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Spice simulation of closed loop response of a linear regulator

Discussion in 'Electronic Design' started by mook Johnson, Mar 4, 2006.

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  1. mook Johnson

    mook Johnson Guest

    I'm spinning my own 5V linear voltage linear regulator. I'm using a NPN
    pass transisitor (non-ldo configuration) and a opamp, and 2.5V reference.

    It is the textbook circuit with a voltage divider on the output (two 1K
    resistors) with the center going to the in- on the opamp and the 2.5V
    reference on in+ and of course the output drives the base of the NPN through
    a current limiting resistor. essentialy an emitter follower.

    What I'd like to do is simulate the loop response because with no
    compensation the output oscillates after a step load change. Also
    oscillates with heavy load.

    How do I simulate the gain and phase in spice so I can check the phase
    margin at various loads and compensate properly.

    thanks.
     
  2. no_one

    no_one Guest

    you probably need to look to the op amp vendor data first and make sure that
    you have the op amp properly compensated per their recommendations.
    otherwise go to the same source to get a best guess at the phase-gain
    characteristics of the amp and plug that into your simulation.
     
  3. Guest

    Guest Guest

    : I'm spinning my own 5V linear voltage linear regulator. I'm using a NPN
    : pass transisitor (non-ldo configuration) and a opamp, and 2.5V reference.

    : It is the textbook circuit with a voltage divider on the output (two 1K
    : resistors) with the center going to the in- on the opamp and the 2.5V
    : reference on in+ and of course the output drives the base of the NPN through
    : a current limiting resistor. essentialy an emitter follower.

    : What I'd like to do is simulate the loop response because with no
    : compensation the output oscillates after a step load change. Also
    : oscillates with heavy load.

    : How do I simulate the gain and phase in spice so I can check the phase
    : margin at various loads and compensate properly.

    : thanks.

    You want to simulate the loop gain, correct?

    There are at least 2 different ways:

    1. Do a google search of Middlebrook (or is it Middlebrock?)
    stability analysis.

    2. Consult your SPiCE documentation about how to use AC resistors
    to break the loop at AC. Basically, you allow the circuit to bias at DC,
    but you break the loop and run it open-loop at AC.

    Joe
     
  4. Bob Monsen

    Bob Monsen Guest

    If you are using LTSpice, there are 'loop gain' circuit elements that you
    can use for this. Check out the yahoo ltspice group.

    http://groups.yahoo.com/group/LTspice/

    Jim Thompson has a version on his website that works with pspice:

    http://www.analog-innovations.com

    However, you can also probably just break the loop using an AC source at
    the V- input, because the impedance at the opamp is high enough.

    Actually, though, folks were talking about this on another thread. A
    longish run from the smoothing caps, added to a very low impedance
    through the opamp, can cause the pass transistor to oscillate. Use a 100
    ohm resistor between the output of the opamp and the NPN base, and make
    sure there is a 0.33uF cap between the collector lead and ground of the
    NPN transistor to shunt any oscillations to ground. I suspect this is the
    reason that 7805s and their brothers need decoupling caps if the leads
    from the power supply are too long; it is to counteract the inductance in
    the supply run.

    --
    Regards,
    Bob Monsen

    The most beautiful thing we can experience is the mysterious. It is
    the source of all true art and all science. He to whom this emotion is
    a stranger, who can no longer pause to wonder and stand rapt in awe,
    is as good as dead: his eyes are closed.
    Albert Einstein (1879 - 1955)
     
  5. Jim Thompson

    Jim Thompson Guest

    [snip]

    Or use the LoopGain part on my website... my adaptation of
    Middlebrook's method to simulation.

    ...Jim Thompson
     
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