The derivative of the current that flows off-package through the power and
ground distribution is what makes the noise when a driver switches, not the
current itself.
In a simple situation of a driver switching a lumped load, or a homogeneous
transmission line, there are two derivative events during the switching. Once
when the driver turns on, and once when the net at the driver end becomes fully
charged. The first event makes noise that collapses the supply, the next event
makes noise that expands the supply. To be conservative, ignore the second
event, measure how long the first event lasts (a good, conservative rule of
thumb is about the delay of 2 inverters built in the same technology) and
switch your driver groups accordingly.
Joe
CEC Analysis and I/O Design
Phone: 507.253.0762 Fax: 507.253.4966
[email protected]