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

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From: Robert Weber (weber@snet.net)
Date: Thu Sep 21 2000 - 19:18:18 PDT


Chris,

You can add high speed decoupling caps (100nf MLC) to filter out the high
frequency components, and you can even leave in a 1uf or 2uf MLC cap, as
long as you have a large cap with enough ESR to kill the Q of the filter
network. This job is easily done with a standard 10uf tantalum cap, although
a large value MLC with a small series resistance will do the same thing.

Bob W.

-----Original Message-----
From: Chris Cheng <chris.cheng@3pardata.com>
To: si-list@silab.eng.sun.com <si-list@silab.eng.sun.com>
Date: Thursday, September 21, 2000 8:02 PM
Subject: RE: [SI-LIST] : ESR and Q factor

>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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