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

Bermensolo, Todd L (todd.l.bermensolo@intel.com)
Mon, 17 May 1999 10:55:10 -0700

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

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