Re: [SI-LIST] : ESR and Q factor

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Fri Sep 22 2000 - 10:53:34 PDT


Well, a linear regulator is nothing else but DC amplifier, using a
internal reference as an amplified signal. In the heart of it is OpAmp
with some correction circuit in its neg. feedback.
If you do not calculate it well, then a load capacitor, introducing a
phase shift, may result in oscillation.

This is especially bad with low-dropout regs, since the output impedance
of an amplifier is inversely proportional to the neg. feedback strength,
and the stronger feedback usually without correction results in
oscillation; the worst case is unity gain feedback (follower).

If a output uses emitter follower, then it is still worse, since emitter
follower itself is prone to oscillation around unity gain frequency.

Anyway, the higher ESR will dampen this effect, since it introduces
lower phase shift immediately at the amplifier output.
And one more:

BEWARE of LINEAR Tech parts ALWAYS!
I experienced problems with them, and then I use their second sources
(BB, MAX) and those problems would dissappear by itself with no actions
on my side. Example - a semiconductor filter by LT, that would start
oscillating for no reason, when connected exactly according to their app
notes.
Or their slow-start circuit for DC-DC converters, that had few problems.

Chris Cheng wrote:

> dear friends,
> this is a strange one. i am using a linear regulator to
> generate 3.3V on one of my board. the app note calls out using
> a 10uf tantalum for output stability. being a simple minded
> person, i decided to use a 10uf ceramic cap instead, thinking
> it has a better esl and esr value. to my surprise, the
> output breaks into huge oscillation. when i switch back to
> using tantalum caps, the output quiet down instantly. the
> same can be achieve if i use a >1 ohm resistor in series
> with the ceramic cap.
> the question is, has anyone experienced similar sensitivity
> in dc linear regulators (the part is lineartech lt1086)? why
> do linear regulator has such sensitivity to q factors ?
> thanks in advanced.
> chris
>
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