Re: [SI-LIST] : Decoupling caps and power plane effects

bgrossma (bgrossma@td2cad.intel.com)
Mon, 17 May 1999 16:47:15 -0700

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Todd,

It is true that we use a number of different tools for modeling our test
interface power distribution systems (by we, I include Arthur as we work
together) though HP is not the only source of our tools. We have used
everything from Spice to Excel to MDS, each with there own application
areas, and each with its' own level of success.

Please contact me offline, and I'll try to provide a condensed
description of what we are doing in ITTO.

Regards,

-Brett

Ron Miller wrote:
>
> > Hi Todd
>
> Within your own company, Intel, a small group is working on test
> interfaces.
> Arthur Frazier , a microwave engineer works in this group, and has MDS
>
> Microwave Development System(simiulator) from HP which has models for
> power/ground in a matrix form. This tool does what you need
> accurately.
>
> Ron Miller
>
>
>
> > To calculate the required number of decoupling capacitance for a
> > given
> > motherboard, I am trying to setup lumped RLC circuit to model the
> > behavior
> > of a realistic capacitor discharging into a power plane. The
> > modeling of
> > the realistic capacitor just involved RLC elements all in series.
> > The ESR
> > is determined from the vendors datasheet. The ESL from the
> > datasheet as
> > well as the loop inductance when placed on the PCB. To model the
> > presence
> > of the power plane is proving more involved.
> >
> > When a chip on a circuit board has its initial current draw from its
> > outputs
> > switching, the power plane is the first to respond with current.
> > This is
> > due to the low inductance of the power plane. Next the ceramic
> > capacitors
> > respond, followed by the higher ESL caps and then finally the power
> > supply.
> > The effect of the power plane responding to the IC's current draw is
> > the
> > topic which I would appreciate assistance on. At time t=0, an IC
> > chip's
> > outputs switch and its power pins will draw a current Io from the
> > power
> > plane of a motherboard. Since the power plane is essentially a
> > large
> > capacitor, its discharging current will decrease the voltage level
> > of the
> > plane until the ceramics respond to stabilize...then the
> > bulks...then the
> > power supply. The rate at which the power plane discharges is of
> > interest
> > to me. If the effective capacitance of the power plane seen by a
> > chip can
> > be gauged, then the discharge rate of the power plane supplying
> > current to
> > some load can be modeled with a lumped capacitave element. With the
> >
> > discharge of the power supply predictable, then decoupling
> > capacitance can
> > be calculated to prevent the power plane voltage from drooping below
> > a
> > specified voltage.
> >
> > Has anyone done any power/ground studies that would shed light on
> > this
> > problem? Are any of my assumptions invalid?
> >
> > Thanks all.
> >
> > Todd Bermensolo
> > Intel Corp
> > High End Server Division
> >
> > **** To unsubscribe from si-list: send e-mail to
> > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
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>
> --
> Ronald B. Miller _\\|//_ Signal Integrity Engineer
> (408)487-8017 (' 0-0 ') fax(408)487-8017
> ==========0000-(_)0000===========
> Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA 95131
> rmiller@brocade.com, rbmiller@sjm.infi.net
>
>
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