[SI-LIST] Decoupling Strategy??
- From: Simba Julian <sjulian@xxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 15 Dec 2006 10:23:23 -0500
Greetings to All,=20
With respect to formulating a decoupling strategy. Assuming that you
have access to frequency domain board analysis simulation tools such as
SiWave, PowerSI or similar SPICE tools, it should be easy to look at the
impedance profile as seen by ICs on your design based on plane shape,
stackup, decoupling capacitor placement, power etch connecting ICs to
planes and Decoupling caps to planes. =20
So I've heard it said that you take the max dV that you can allow for
ripple on your power rails and divide it my the dI mimicking the
switching transients of your IC (i.e. processor switching from low
power state to fully active). While the dI is not always known to the
SI engineer using off the shelf parts, I've seen a presentation that
suggests a method of taking the Max current (typically known) and
dividing it in half and assume that as the dI, implying that my IC uses
half the power/current in a low power state.... I can buy that. With
this new dV and dI I can come up with an impedance target.
Here's the question: =20
How do you determine what max frequency is this max impedance target a
requirement? It's a trivial exercise to verify through simulation the
effects of adding more caps, and staying under this impedance target for
low/mid frequencies, which may be good enough....or.... maybe I need to
add a few more caps to keep my impedance under the target at slightly
higher frequencies..... or worst yet... maybe I need to consider
changing my stackup to have better use of buried capacitance between the
planes....or .... thin core dielectrics.... or other exotic
materials..... or..
Point is, when making this analysis based on impedance target, you
eventually move beyond the right side of the resonance curve and will
probably violate your impedance target at some frequency. I'm quite
sure it has something to do with the dT of the dI that we have come up
with a rule of thumb for but I'm interested in knowing how do you
determine this target from datasheet specs or any other practical
means(Unless I have this all wrong :)).
Simba
EMC Corporation
------------------------------------------------------------------
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:
