Vick:
You have the right idea. Clamps are required to protect chips from
issues caused by power rails being out of spec.
When a chip starts up, or when a power domain/rail has an event occur
that causes it to leave its specified voltage range,
then the clamps are designed to shunt as much of the damaging excess
voltage as possible away from the sensitive circuits.
In normal operation, the clamps do not operate in this manner.
One issue is that the over/under voltage conditions are often not
modeled accurately and simulators will often not predict
how the clamps will perform.
The design of clamps (and other protection devices) is somewhat of a
black art, rather than a science. I have spent time
examining failed devices to attempt to codify some bad practices (i.e.
write bizzare looking DRC rules ) to make things more robust,
but clamps and other protection device designs are always a tradeoff
between various factors of Size, Protection, and is also limited by the
need to minimize any degredation in normal (non-protecting)
performance.
