[SI-LIST] Re: Linear Regulator Problems
- From: steve weir <weirsp@xxxxxxxxxx>
- To: chris_landrum@xxxxxxxxxx
- Date: Thu, 23 Sep 2004 14:41:52 -0700
Chris, some more detailed answers-
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.
>
Not just any value will do. Also, the operating point of your transistor
matters. Have you put this through SPICE?
>
>
>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.
If you have models for the transistors, and know your operating currents,
this is straightforward in SPICE. You can also do it the old fashioned way
with graph paper if you account for the operating point(s) of your transistor.
>
>
>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?
Yes, a bigger capacitor does slow down the response, but it lowers the
amplitude as well. A problem can occur when the compensated amplifier
response which has inductive characteristics, crosses the capacitor
response. This is the evil anti-resonance. A really big, low ESR
capacitor and an amplifier with a low frequency audio pole can result in
nasty behavior down in the low KHz.
I assume that you are rolling your own regulator due to cost
considerations. Since you will be mass producing, you really must push
this through SPICE and do a Monte-Carlo analysis for your four corners, or
expect to see unwelcome surprises. In a plug for some of those ICs that
you probably don't want to spend money on, the people at LTC, Maxim, etc,
have solved the problems that you are dealing with. LTC has a very nice
low dropout linear controller in an SO8. You supply the external
FET. These were developed for PII systems and have closed-loop response
out to 1MHz or better.
>
>
>Thanks.
>
>Chris
>
>
>
>
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