[SI-LIST] Re: Linear Regulator Problems
- From: "Landrum, Chris" <chris_landrum@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 24 Sep 2004 09:10:41 -0400
Steve,
No need for a picture. I have dead-bugged a transistor on a board and
then "wired" it in. Input is a 12 AWG wire about 0.5in long (2) 10uF
caps right at the pin. Output is much better with the pin being
directly soldered to the PCB. The output cap is very close to the pin.
I would immediately suspect the layout, however, if it were at fault, I
would think I would see inductive voltage spikes on the input or
something. I don't see any misbehaving other than the slow response
time. Also, this very same regulator has been attempted before (i.e. a
proper layout was completed) with the same, poor results. However, this
ckt works perfectly on the vendor's eval system. I am in the process of
finishing a layout to implement the ckt, however I was hoping to have
concrete evidence that it will work before I build boards. I realize
this is difficult without a proper layout, but it would help give me
that warm and fuzzy feeling inside.
-----Original Message-----
From: steve weir [mailto:weirsp@xxxxxxxxxx]=20
Sent: Friday, September 24, 2004 8:59 AM
To: Landrum, Chris
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Linear Regulator Problems
Chris, if you cannot get models, then you can set-up some
characterization=20
to obtain the response. I think one of the links posted a bit earlier=20
pointed to an article that has examples for the case of the once
ubiquitous=20
LM317.
When you say that the layout has changed for the worse how bad are we=20
talking about? Can you send me a picture of just that section without=20
getting into trouble?
Regards,
Steve.
At 08:51 AM 9/24/2004 -0400, Landrum, Chris wrote:
>Steve,
>
>Thanks for the suggestions. I most likely can get models of the
>transistor, but I assume I will need a model of the internal error amp,
>etc.. I would love to run this through Spice, however, I do not have a
>model for the "feedback control". As you have guessed, my regulator is
>home grown, however it is within a custom ASIC designed by another
>vendor. I will attempt to get models for it, but it is probably not
>likely I will be able to get one. =3D20
>
>Thanks.
>
>Chris Landrum
>PCBA Design Engineer
>Maxtor Corporation
>333 South Street
>Shrewsbury, MA 01545
>508-770-2282 (d)
>
>-----Original Message-----
>From: steve weir [mailto:weirsp@xxxxxxxxxx]=3D20
>Sent: Thursday, September 23, 2004 5:42 PM
>To: Landrum, Chris
>Cc: si-list@xxxxxxxxxxxxx
>Subject: Re: [SI-LIST] Linear Regulator Problems
>
>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=3D20
>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 =3D3D 2V and Ic of =
1A.
=3D
>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,=3D20
>this is straightforward in SPICE. You can also do it the old fashioned
>way=3D20
>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=3D20
>amplitude as well. A problem can occur when the compensated
amplifier=3D20
>response which has inductive characteristics, crosses the =
capacitor=3D20
>response. This is the evil anti-resonance. A really big, low ESR=3D20
>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=3D20
>considerations. Since you will be mass producing, you really must
push=3D20
>this through SPICE and do a Monte-Carlo analysis for your four corners,
>or=3D20
>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,=3D20
>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=3D20
>FET. These were developed for PII systems and have closed-loop
response
>
>out to 1MHz or better.
>
> >
> >
> >Thanks.
> >
> >Chris
> >
> >
> >
> >
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