Re: [SI-LIST] : Best type of models, edge rates & load

Adrian Shiner (adrian.shiner@virgin.net)
Wed, 28 Jul 1999 19:04:27 +0100

Guys, you need to read ALL that I wrote very carefully, also not read what I didn't write! I did not say that the output driver has a fixed characteristic and the load is a simple capactive integrator type. The variation in impedance
at the output as it switches (infinite to zero or somewhere between) is considered in "..approximation to current or voltage source". How the driver impedance and the driven impedance interact when combined can't be determined
properly unless the driver impedance is defined. If the driver also sinks current, the same issue pertains. If the semiconductor manufacturers don't characterise this properly then simulations are going to be unreliable as switching
times shorten.
.

"D. C. Sessions" wrote:

> Adrian Shiner wrote:
> >
> > Surely all this data sheet spec rise & fall times info is becoming "foo dust" as switches become faster? The slew rate of a voltage waveform at a given point on a printed circuit, driven from a perfect switch (zero switch time)
> > depends solely on the current source/ sink capability of the generator being switched, its approximation to a current or voltage source and the circuit impedance.
>
> Not really. The output current doesn't go instantaneously from full-drive
> sink (or off) to full-drive source, so the output waveform isn't *entirely*
> a capacitive-charging curve. In fact (as we've been discussing) real loads
> are close to the real axis, so the I/T curve dominates.
>
> > Why don't the IC manufacturers defne the output driver broadly in line with the above?
>
> Actually, that's what too many do and what we've been griping about.
>
> --
> D. C. Sessions
> dc.sessions@vlsi.com
>
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