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LM339: schematic review

Michael T Schneider
Expert 3790 points
Part Number: LM339
Other Parts Discussed in Thread: , TLV1704

Team, 

can you help explain why we get 0.79V on the output regardless of input in this schematic?

  • 0
    TI__Guru 60786 points

    Hello Michael,

    Yes. If all the inputs are "high", you are violating the input voltage range on both inputs. For the LM339, the expected result when both inputs are exceeded is that the output goes low.

    The valid input range is Vcc-1.5V at room, or 1.8V. Vcc-2V over temp, or 1.3V.

    Please see the LM339 family application note, sections 2.2 and 2.4, it will explain it all:

     Application Design Guidelines for LM339, LM393, TL331 Family Comparators

    Quick fix is to use a R-R input device such as the TLV1704 or LM339LV, or attenuate the inputs to below 1.3V.

  • 0
    TI__Guru 60786 points

    Hi Michael,

    I missed that the inputs were pulled-up to 5V on a 3.3V comparator supply. The TLV1704 will not tolerate the inputs above the supply. The LM339LV will allow the inputs to go up to 6V, regardless of supply, so it should work.

    After reading the appnote, and you wish to rely on "The Feature", then just change 2.5V reference to <1.3V, and the outputs should respond, but realize that it is still working in an unspecified range and the propagation delay may be worse (slower).