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LM2901B: VCM

Hailiang Zhang
Expert 6825 points
Part Number: LM2901B
Other Parts Discussed in Thread: LM339

Hi Team,

From the schematic view, the VCM may greater than VCC. Is there any risk? LM2901B is used to monitor whether the 54V Bus is correctly connected. Judging from the test results, it is feasible. Is it necessary to modify and make VCM<Vcc-2V?

Regards,
Hailiang

  • 0
    TI__Guru 53826 points

    Hello Hailiang,

    This will not work correctly and could even have unintended operation. It may seem to work, but they are relying on a lot of variables that may change.

    Please see sections 2.5 through 2.8 of the LM339 family application note:

     Application Design Guidelines for LM339, LM393, TL331 Family Comparators

    The supply voltage is 3.3V, which makes the input limit 1.3V, They have the 3.3V on the negative input as a reference, which grossly violates this limit.

    Theoretically, the output should go high when the positive inputs exceed 3.3V. THIS WILL NOT HAPPEN. Both inputs will be above VCM and the output will revert to the "both inputs above VCM" behavior and the output will go either high OR low.. Please read section 2.8 of the appnote, 3rd and 4th paragraphs.

    In short, the output will be correct as long as the positive input is below 1.3V (the "feature" of one input within range). But when the positive input exceeds 1.3V, the output reverts to the "both inputs above VCM" behavior, which could be output high or low.

    The reference needs to be brought below 1.3V and the divider resistors re-calculated to keep ALL voltages below 1.3V. Another option is to supply the comparator VCC with a voltage greater than 5.3V. Is there another supply voltage available between 6V and 36V?