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Original question:

LM317A: LM317A : v oltage on "Vout" PIN if "Vin" pin is not connected to input voltage

LM317A: Reverse bias on turn-off

David Curtis
Prodigy 10 points
Part Number: LM317A

I have a circuit in which the input goes to high impedance on turn-off.  I see a brief (10ms) excursion of the input-output voltage to -0.39V, exceeding the -0.3V datasheet spec.  I am trying to decide how concerned I should be about this.  From looking at similar questions it appears that I am biasing an internal parasitic path not meant to carry current with this reverse bias.  Is there a spec for how much current is an issue?  The datasheet talks about protection diodes, but the diodes indicated will not carry much current at 0.3V so it is not clear they would keep the reverse bias below 0.3V.

  • 0
    TI__Genius 12040 points

    Hello,

              This is a relatively old device and, at this time, we do not have the information on the I-V characteristics of the protection ESD diodes used internal to the device. If the magnitude and duration of the voltage excursion (beyond the Abs Max rating) is very small, the risk of damage to the device is probably not very high. External diodes (as shown in Figure 17 of the datasheet) may be used to help protect the device. However, we strongly recommend that the Abs Max rating not be violated to ensure long term reliability of the device. Thanks!

    Regards,

    Srikanth