Subject: Re: [IBIS-Users] IBIS model for FET switch
From: Ross, Bob (bob_ross@mentorg.com)
Date: Fri Sep 06 2002 - 11:57:00 PDT
Moshiul:
Some responses are below in your text.
Bob Ross
Mentor Graphics
"Haque, Moshiul" wrote:
>
> IBIS Gurus,
> I am trying to model a switch with parallel NMOS and PMOS. I am kind of
> confused about the description of Series Mosfet keyword in IBIS Version 4
> specification. The description says that either of the FET's could be
> removed. Thus this model covers all four conditions , off, single NMOS,
> single PMOS and parallel NMOS/PMOS.
>
> 1) Does that mean that I have to model NMOS, PMOS and off condition
> separately?
No, do the I-V extraction on the total switch for a given Vgs.
Or only one Series Mosfet keyword with different VDS is
> sufficient to model the switch.
Yes.
> 2) How to model the off condition? I thought to model off condition series
> current keyword with off sub parameter is necessary.
Actually the [Off] keyword is necessary. Also, the Off subparameter
is normally listed in the [Series Switch Groups] table if a completely
off (High-Z or whatever) condition exists.
> 3) What would be appropriate keyword to model parallel FET switches, series
> current or series mosfet?
[Series MOSFET] since it captures some "across" impedance changes as a
function of actual input voltage at one terminal.
>
> I have tried to model the switch with series switch keyword. One problem
> with this type of model is the error generated due to decreasing current. In
> the IBISCHK3.2.9 version the error has been changed to warning. But lot of
> the simulator uses the older version of IBISCHK and my concern is that will
> this error prevent the simulator from simulating?
No. Most simulators will allow Warning messages, but will reject models
with Error messages. Some Warnings are informational to alert the user
of some internal values or characteristics in an otherwise technically
correct model. There might be philosophical arguments about allowing this,
but that is currently the reason some messages (such as large package
model values) exist. The non-monotonic I-V characteristic is technically
correct, but some EDA tools might be limited. So the Warning message
give some guidance to the user that this might be an issue.
>
> Appreciate any suggestions.
>
> Regards
> Moshiul Haque
> Standard Linear and Logic Marketing
> Texas Instruments Incorporated
> Ph: (903) 868-7153
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