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7805 regulator stable?

Discussion in 'Electronic Design' started by Yzordderrex, Aug 26, 2005.

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  1. Yzordderrex

    Yzordderrex Guest

    A co-worker just asked me what capacitor he should put on the output of
    the regulator in order to keep it from oscillating.

    I recall that many years ago the TI or National data books had some
    kind of curves suggesting the value had to be less than one value or
    greater than some other value in order to assure the thing didn't
    oscillate. Seems most data sheets I can find today are abbreviated and
    not so much info. The TI data sheet I downloaded just says all testing
    done with 0.33uf on input and 0.1uf on output.

    Can anyone shed some light on this concern?

    regards
    Bob
     
  2. I've never seen one oscillate with zero capacitance on the output,
    provided the input is reasonably bypassed. Some people claim they've
    seen it with some brands, but I'm very, very, very dubious. In fact
    some, if not most, data sheets actually say that no output cap is
    required for stability. Eg. http://cache.national.com/ds/LM/LM340.pdf

    "It is not necessary to bypass the output,
    although this does improve transient response."

    As they say, the reason to put a cap on the output has more to do with
    transient response-- so 0.1uF or 1uF or 0.01uF or whatever will work
    okay depending on what's connected to it.

    LDOs, OTOH, will merrily oscillate away if you don't get the output
    cap in the right range.


    Best regards,
    Spehro Pefhany
     
  3. John  Larkin

    John Larkin Guest


    And 7905's will definitely scream with no output cap.

    John
     
  4. Pooh Bear

    Pooh Bear Guest

    After having experienced occasional batch related issues with 78xx and 79xx
    regulator stability, I always ensure that there's decoupling close to the
    device on both input and output these days. A 10u electrolytic in parallel
    with 100n ceramic on the out and 100n close to the part on the in ( the
    bulk cap is rarely far away ) seems to work just fine. Never had any
    trouble since.

    Graham
     
  5. Ol' Duffer

    Ol' Duffer Guest

    I've seen them oscillate if the output cap is bigger than the input.
    Aside from that, they usually seem happy with anything within a
    couple orders of magnitude of 1uF.
     
  6. mc

    mc Guest

    My recollection is that it needs a capacitor on its input (which can be the
    big filter capacitor if necessary) but not on its output.
     
  7. Fred Bloggs

    Fred Bloggs Guest


    The output capacitor is not chosen on the basis of stability. It is
    chosen based on maximum percentage error in output voltage tolerable due
    to a no load to full load instantaneous load transition and vice versa.
     
  8. Ben Bradley

    Ben Bradley Guest

    Bob Pease's "Trobleshooting Analog Circuits" says something about
    this, but ISTR it's with the adjustable regulator, the
    LM3-something-something that I can't remember right now. He says use
    an electrolytic on the output, it's not made to go into the very low
    impedance of a tantalum.
    Regardless, you can do Pease' 'ping' test (maybe he doesn't call it
    that, but I think of it that way. Maybe I've been on the Net too
    long), couple a square wave through a cap and a 100 ohm or so (perhaps
    higher to reduce damping effects) resistor into the output, and look
    at it on a scope. If it has overshoot or especially if it shows
    ringing, then you know it's near oscillation and less stable than you
    want it to be. Reading the other responses suggests doing this with a
    few off the production line too, especially after changing production
    runs or brand names. Just add it to the things on your "If don't check
    these things out it could come back to bite my ass" list.
     
  9. Rich Grise

    Rich Grise Guest

    Yeah. I'd put a 0.33uf on the input and 0.1uf on the output, as close to
    the regulator as physically possible. ;-)

    Cheers!
    Rich
     
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