[SI-LIST] Re: Voltage droop analysis_ load current model
- From: Larry Smith <ldsmith@xxxxxxxxxxxxxxxxxx>
- To: Aubrey_Sparkman@xxxxxxxx
- Date: Tue, 7 May 2002 15:39:45 -0700 (PDT)
Aubrey - Yes, thanks for catching this. Zero current is a result
of a very high (infinite) resistance. The first example below
should have had infinite resistance instead of zero resistance.
regards,
Larry
------------- Begin Forwarded Message -------------
From: Aubrey_Sparkman@xxxxxxxx
To: ldsmith@xxxxxxxxxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Voltage droop analysis_ load current model
Date: Tue, 7 May 2002 13:59:41 -0500
MIME-Version: 1.0
Larry,
I'll bet you meant to change your resistor (for 0 to 1 amp) between 1 ohm
and infinite resistance instead of between 1 ohm and 0 ohms... :-)
Aubrey Sparkman
Signal Integrity
Aubrey_Sparkman@xxxxxxxx
(512) 723-3592
> -----Original Message-----
> From: Larry Smith [mailto:ldsmith@xxxxxxxxxxxxxxxxxx]
> Sent: Tuesday, May 07, 2002 10:55 AM
> To: si-list@xxxxxxxxxxxxx; sghsu55@xxxxxxxxxxxx
> Subject: [SI-LIST] Re: Voltage droop analysis_ load current model
>
>
>
> 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
>
> > Delivered-To: si-list@xxxxxxxxxxxxx
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> > Date: Mon, 06 May 2002 04:06:01 -0000
> > From: "sogo_hsu" <sghsu55@xxxxxxxxxxxx>
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Voltage droop analysis_ load current model
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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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