RE: [SI-LIST] : To overshoot or undershoot: ( to diode or not to

Andrew Ingraham (Andrew.Ingraham@digital.com)
Mon, 23 Feb 1998 09:09:08 -0500

I take it you are talking about the currents into or out of clamp diodes
that is doing bad things.

Some ICs specify a limit on the currents into those clamp diodes. Have
you seen problems where this current limit was not exceeded?

I believe the mechanisms for overshoot and undershoot clamps are totally
different (but I am not an expert on this). Undershoot clamp current
goes into the substrate where it affects neighboring transistors, and
has the potential to activate those parasitic SCRs that causes latch-up
and total chip failure if stressed enough. Overshoot seems to be
different.

I have been told of a situation where some sort of programmable IC, was
seen going into program mode, on account of overshoot voltage magnitude.
These ICs typically use something like +12V applied to one or two pins,
to tell them to go into program mode. The overshoot on some pin went to
something like +8V, which was just enough. I think it wasn't even the
pin you're supposed to use to activate program mode (but I'm not sure
about that).

Sometimes there are huge differences in the clamp diode characteristics
even between the "same" components from different vendors. We recently
looked at one such part, a simple logic IC. SPICE models from the two
vendors showed an enormous difference; on one model they were brick
walls, on the other, there was significant resistance. So we measured
parts from both vendors, and indeed, they were different. Same generic
part number.

We have had situations with RAM chips (different part numbers) where one
vendor's product had no overshoot clamps, and exhibited significant
ringing and timing problems due to ringback.

Regards,
Andy