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lm139aqml: oscillation below 0C.

Todd Reed
Prodigy 100 points
Part Number: LM139AQML
Other Parts Discussed in Thread: LM139

Hello,


We have an LM139AWG/883 used to create an edge from a power on reset timer RC circuit and it is failing below 0C, even though the part is supposed to be rated -55 to 125C.  The output starts to glitch low a little under 0C and then as it gets colder, the output more resembles a clock.

I see two potential issues in the circuit and wanted to see if these could be contributors or if we could have a bad part.

1)  The VCC is 3.3V, which is within the absolute ratings, but not within the 5-30V recommended input voltage.

2)  The circuit needs some hysteresis, as seen by some bouncing while transitioning.

Thank you,

Todd

  • 0
    TI__Genius 17035 points

    Hello Todd,

    The LM139 is typcially functional with a supply voltage as low as 2V over the full temperature range.   However, it is only tested with Vcc down to 5V, which is why the minimumn recommended supply voltage is 5V.

    There isn't a designed in hysteresis.   But, if your input differential is inside the Vos limits, you are in a region where the output state would be unknown and it is possible that the output could oscillate.   If the input differential is greater than the Vos limits, the output should be stable.

    Kirby

  • 0 in reply to Kirby Kruckmeyer96848
    Prodigy 100 points

    Kirby,

    Thanks for the response.  I have tested and verified that our circuit starts oscillating at cold (@-10) when the LM139 is powered at 3.3V, but it works fine when powered at 5V.

    The circuit is just a power on reset timer where the delayed rail is compared to 2V, so it would be 3.3V vs 2V at steady state.  It looks like we are violating the Vicr range on both inputs as both voltages are above (Vcc-1.5) when VCC is 3.3V.  When VCC is 5V and at temperature, one of our inputs violates the (Vcc-2) Vicr range and the other one does not.  Does this explain what I am seeing?  Is it ok to run with the IN+ above the Vicr and the IN- within the Vicr, as it gives us the desired high impedance output?


    Thanks,


    Todd

  • 0 in reply to Todd Reed
    TI__Genius 17035 points

    Hello Todd,

    Violation of the common mode range likely explains what you are seeing.   It doesn't matter what the absolute values of the inputs witthin the abs max limits.   What is important is where your common mode is (the halfway point between the two input voltages).   That is why you see an issue with Vcc and one input at 3.3V (common mode = 2.65V) where you are 0.65V from the top rail while you don't at 5V (common mode = 3.5V) where you are at 1.5V from the top rail.