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LM139: Comparator output seems to be oscillating at lower temperatures

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Replies: 3

Views: 185

Part Number: LM139

Hi Team,

I would like to ask some help if you can contact the customer offline with his inquiry below:

Our customer is using LM139 as a cutoff switch.

The output of the comparator output seems to be oscillating at lower temperatures.
The circuit is supposed to turn off the MOSFET if the power supply droops due to a high load.
They are using a unregulated 15V to power the comparator as well as the positive comparator input: is not a regulated load;
When the 15 droops, it's supposed to shut off the MOSFET.
There is a volt between the comparator inputs, with the positive input very very close to the positive rail. The customer thinks that a volt should be plenty of margin.
Their simulations do not show any oscillation on the output.

Additional information:
The reference is a regulated output. So is the +28
All four outputs are oscillating. Also this IC is used for two identical circuits (Two amps are spares).


I can't share here the customer's schematic due to propriety reasons. I will give the customer's information offline.

Thank you very much.

Jonathan.

  • Hello Johnathan,

    How "close" to the positive rail? The upper input range specification is Vcc-2V over temperature. The upper common mode range shrinks at cold.

    Please see section 2 (particularly section 2.4) of the following Appnote:

     Application Design Guidelines for LM339/LM393/TL331 Family Comparators

    One common problem with simple overload circuits is that as soon as the load is disconnected, the circuit then turns back on - then overloads again...then turns off...then turns back on again...then overloads again....you get the idea... Hysteresis or a latch are needed to prevent this, particularly if there are peak start-up currents.

    Please send the schematic to  for review.

    Regards,

    Paul Grohe

    TI Comparators (CMPS) Applications Group

  • In reply to Paul Grohe:

    Hi Mr. Paul,


    Thank you very much for your immediate response and for your help.
    I will send you an email and I will include our customer so he can add more information if necessary.

    Regards,

    Jonathan

  • In reply to Jonathan Geronga:

    Jonathan

    I am closing this thread since the discussion is being taken offline.

    Thanks

    Chuck

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