I have found that driver current ratings don't mean a whole lot today.
They are a hold-over from TTL days, when each receiver drew a certain dc
current and a driver had to be able to source the dc current of all its
loads. In a CMOS environment, they are useful in a relative sense, i.e.
you can guess that a 4 mA driver will probably have a lower output
impedance than a 2 mA driver - within the same part family and the same
vendor. However, don't expect vendor A's 4 mA driver to have the same
output impedance as vendor B's 4 mA driver.
Output impedance tends to be a much more useful parameter in a
transmission line environment. Unfortunately, most vendors don't spec
output impedance. If you have a non-linear output IV curve, it does
vary over loading, making it a little difficult to spec. The vendor
would have to guess what kind of environment driver will see and spec,
say, a 60 Ohm resistive load to ground and Vdd. Even this information
is more useful than nothing.
The best thing you can do is to ask for an IBIS model of your driver
that is verified against lab data.
How many loads you can expect to drive varies widely with load
capacitance, net topology, and timing. A behavioral simulator is an
excellent tool for prototyping nets and answering these kinds of
questions. It's important not to rely on the simulator to do all your
work for you, though. I find it necessary to go back to transmission
line theory (reflection coefficients, etc.) and make sure what I'm
simulating makes sense!
Greg Edlund, Principal Engineer
Server Product Development
Digital Equipment Corp.
129 Parker St. PKO3-1/20C
Maynard, MA 01754
(978) 493-4157 voice
(978) 493-0941 FAX
From: Mark Nass[SMTP:email@example.com]
Sent: Monday, March 02, 1998 10:35 PM
Subject: [SI-LIST] : Driver Strength
Can someone explain to me what driver strength means? When
a driver is spec'd by an ASIC vendor as a 12ma, 6ma, etc what
that mean as far as its expected VI curve and how many loads
I can expect it to drive?