FreeLists / si-list / [SI-LIST] Re: Frequency criterion in power plane_Power integr ity

[Larry Smith <Larry.Smith@xxxxxxx>]

Chris - be careful with that one.  Buried capacitance for the core
power planes probably does not help SI but it may help EMI.

The loop inductance is very important for the core logic supply.  It
essentially determines the amount of capacitance that must be put on
the package or into the chip itself.

It turns out that the dominant inductance in this loop is often not in
the package but in the spreading inductance of the perforated power
planes on the PCB.  That inductance is proportional to the dielectric
thickness but not related to the dielectric constant.

PCB dielectric thinness is the key to spreading inductance.  Power
plane capacitance is inversely proportional to dielectric thickness and
power plane inductance is proportional to dielectric thickness. The 
inductance property is important, the capacitance property is of
less importance.

regards,
Larry Smith
Sun Microsystems

> Delivered-To: si-list@xxxxxxxxxxxxx
> From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Frequency criterion in power plane_Power integr ity
> Date: Thu, 9 May 2002 14:10:38 -0700 
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> 
> 
> 
> >I think that what some folks may be asking to themselves,
> >is, if in fact I am only needing to be responsible up to 
> >7 MHz, why would I ever need buried capacitance, and why 
> >is there such a fanatical focus on reduced loop inductance.
> 
> The answer is you don't need buried capacitance.
> And the loop inductance management is between the I/O
> signals and their reference plane. Nothing to do with
> core power distribution or even power doupling since
> you can't "decouple" the signal and its return path.
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