[SI-LIST] Re: VRM
- From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
- To: mohaghtalab@xxxxxxxxx
- Date: Mon, 17 Dec 2007 17:39:07 -0500
Mohamad,
Unfortunately VRM models are not easy to get. You usually can get
models for controllers of VRMs, but this helps you only if you build
your own VRM or know the entire schematics of the VRM you use. If you
use or plan on using off-the-shelf DC-DC converters, you may very well
need to do the characterization yourself and from the characterization
data, create your own model. You can do the characterization in the
time domain with an electronic test load that you can modulate, or do
small-signal characterization in the frequency domain with a suitable
Vector Network Analyzer setup. In either way you need to remember that
the converter has an active loop in it, where your external output
capacitors are within the feedback loop. This means that your best
chance to get the characterization correct is to use approximately the
same capacitors that you intend to use with the converter in your
application. For this reason sometimes you need to repeat the
characterization with different sets of output capacitors.
You can get some ideas about the small-signal frequency domain
characterization from
"Frequency Domain Power Distribution Measurements - An Overview" posted
on http://home.att.net/~istvan.novak/papers.html. You may also find
paper 11-WP1 from DesignCon 2007 interesting: "Characterization and
Output Filter Optimization Methods for Voltage Regulator Modules". It
describes a time-domain approach.
Once you have the characterization data, you will have to fit your
selected model onto the data. In the simplest for, a single inductor or
series inductor-resistor may. Dr. Howard Johnson described a simple
three-element model in his "Voltage Regulator Droop" article, EDN,
September 14, 2006. These simple models have limited bandwidth and they
usually severely overestimate the peaking between the converter and
external capacitors. If you want more accurate, but still easy-to-use
linearized small-signal models, you can follow the procedures in Section
9.2 of the book "Frequency-Domain Characterization of Power Distribution
Networks". All of these models would run equally in transient or AC
simulations.
BTW: thanks everyone giving feedback on the above book; now in addition
to the extended table of contents, an errata sheet is also posted at:
http://home.att.net/~istvan.novak/Novak-Miller_TOC_v4.pdf.
Regards,
Istvan Novak
SUN Microsystems
Mohamad Haghtalab wrote:
>
> Hi all
>
> for PI(power integrity) simulation I need VRM parameters of voltage
> regulators in my PCBs
> How can I extract them?
>
> Thanx for your help
>
> ---------------------------------
> Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it
> now.
>
>
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