Is there a point to output ESD diodes?

From: Chris Carlen (crcarle_at_BOGUS.sandia.gov)
Date: 01/19/05 

Date: Tue, 18 Jan 2005 16:32:07 -0800

Hi:

I have some circuits where I decided to put reverse biased 1N4148 or
SD103 type diodes to the power rails at the output connector pin of a
50ohm cable drive circuit composed of a pair of HC gates in series with
a 27R resistor.

If the diodes are placed at the output (after the resistor from the
gates' perspective) then they take the full brunt of any ESD surge
applied to the output. Any constant voltage source greater than 5.7V or
so would of course be shorted and potentially smoke the diodes.

But the main point is ESD not overvoltage protection. I understand the
CMOS *input* diodes and FET characteristics fairly well, and have SPICEd
these in considerable detail to come up with my favorite circuit for
protecting inputs from typical ESD models while retaining most speed.

But the outputs are a different animal. I have no basis for knowing
what surge currents and voltages the outputs can tolerate or enough
about the FET characteristics to do meaningful modeling. Thus, my
application of diodes is "seat of pants" design.

Do others bother with this or are outputs just considered not delicate
enough to worry about?

What about when the power is off? There the diodes do come in handy for
diverting current to the rails rather than through the device.

Thanks for comments.

Good day! 

-- 
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarle@sandia.gov -- NOTE: Remove "BOGUS" from email address to reply.
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