FreeLists / si-list / [SI-LIST] Re: Post Designcon thread

[Steve Weir <weirsi@xxxxxxxxxx>]

Jim, I had a blast at DesignCon, and want to thank everyone who attended my 
paper.

Larry and company's work on the subject of power delivery has led the way 
for many of us.  Introduction by SUN of the F^N method really got a lot of 
people thinking about this issue.

There are trade-offs to both methods, and Larry correctly pointed-out what 
is one of the greatest strengths of the F^N method that he advocates, 
against what may be considered the single greatest weakness of the big "V" 
( single value capacitor ) that I advocate.  I think it would be great to 
air-out what both of these methods do and do not do, and then the informed 
reader can use either method properly.

Basically, what is at issue is the anti-resonance that occurs between the 
apparent plane lumped capacitance and the discrete capacitor array in the 
common case of boards that are not huge with high Er power dielectric.  The 
advantage that Larry sees to the F^N networks is that is loads the bypass 
network with capacitors in the smallest values.  Capacitor ESR depends on 
the package size and the capacitance.  So, the upshot is that for a given 
closing frequency and impedance an F^N network has a higher resistance than 
the big "V".  Using Larry's 1mohm 100MHz 300pH  example with 500pH 0603 
capacitors I get an F^N network with roughly 150uohms versus about 60uohms 
for the big "V".  Consequently, plane skin effect aside, the AR Q with the 
planes is about 2.5X times higher than with the F^N method.  All thngs 
considered equal, ( such as ignoring plane and at these extremely low 
impedances even via skin effect ) this means the impedance peak at the AR 
with the F^N method will be only about 40% that of the big "V".

At this point, hopefully Larry will chime in if he feels I have gotten 
anything wrong.

My position is that both methods suffer large impedance departures due to 
the AR.  The salient questions are:  what is the impact, and what can we do 
about it?

Assuming that the AR is well above the IC package power delivery cut-off, 
the AR does not materially affect power quality to the IC die.  What it can 
affect is EMC from IC power, as even though the percentage power is way 
down, substantial currents can still flow above the package cut-off.  For 
signals running in the offset stripline sandwich, the potential impact is 
increased:  signal crosstalk and EMI.

How bad any of these problems are for either method is a matter of 
coefficients.  I consider the signal cross talk issue the more serious of 
the problems.  We can deal with that problem in two ways:  First, reference 
signals correctly so that we do not rely on the ancient CMOS power split at 
the package / PCB boundary, and Second as necessary eliminate large Vcc 
planes.  The many voltages commonly needed often already do this to/for us 
anyway.


  Regards,


Steve



At 06:26 AM 2/4/2005 -0700, Peterson, James F (FL51) wrote:
>  I too enjoyed DesignCon and running into some people from the SI-list. I
>actually was keeping an eye out for the infamous Chris Cheng, but never saw
>him.
>
>It was interesting to see Larry Smith and Steve Weir spar over Steve's one
>decoupling cap value only position. Too bad it had to end so quickly - it
>was just getting interesting.
>
>I think an SI-List DesignCon get-together is a fantastic idea. We should try
>to do this next year.
>
>Jim Peterson
>Honeywell
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
>Behalf Of Chris Cheng
>Sent: Thursday, February 03, 2005 8:03 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Post Designcon thread
>
>Sorry for wasting the bandwidth.
>But its sure nice to put faces on people I only 'meet' in Si-list. Glad to
>meet the new faces.
>And for the old ones I know, it sure nice to know your grey hair has grown
>just as much as mine. :-D.
>Next time we should have a Si-list get together lunch instead of walking
>around looking for strange faces ;-D. I'll bring the pie so those who feel
>like it can throw at me.
>I learned a few new things and refresh a few old ones.
>Cheers.
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