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# Modeling Large CMOS Circuits

IP.com Disclosure Number: IPCOM000120616D
Original Publication Date: 1991-May-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 1 page(s) / 46K

IBM

Raver, N: AUTHOR

## Abstract

Large aggregates of CMOS circuits will cause noise when operating because of the current surges acting on the power ground inductance loops. Instead of modeling the individual circuits, a method has been found which permits the large number of circuits to be represented by a number of simple equations. Extensive simulations show that the resultant noise is the same with both approaches. The equations, called the synthetic model, are summarized below:

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Modeling Large CMOS Circuits

Large aggregates of CMOS circuits will cause noise when
operating because of the current surges acting on the power ground
inductance loops.  Instead of modeling the individual circuits, a
method has been found which permits the large number of circuits to
be represented by a number of simple equations.  Extensive
simulations show that the resultant noise is the same with both
approaches.  The equations, called the synthetic model, are
summarized below:

The device current is linearly dependent on local voltage Vl:
Il = Io(Vl-Vo)                     (1)

The constant Io is linear with device size.  The constant Vo is
approximately 0.5 to 1.5 volts, depending on the technology used.
The actual current drawn by the circuits in a drop-point area is a
high- frequency sequence of overlapping pulses.  However, because of
the decoupling capacitance, these pulses are smoothed out.
Simulations have shown that substituting an average smooth current
instead of the overlapping pulses gives the same noise.

The total current flowing locally must charge a
pseudo-capacitor Cx to VDD .  Alternatively, this is equivalent to
accumulating a pseudo-charge Qx = VDDCx . Thus, the duration of the
local current transient can be formulated as:

(Image Omitted)

If the local voltage is time dependent, then the accumulated
charge Qx = ?idt...