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LM311: LM311 behavior in failure

Keith Keller
Expert 7260 points
Part Number: LM311


Hello,

We are wondering about the LM311:  can you provide general high level info on the worst case output voltage of the comparator.  Is there a failure mode in the comparator (some internal FETs/BJT failure) which can result in +15V or -15V on the comparator output (assuming +/-15V is used for supply.

Thank you, Keith

  • 0
    TI__Guru 61156 points

    Hi Keith,

    The output is an open collector output. It can only sink current. So the maximum output voltage would be determined by the voltage applied to the top of the pull-up resistor (assume +15V in this case) and any load resistance (that creates a voltage divider). The comparator itself has no control of the "high" level. The output is not (normally) capable of setting a voltage somwhere between the rails.

    For low - you would need to be specific about the type of failure.

    The output transistor can either:

    1. Melt - creating a short to GND,
    2. Be damaged and go resisitive - increasing the saturation voltage, which can vary the "low" level
    3. Blow open - which means the output will be pulled-high continuously by the pullup resistor (as mentioned above)

    There are about 25 transistors internal to the 311, and a failure of each one will have differing effects. We cannot say which way the output will fail unless we know which device has failed. There is no "default" (failsafe) failure mode.

    Hope that helps?