[SI-LIST] Re: Linear Regulator Problems

----- Original Message ----- 
From: "Landrum, Chris" <chris_landrum@xxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Thursday, September 23, 2004 4:49 PM
Subject: [SI-LIST] Linear Regulator Problems


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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