[SI-LIST] Re: Linear Regulator Problems

Chris, the problem is most likely instability caused by INSUFFICIENT 
ESR.  Most older linear regulators require a certain amount of ESR for 
stability.  Those high quality ceramic caps make the regulator 
conditionally stable, or outright unstable, leading to the ringing.  You 
have a couple of choices:

1) Change your capacitor.
2) Add a bigger capacitor that has ESR, or a discrete resistor.
3) Insert a discrete resistor in series with your ceramic cap.
4) Change your regulator to a type that can tolerate the very low ESR of 
ceramic output filters.

Regards,


Steve.
At 10:49 AM 9/23/2004 -0400, Landrum, Chris wrote:
>Alright, I'll start with a new question.  Based on the recent emails, I
>have no idea whether or not this belongs in this forum, but I will ask
>anyway.
>
>
>I am working on a linear regulator that is exhibiting very poor
>transient response.  At first I assumed that the amount of output
>capacitance was to blame.  After dropping the amount of output
>capacitance, I found little change.  I have another board that uses the
>same circuit (same NPN-BJT, different decoupling, same load, better
>layout) which does not have the problem.  I attempted to mimic the good
>board's output decoupling, but the result was the same.
>
>
>
>I then noticed that the good board was using a tantalum output
>capacitor, while I am using Ceramic.  I am aware that tantalums do have
>a higher ESR than ceramics, so I attempted to use a tantalum.  The
>result was that the base started to oscillate (became un-stable).  I
>then tried adding a resistor in series with a ceramic cap to "simulate"
>a higher ESR value.  This actually seemed to help a little, but not
>enough to declare any type of success.  Since the base does not
>oscillate when using the ceramics, I assume that the regulator is
>stable.
>
>
>
>I began to suspect a problem with base drive current.  The transistor I
>am using is an NPN that has a Hfe of 100 at Vce = 2V and Ic of 1A.  My
>measured transient is less than 1A (~850mA).  The base driver is capable
>of sourcing 20mA.  I am using 3.3V to make 1.2V.  With my calculations,
>I don't see a problem with base drive vs. gain.
>
>
>
>Some questions:
>
>What else might be contributing to my slow response times?
>
>Does anyone have a suggestion for obtaining an accurate bode plot of a
>linear regulator freq response?  I would like to figure out a way to
>check phase margin.
>
>
>
>On the same topic... I have learned that a good method for estimating
>the variety of decoupling caps required in a PDS is to look at the
>problem in the freq domain.  Based on my max transient, I have
>calculated that I would need about 30uF of capacitance on the output.
>But this contradicts what is recommended on the output of a linear
>regulator doesn't it?  Most of the time, only values between 4.7uF and
>10uF are used.  Wouldn't 30uF on the output slow the response time?  In
>my circuit, I have noticed the circuit does respond faster with less
>capacitance, but the overall ripple is greater....   What is the best
>way to balance these conflicting requirements BEFORE I get in the lab?
>
>
>
>Thanks.
>
>Chris
>
>
>
>
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