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

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From: Chris Cheng (chris.cheng@3pardata.com)
Date: Thu Sep 21 2000 - 16:48:39 PDT


larry,
here's the problem, i can certainly put series resistor in front
of the bulk caps. however, i cannot control the q on the high
speed decoupling caps which are needed for medium range decoupling.
lt1086 is such a common part its hard for me to imagine this is
not a problem people has seen before.
chris

-----Original Message-----
From: Larry Smith [mailto:Larry.Smith@Eng.Sun.COM]
Sent: Thursday, September 21, 2000 4:17 PM
To: si-list@silab.eng.sun.com; chris.cheng@3pardata.com
Subject: Re: [SI-LIST] : ESR and Q factor

Chris - at frequencies near the bandwidth of the regulator loop, the
open loop gain is usually going down at 20 dB per decade on a bode plot
(assuming a single pole roll off). Therefor, the output impedance of
the regulator (with no capacitor) is going up at 20 dB/decade. This
looks very much like an inductor.

If you put a capacitor in parallel with the (inductive) output of the
regulator, you have a nice LC tank circuit. If the capacitor has high
ESR (tantalum), there may be enough damping. If the capacitor has low
ESR (ceramic), watch out! It can ring like crazy. You may even turn
the regulator (which is really a DC amplifier) into an oscillator.

The stability of the regulation loop is usually a strong function of
the output capacitor. Too much low ESR capacitance on the output of a
regulation loop will alter the phase margin and gain margin such that
the loop becomes unstable (oscillates). Who ever is responsible for
the loop stability of your regulator could probably re-compensate the
loop so that it will tolerate low ESR capacitors on the output.
Otherwise, stick to the tantalums.

regards,
Larry Smith
Sun Microsystems

> From: Chris Cheng <chris.cheng@3pardata.com>
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : ESR and Q factor
> Date: Thu, 21 Sep 2000 15:08:10 -0700
> MIME-Version: 1.0
>
> 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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