FreeLists / si-list / [SI-LIST] Re: Voltage droop analysis_ load current model

[Larry Smith <ldsmith@xxxxxxxxxxxxxxxxxx>]

Sogo - A good way to model a CMOS load is with a resistor.  The current
drawn from the power supply by a bunch of switching CMOS gates is
proportional to the power supply voltage.  A time varying resistor
represents this load accurately.

Please note that transient current is not the only important factor.  A
system load that changes between 0 and 1 amps is a lot different than a
system load that changes between 10 and 11 amps.  Both have a 1 amp
transient current but one system has far more damping than the other.

Suppose the power supply voltage is 1 volt.  A resistor that changes
from 0 Ohms to 1 Ohm as a function of time will represent the first
load.  A resistor that changes from 0.100 to 0.091 Ohms would represent
the second load.  The second system has a lot more damping provided by
the load than the first system.  The first system has a small amount of
damping from the load on the 0 to 1 transition but zero damping when we
make the transition from 1 to zero amps.  If the simulation has ideal
inductors and capacitors, they may ring for a very long time.

A current source is not as good for modeling CMOS loads.  Current
sources are high impedance and will draw the same amount of current,
independent of terminal voltages.  This is not the way that CMOS
circuits operate and will be a source of error for your simulation.
Current sources do not absorb energy or provide system damping the way
that a resistor and a real CMOS load do.

regards,
Larry Smith
Sun Microsystems 

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> 
> Hi all,
>   I would like to simulate voltage regulator module connect to a 
> power plane pair by equivalent circuit model. The equivalent model of 
> VRM and bulk capaciators were built up. But, how to model the current 
> load? Opt pulse function and estimate the maximun/minimun surge 
> currents or something else?
>   Thanxs in advance!
> 
> Sogo 
> 
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