Subject: Re: [IBIS-Users] FW: Extreme currents????
From: Scott McMorrow (scott@teraspeed.com)
Date: Wed Jul 10 2002 - 16:22:51 PDT
Kim and all,
In many cases these are not ideal diodes. The IBIS spec suggests that
the input voltage for model creation be swept from -Vcc to 2Vcc. It
turns out that for many real devices of small geometry that the Spice
simulation accurately predicts extremely high currents. The real device
would have blown up in a lab, which I know that Tom Dagastino of Mentor
can attest to in creating models from measurement.
Spice to ibis does not allow the user to change the input voltage sweep
range. These excessive voltages, especially on devices with low core
voltages, will cause all sorts of problems in I/V curve extraction. The
least of which is excessive currents. I have had a wonderful time with
convergence issues under curve extraction, too. The solution I found
was to write a bit of script code to place a limiting amplifier in the
path of the voltage sweep for spice to ibis. (This was much easier than
going into the spice to ibis code, finding the many offending sections
and assumptions, and fixing it.)
Generally one can remove the offending high current table entries from
the IBIS file and perform the limiting yourself with no ill effect in
the actual active region of the device.
Besides, in IBIS simulators that use a universal "perfect ground" (which
doesn't exist in reality) reference, the reference for the clamp diode
(and output I/V curves) on the silicon is all wrong anyway. Packages
and boards are much more perverse than we like to imagine. And with
high enough clamping currents, the simultaneous switching input (SSI)
power/ground rail bounce can be quite significant.
My best advise to all, after a couple years of experience, is to never
count on device clamp diodes, unless you are willing to perform some
substantial Spice simulation runs, including power and ground rail
package modeling ... or are prepared for some interesting surprises in
the actual system.
Best regards,
Scott
-- Scott McMorrow Teraspeed Consulting Group LLC 2926 SE Yamhill St. Portland, OR 97214 (503) 239-5536 http://www.teraspeed.comKim Helliwell wrote:
> Frankly, I wish that suppliers of models (whether IBIS > or SPICE) would stop using these "ideal" diodes that > allow such large currents. I don't understand how anyone > can look at such results and think that's OK to ship to > customers! > > Maybe they just don't look, but that's hardly an acceptable > excuse, either. > > There, I feel better now! > > Kim > > > On Wednesday, July 10, 2002, at 03:19 PM, Lynne Green wrote: > >> Hello, Chris, >> >> Different EDA tools have different upper limits on currents that can >> be handled >> by the simulator. >> >> The warning is needed because "Spice" models that use the default series >> resistance (0.0 ohms) for diodes and junction leakage currents result >> in large >> IBIS table currents (often above 1 MegAmp). Most EDA tools don't go >> quite >> _that_ high. >> >> Best regards, >> Lynne >> >> >> >> -----Original Message----- >> From: John Angulo [mailto:jangulo@innoveda.com] >> Sent: Tuesday, July 09, 2002 6:02 PM >> To: ibis-users@eda.org >> Subject: [IBIS-Users] FW: Extreme currents???? >> >> >> The following is forwarded on behalf of Chris DeWitt at Motorola TCG. >> Please send all private replies to Chris.Dewitt@motorola.com. >> >> John Angulo >> IBIS Open Forum Postmaster >> ----------------------------------- >> >> If I run IBISCHK3 on many models I download I get an "Extreme currents >> present" warning. >> >> What are the limits on currents in the VI tables? I can't find it >> documented anywhere. >> >> Thanks >> >> Chris DeWitt >> Principal Staff Engineer >> Motorola TCG >> |------------------------------------------------------------------ >> >> |------------------------------------------------------------------ >> |For help or to subscribe/unsubscribe, email majordomo@eda.org >> |with just the appropriate command message(s) in the body: >> | >> | help >> | subscribe ibis <optional e-mail address, if different> >> | subscribe ibis-users <optional e-mail address, if different> >> | unsubscribe ibis <optional e-mail address, if different> >> | unsubscribe ibis-users <optional e-mail address, if different> >> | >> |or email a written request to ibis-request@eda.org. >> | >> |IBIS reflector archives exist under: >> | >> | http://www.eda.org/pub/ibis/email_archive/ Recent >> | http://www.eda.org/pub/ibis/users_archive/ Recent >> | http://www.eda.org/pub/ibis/email/ E-mail since 1993 >> >> > Kim Helliwell > Apple Computer > kimgh@apple.com > 408 974 9936 > > |------------------------------------------------------------------ > |For help or to subscribe/unsubscribe, email majordomo@eda.org > |with just the appropriate command message(s) in the body: > | > | help > | subscribe ibis <optional e-mail address, if different> > | subscribe ibis-users <optional e-mail address, if different> > | unsubscribe ibis <optional e-mail address, if different> > | unsubscribe ibis-users <optional e-mail address, if different> > | > |or email a written request to ibis-request@eda.org. > | > |IBIS reflector archives exist under: > | > | http://www.eda.org/pub/ibis/email_archive/ Recent > | http://www.eda.org/pub/ibis/users_archive/ Recent > | http://www.eda.org/pub/ibis/email/ E-mail since 1993 > >
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