Sean,

   I found that LM 1117 works well for me. I had no
problems so far on my side with the regulator. The
list did mention some other regulator but I can't
remember the name. Try to check the archive for LM
1117 and you would see some pretty interesting
suggestion.

John

--- Sean Breheny <shb7@cornell.edu> wrote:

> Thanks to everyone who replied to this. I'm still
> playing with the
> circuit. The problem I'm having is this:
> 
> I'm trying to design a linear bench power supply.
> Given that the bench
> supply can be connected to any circuit (which may
> have an arbitrarily
> high capacitve load), I need it to be stable. In
> addition, I'd like it
> to settle within 100 microseconds after a
> disturbance (either line or
> load).
> 
> It seems that it is not possible to achieve
> stability with very large
> capacitive loads AND guarantee fast settling times,
> even when the
> capacitve load is less. (This is the way it seems to
> me after playing
> with it, I have no proof of this yet). This is
> because you essentially
> have to either design for a particular capacitve
> load (where you get
> fast response but there is then some max capacitance
> you can tolerate
> and still be stable) or you have to let the output
> capacitance
> dominate the response (dominant pole compensation),
> in which case you
> are essentially designing for the worst case
> (highest) output
> capacitance and response will be slow.
> 
> I guess this isn't too surprising a result, but I
> was just wondering
> why I'd never seen this discussed before. I've seen
> linear regulators
> with a MINIMUM output capacitance spec, but never a
> maximum output
> capacitance spec.
> 
> Sean
> 
> 
> On 9/11/06, Vasile Surducan <piclist9@gmail.com>
> wrote:
> > 1000uF on a low output impedance emiter circuit ?
> > The output capacitor should be low, 10uF up to
> 100uF..
> > You must have resistors on both OA inputs.
> >
> > Vasile
> >
> > On 9/11/06, Sean Breheny <shb7@cornell.edu> wrote:
> > > Hi all,
> > >
> > > As you all know, linear regulators essentially
> consisting of an
> > > op-amp, voltage reference, and pass transistor
> are very common. One
> > > drawback is that they cannot sink current, only
> source it. The first
> > > time I tried to design such a regulator, I had
> an awful time trying to
> > > get it to stop oscillating. I eventually
> discovered that this was
> > > because of the nonlinear behavior of the pass
> transistor at low
> > > currents (essentially because if the capacitance
> on the output got
> > > charged to even slightly too high a voltage and
> there was no load on
> > > the output, the op-amp's output would swing all
> the way to the
> > > negative rail untl the cap drifted down a bit in
> voltage).
> > >
> > > I've attached a small PDF of a circuit to
> illustrate this. An LTSpice
> > > simulation of this circuit shows it to be
> unstable when the current
> > > being drawn from the output jumps from 10mA to
> 1A.
> > >
> > > One way to mitigate this is to put a resistor in
> the feedback path and
> > > a capacitor directly from the op-amp output to
> its inverting input,
> > > thus slowing down the response. However, no
> matter how large I make
> > > this compensation capacitor, large enough output
> capacitor values will
> > > eventually cause instability. The same is true
> if I put a resistor on
> > > the output to draw a minimum load (to reduce the
> nonlinearity of the
> > > pass transistor at low currents).
> > >
> > > I've often heard of regulators being unstable
> with too little
> > > capacitance on the output, but I've never heard
> anyone complain of
> > > what happens when you have too much capacitance
> on the output.
> > >
> > > Is there something I'm missing here?
> > >
> > > Thanks,
> > >
> > > Sean
> > >
> > >
> > > --
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> > >
> > >
> > >
> > >
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