From: sweir ([email protected])
Date: Thu Sep 21 2000 - 20:42:38 PDT
Chris,
The situation is pretty easy to deal with. If you look at the gain / phase
plot for the regulator with a 10 uF load, you can see where the gain
crosses the 0db point. You want to have at least 45 degrees of phase
margin at that point. Once, the gain is -20db or less, it is ok to let the
phase go pretty much where ever it wants. From the data sheet it looks
like this guy crosses zero at a couple of hundred kilohertz. With a 10uF
cap, .3 ohms will set a zero at 50 KHz which should leave plenty of phase
margin.
Regards,
Steve Weir.
At 04:48 PM 9/21/00 -0700, you wrote:
>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:[email protected]]
>Sent: Thursday, September 21, 2000 4:17 PM
>To: [email protected]; [email protected]
>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 <[email protected]>
> > To: [email protected]
> > 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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