RE: confusion about c_comp

> I am a bit confused about c_comp. C_comp is the silicon die capacitance,
> which includes the parasitic capacitance of the buffer, the bond wire and
> the die pad capacitance.
 
I would not have included the bond wire capacitance, because the bond wire
is part of the package (although the position of the capacitance relative to
L_pkg may make this debatable). I think c_comp would apply to just the bare
die, even though the IBIS spec does not explicitly say so.

> When the Spice simulations are run to generate the IBIS data, the effects
> of the c_comp parameter should already be included (since you can't take
> away the parasitic capacitance of the circuit). So when the [c_comp]
> keyword is specified, isn't the effect of c_comp double-counted?
 
By specifying the c_comp keyword, you are providing the IBIS-aware simulator
with the equivalent capacitance that you had in your SPICE simulations.
That is the only place in the IBIS data where this capacitance exists! I
don't see how there is any double-counting.

> For example, when I generated my IBIS model, I estimated the value of the
> bond wire and die pad capacitance, and added that as an explicit
> capacitance to the buffer output before using Spice to extract data. Then
> I estimated the value of parasitic capacitance of the buffer circuit (as
> described by Arpad Muranyi). The total [c_comp] value is the sum of bond
> wire and die pad capacitance, and the parasitic capacitance of the
> circuit. Is this approach correct?
 
If your SPICE model is complete enough to include die bonding pad
capacitance (and bond wire capacitance if you determine that it should be
part of c_comp too), then whatever value of capacitance you extract from
your SPICE simulations *IS* the value for c_comp. You would not add a
correction factor to account for the die pad capacitance. If, on the other
hand, your SPICE simulation doesn't include the die pad, then you should add
it to the extracted capacitance to get the c_comp value for the IBIS data.

However you do it, c_comp should represent the effective on-die capacitance.

Andy

 
Received on Wed Dec 20 12:11:39 2000