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LM7705 Shutdown

Brian Tate
Intellectual 355 points
Other Parts Discussed in Thread: LM7705

I have a customer that has been using the LM7705 for some time with the SD pin floating and is now seeing the following issue:

In this latest run of field modules we are experiencing intermittent failures of our LM7705s.  They appear to regulate (-0.200V) at power up for several seconds, then they stop regulating.  If I put my finger on the part it starts back up, then stops several seconds after removing it.  We have not seen this behavior on the other 100 modules we’ve built and tested across temperature for the past 6 months.  The only thing that stands out, is we have the shutdown pin floating and maybe the latest silicon is behaving a bit differently.  Is a pulldown required on this pin, or is it valid to have it floating?

Is leaving this pin floating acceptable?

  • 0
    Prodigy 5 points

    The shutdown pin input current Ic is 50pA, according to the datasheet page 3 and 4.
    This means it does not have an active pulldown or pullup.
    If it did the input current would show several mA positive for active current sink, or mA negative for active pullup.

    The EVM shows the pin either pulled high directly or to ground (for operation).

    See if the problem goes away by grounding the Shutdown pin 3.
    The pin needs to be grounded for reliable operation.