said - . "When IN- and IN+ are both higher than common-mode, the output is low and the output transistor is sinking current")., So I understand what my solution is not working very well.

    The only solution to solve the problem is what you suggest, is there no references of amplifiers that can accept these voltages ?

    Best regards,

    Fabrice Péden
  • 0 in reply to fabricepeden
    TI__Guru*** 139956 points

    Fabrice,

    Here are two.

    TLV3202 - Dual, 40ns, microPower, Rail-to-Rail input Comparator with Push-pull outputs

    TLV1702 - Dual, 2.2-V to 36-V, microPower Comparator

    More here;

    http://www.ti.com/lsds/ti/amplifiers/comparators/comparators-products.page#p480=2;2&p78=In

  • 0 in reply to Ron Michallick
    Intellectual 495 points
    Ronald,

    Thank you for your quick reply but I choose to modify some resistors to decrease my voltage values. It seems to work well.

    Once again, thank you.

    Regards,

    Fabrice Péden