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

Mon, 17 May 1999 19:34:19 +0100

Would it be beneficial to treat the power plane as a transmission line with impedance Zo? If it is of a sufficiently large area, this can be a valid approach.
Best wishes

"Bermensolo, Todd L" wrote:

> 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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