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Wanna see what happens when you don't read the data sheet?

Discussion in 'Electronic Design' started by Christopher Ott, Feb 28, 2007.

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  1. Making a long story short, the LM2940 does not like running with a 1uf
    output cap. It needs at least 22uf. This was a bugger to track down (I
    thought it was related to the new switcher circuit I had designed, and
    incorrectly focused my efforts there.)

    http://www.ottelectronics.com/photos/LM2940.JPG

    Yellow trace is input voltage (from the switcher), blue trace is output. The
    oscillation was making my A/D's go haywire. It seems such a simple thing to
    miss.

    Originally I was P.O.'d because this LDO should be able to handle 0.5v
    dropout, and the scope clearly shows that I was above that.

    My lesson for the week: Read the "f-ing" data sheet!
     
  2. (snip)

    Based on the capacitor ESR stability graph on page 10 of:
    http://cache.national.com/ds/LM/LM2940.pdf
    I would use a low ESR 22 uF capacitor in series with a .33
    or .47 ohm resistor to make sure. If you can generate a
    pulse load for your regulator, you can easily see the
    improvement in stability the series resistor gives you.
     
  3. MassiveProng

    MassiveProng Guest

    The word for today is "Overshoot".

    Don't you feel critically damped now? :-]
     

  4. I removed the soft start cap and, volla! The overshoot is gone.

    The switcher is an LM2672. They've been pretty forgiving so far...

    Chris
     
  5. Hawker

    Hawker Guest

    Forgive my stupidity, but why should I bother with a low esr cap if I am
    adding .33ohm series resistance? The difference in ESR between say low
    ESR tant and say a generic electrolytic will be minimal compared to the
    ..33ohm resistor you mention.
    For that matter a real cheap cap, say 22uF at 16V is going to have an
    ESR around around that point anyway. A Nichicon PW series (my cheap
    client loves these for cost) is 22uF 16V = 0.60 ohms @100k @ 20c

    Hawker
     
  6. The low ESR capacitor plus resistor guarantees a stable RC
    zero over the life and operating conditions of the system.
    Getting the zero from some poorly controlled and unstable
    resistance from a high ESR capacitor is a ticking time bomb.
    By the way, "low ESR" in this contest means a small (1/4
    to 1/10th, say), compared to the added series resistor, so
    that the fixed resistor dominates the total resistance, even
    if the ESR varies quite a bit.
    Does the client expect the circuit to work for several years
    and over a range of ambient temperature beyond normal room
    temperature? If so, proving that the cheap capacitor is
    stable enough to expect success may cost more than using
    more predictable parts.

    If this circuit is part of a greeting card or toy with a one
    month expected life time, then the cheap capacitor is
    definitely the way to go.

    What is important is to understand the problem and the
    stability and cost of its possible solutions. In this case,
    the solution not only includes the capacitor and series
    resistance at the regulator, but the additional parallel
    bypass capacitance distributed at the loads. The data sheet
    isn't really helpful about setting up this series parallel
    combination inside the stability box. But it has been my
    experience that additional parallel capacitance is tolerated
    better if you are in the higher resistance half of the graph
    shown on the data sheet. But a pulsed load test of the
    alternatives is still a good thing to do. The one that
    rings the least (in radians of oscillation) is most stable.
     
  7. Hawker

    Hawker Guest

    Gotcha. Do you have any data to show trends of ESR change over time and
    temp. In the case of the regulator in question it can use a fairly wide
    range so if you hit in in the middle I wonder if it is overkill to
    assume it could fall outside that range.
    Oh I am with you on that but you gotta understand clients.

    I do contract engineering and while I can advice I have to accept what
    my clients want in the end. Most clients are reasonable in there desires
    although some want me to over analyze things beyond nessasary and some
    the other way. Usually the other way is because they are bean counters,
    or non technical or because they can't "see" theory.

    In the case of this "cheap" client it has been a real struggle to try to
    get them to understand why just because I power it up on the bench and
    it appears to work does not mean the design is ready to ship.
    I can't tell you how many times they have built a product with parts
    other than I specified and even if I find it has issues with those parts
    (that I can prove on the bench) if they don't see it then it isn't real
    to them.

    Reality is these kinds of clients are not uncommon. They are not
    technical and run by bean counters. I get so sick of "well the
    capacitance and voltage are the same - who gives a [email protected]#$ about other specs".

    Hawker
     
  8. Guest

    (snip)

    It varies a lot from manufacturer to another, and from one product
    line to another. But you can rely that almost any change from new and
    room temperature ambient will increase the ESR. With most LDO
    regulators the problem is that all but the lowest cost capacitors are
    too low in ESR, so either you pick an unstable cheap one, or a little
    more costly, more stable one and add a 2 cent resistor to make it
    work, now and over the long term. If the load includes lots of little
    bypass capacitors in parallel to the one at the regulator output,
    having a lot more than the absolute minimum resistance in series with
    the main filter capacitor may be essential for initial stability.
    Then you don't have much margin for having the ESR of the capacitor go
    up as that capacitor ages or gets cold.

    I have given up on hitting a good zero in the response just by
    selecting a capacitor for LDO regulators. Fro the price of a
    resistor, I sleep better and get less aggravated.
     
  9. MassiveProng

    MassiveProng Guest

    I cannot. SO STOP TOP POSTING, DIPSHIT!

    This ain't your friggin e-mail! Learn about the forums you invade,
    and learn the word CONVENTION, you TOFU RETARD!
     
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