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LM2901: Check the LM2901 output

Eric Shen42
Expert 2750 points
Part Number: LM2901
Other Parts Discussed in Thread: TLV1704

Dear TI team:

     Please help to check LM2901 output level, the VCC is 5V, the test data:

     1:  IN+ = 4.31V  IN- =3.85V   OUT=H

     2:  IN+ = 4.31V  IN- =4.35V    OUT=L

     And I check the information from datasheet  as below, I think the 3.85V/4.31V/4.35V are>3V, so it would be output L, but it output H when "IN+ = 4.31V  IN- =3.85V "

     

   Thanks!

Regards!

Eric

  • 0
    Guru 140200 points
    Hi Eric,

    the common mode input voltage limit where the LM2901 stops working properly when furtherly increasing the input voltage differs from chip to chip. This need not exactly to be 3V (for 5V supply voltage) but can be higher. 3V is only the guaranteed limit but the typical limit can be higher.

    The best idea is to keep the input voltages always within the specified common mode input voltage range. This not only prevents these undesired borderline cases but also preserves the dynamic performance of chip (switching times, etc.)

    Use voltage dividers, if required, to fullfill the common mode input voltage range requirements of LM2901.

    Kai
  • 0
    TI__Guru 59806 points

    Hi Eric,

    As Kai siad, you are operating outside the specified input voltage range. At 4.3V, you are in the "teetering-over-the-edge-about-to-die" range. Hit it with cold spray and see what happens...

    See the following Appnote, section 2.2:

     Application Design Guidelines for LM339/LM393/TL331 Family Comparators

    It will *seem* to work now, but you will eventually get failures.

    I was never fully on-board with the last bullet point that you highlighted. There could be bias points where there is *juuust* enough leakages to push the output in one direction or the other - and it may not be in the direction you want. I think you found one of those points....

    Please re-adjust your input voltages so they fall <3V, or increase the comparator supply voltage to >6.5V to ensure proper operation. If you cannot change the input voltages, replace the LM2901 with a TLV1704 or similar rail to rail input device.