[SI-LIST] Re: resend: s2ibis2 extreme currents

Don't know if this is necessarily PC, but in order to get rid of the warnings 
in the past I have replicated the spice diode models locally and added some 
(0.3 - 0.5 Ohms) series resistance to the model - whatever value gives you an 
"expected" value of current at a slightly greater value of Vt above/below the 
rail.  The problem is that in most digital CMOS processes the forward biased 
diode characteristics are not modeled realistically (or at all) because in 
digital circuits this isn't an important characteristic.  The "knee" or turn-on 
characteristic is important, but beyond that it's not important to the digital 
library. Regards, 
--Rob Goodrich, Freescale Semiconductors, Inc.

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Ingraham, Andrew
Sent: Thursday, June 17, 2004 11:44 AM
To: steimle@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] resend: s2ibis2 extreme currents


(Sorry if this is a repeat ... my first reply doesn't seem to be getting to the 
list, so I'm sending it again.)

Your IBIS model faithfully duplicates what the SPICE model did under those 
circumstances.

It is probably just a warning (not an error), letting you know that the 
currents were on the high side for real silicon, but it doesn't necessarily 
mean something is wrong.  That part of the SPICE-to-IBIS translation is 
stressing the silicon in a way that it may not normally be used.

If you were happy with the SPICE model before, then be happy with the IBIS 
model too.

On the other hand, this experience has probably taught you that your old trusty 
SPICE model was probably somewhat limited in its handling of extreme 
overshoots.  Most of the time this isn't an issue for anybody, because the 
model (even though its clamp currents may have been impractically large) did 
direct the simulation to do the right thing (i.e., clamp the voltage), and the 
waveform might be essentially the same whether the clamping current was 0.7A or 
70A.  IBIS won't care (in spite of its warning).  However, in some cases the 
time-domain results might not correlate well with real silicon, and then you 
would want to fix the SPICE model.

The justification for fixing it is NOT because the IBIS model causes those 
warnings.  It's because the SPICE model wasn't accurate in the first place.

Regards,
Andy



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