What -Vcc to +2Vcc is for...

Hi Stephen,

>Actually, the -Vcc to 2Vcc requirement comes from the fact that this is the
>maximum signal swing on an unterminated transmission line. If the output
>swings 0 - 1.5v then the maximum voltage on the output would be -1.5v to
3.0v.
>However, if the part is intended to live in a 5V signaling environment then
>the V/I curves ought to to go from -5v to 10v.
>
> You know... this is really a defect in the spec. V/I table endpoints
>should be determined not by VCC, but by output swing, as described above.
>I feel a bird comming on....

Stephen I think I agree with you, I would explain it as follows:
The -VCC to 2VCC requirement was put in to insure that over
the broadest expected range for which the device might need to
be simulated there would be valid V/I entries so the simulators
did not extrapolate to current levels the device would not deliver.
For most devices this would include the max. input range generated
by worst case reflections as Stephen indicated.
However if you have a part that runs on a core switching range of
say 0 to 3 and has a clamp at 5V or a weak pull up to 5V the
range should probably be -5 to +10 not -3 to +6 (my opinion).
Discression is left to the model developer on this one but the
boardest possible range for which the device could conceivable
operate should be covered with a minimum of -Vcc to +2VCC.
Another example would be an old 30V open collector 7406.
The V/I table for this device should extend to at least +30V and -5V.
When in doubt use a broader range.

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Have a great day....
Kellee Crisafulli at HyperLynx
SI,EMC,X-talk and IBIS tools for the Windows platform
E-mail: <mailto:kellee@hyperlynx.com>
web:   <http://www.hyperlynx.com>
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Received on Wed Nov 4 16:58:22 1998