Re: IBIS Series MOSFET Models - questions

Hi Mike,

I discussed this drawing with it's creator (Bob Ross) on the phone yesterday. It
appears that it was the nomenclature in the drawing that I was misunderstanding.
Here is another suggested improvement that I provided:

| +----------------------------------------+
| | |
| | Ids = Table Current |
| | ---> |
| +---<---| |--->----------+ |
| d |_____| - s | + |
| --+-- Vgs +---+---+ +----+----+
| | g + | Sweep | | Sweep |
| | Vs | | Vs+Vds |
| +---+---+ +----+----+
| | - |
| GND GND
|
| Example of Series MOSFET Table Extraction

The way that the power-supply on the drain was originally labeled, I had
misunderstood it to be a FIXED supply (like a Vdd). With the labeling I propose
above, I think it is more clear that this is a SWEPT supply that is always Vds
above Vs.

I agree with you that the gate supply should be illustrated also.

There is an example model (Created by Mentor Graphics) of a TI CBT3383 part
available at: http://www.eda.org/pub/ibis/samples/ver3.1/cbt.ibs

Now another question:

What table voltage do I use for Vgs, when the series switch is a parallel NMOS
and PMOS?

| gateP=GND
| |
| o
| ======= Vgs-p
| | |
| ----<---+ +--->----
| d |_____| s
| --+-- Vgs-n
| |
| gateN=Vcc

-- 
Regards,
Stephen M. Nolan
Mike LaBonte wrote:
> 
> Stephen,
> 
> The figure in the IBIS specification is electrically equivalent to yours,
> although your version is easier to understand, in my opinion. The spec
> shows two swept sources with a DC offset between them. I prefer your notion
> of one fixed source and one swept source, closer to the hardware that would
> actually be used.
> 
> I would like to see the diagrams also showing the attachment of the gate to Vcc,
> since it currently appears that the entire circuit just floats up and down with the
> sweep voltage. Then again, maybe the swept source should be attached between
> the source and gate, since we are attempting to sweep Vgs.
> 
> By the way, has anyone found one of these series switch models available for
> public use?
> 
> Mike
> 
> Stephen Nolan wrote:
> >
> > Question 1,
> >
> > In IBIS Version 3.2, Section 6, "Keyword:  [Series MOSFET]" it states that
> > "C_comp values are ignored for [Series MOSFET] models." How do you account for
> > the Cio(off) and Cio(on) characteristics of the FET in the IBIS model?
> >
> > Question 2,
> > In IBIS Version 3.2, Section 6, "Keyword:  [Series MOSFET]", and Section 9.5,
> > "Series MOSFET Table Extractions", it says that the voltage applied to the drain
> > of the MOSFET is Vds, a fixed voltage difference between the drain and the
> > source. Quoting from section 9.5:
> >
> >    Vds is held constant by having a fixed voltage Vds
> >    between the drain and source nodes.
> >
> > However, the drawing shows a fixed voltage source between the drain and GROUND!
> >
> > |               +----------------------------------------+
> > |               |                                        |
> > |               |                   Ids = Table Current  |
> > |               |                   --->                 |
> > |               +---<---|     |--->----------+           |
> > |                   d   |_____| - s          | +         |
> > |                        --+-- Vgs       +---+---+  +----+----+
> > |                          | g  +        | Sweep |  |   Vs +  |
> > |                                        |   Vs  |  |Fixed Vds|
> > |                                        +---+---+  +----+----+
> > |                                            | -         |
> > |                                           GND         GND
> > |
> > |                  Example of Series MOSFET Table Extraction
> >
> > I could see how this would result in a Vds voltage if Vs + was swept, so that it
> > was always equal to the swept Vs+Vds, but it says "Fixed", not "Sweep".
> >
> > Shouldn't this drawing be as follows for a true Vds supply?
> > |                           +---------+
> > |                         + |   Vs +  | -
> > |               +-----------+Fixed Vds|------+
> > |               |           +---------+      |
> > |               |                            |
> > |               |   Ids = Table Current      |
> > |               |         --->               |
> > |               +---<---|     |--->----------+
> > |                   d   |_____| - s          | +
> > |                        --+-- Vgs       +---+---+
> > |                          | g  +        | Sweep |
> > |                                        |   Vs  |
> > |                                        +---+---+
> > |                                            | -
> > |                                           GND
> >
> > --
> > Regards,
> > Stephen M. Nolan