[SI-LIST] Re: Linear Regulator Problems
- From: steve weir <weirsp@xxxxxxxxxx>
- To: chris_landrum@xxxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 23 Sep 2004 14:29:17 -0700
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
>
>
>
>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List FAQ wiki page is located at:
> http://si-list.org/wiki/wiki.pl?Si-List_FAQ
>
>List technical documents are available at:
> http://www.si-list.org
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
