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LM317 Error correction amplifier

Richard Meidl
Intellectual 370 points
Other Parts Discussed in Thread: LM317, LM317-N

Hello,

I have an old design that uses eight LM317T regulators to regulate DC voltages to eight separate audio channels.  This old design is from the early 1990's and the board layout was done with velum and tape.  We are now seeing the LM317's oscillate in some of the production units.  We have not seen this in the earlier builds.  What has changed in the LM317 in the past 15 years or so?  Has the error correction amplifier gotten faster?  Has the gain bandwidth of any of the internal components improves?  The oscillations that I am seeing are 10 to 20 MHz and because of the poor layout, the noise is easily coupled from one channel to another.  Thank you for your help.

Rich

  • 0
    TI__Expert 3920 points

    Hi Rich,

    As far as I am aware, there were no major changes made within this time period.

    That being said, it is possible that external factors may be the cause.

    Tim

  • 0 in reply to Tim Reyes
    Intellectual 370 points

    Hello Tim,

    What external factors would you look at that may cause an LM317 to oscillate?  Are there particular things that I should look for that may cause the LM317 to oscillate?  Do you think that something else has had to change or do you think that because my layout is poor, I may have been conditionally stable in the past?

    Thanks,

    Rich

  • 0 in reply to Richard Meidl
    TI__Expert 3920 points

    Some things I would take a look at are:

    - Has the BOM changed since before the oscillation was seen? For example, a new capacitor value, or different ESR cap might change the behavior of the overall board.

    - Can you test the LM317-N by itself, while removed from the main board? Are any abnormal oscillations seen that are similar to what you are seeing on the board? This will isolate the LM317-N and we can determine if the oscillations are being caused by the LM317-N. I think this may be the best test to see if the oscillations are really coming from the LM317-N.

    - If the board does in fact have a "poor layout" as you describe, it may be possible that noise is coming from another source.

    - It might be good to find the source of the oscillation. Is it seen at both the input and output of LM317-N? At the ADJ pin? Only one of these pins?

    - Check the dropout voltage from Vin to Vout. If it is very near the minimum value, typically, your PSRR will be low.