Severe Overshoot

	Home Contact

Public Seminar Schedule, taught by Dr. Howard Johnson
High-Speed Digital Design	U. of Oxford, UK Portland, OR Huntsville, AL	June 29-30, 2010 May 3-4, 2010 coming Fall 2010
Adv. High-Speed Signal Propagation	Huntsville, AL	coming Fall 2010
High-Speed Noise and Grounding	U. of Oxford, UK Portland, OR	July 1-2, 2010 May 5-6, 2010

Severe Overshoot

*---------------------(QUESTION)---------------------*

SEVERE OVERSHOOT AND UNDERSHOOT

J.J. Leu writes:

I have a question about overshoot and undershoot. As we know, if source impedance of a driver mismatch trace impedance, overshoot and undershoot will exist in a receiver end, assume that source impedance is smaller than trace impedance, and the driver and receiver devices are CMOS type. What is the influence of overshoot and undershoot in the receiver end? That is, will overshoot and undershoot impact receiver end, damage it or cause excessive recovery time? Is there a maximum overshoot voltage or a maximum undershoot voltage for 3.3V or 5V devices? Does the maximum voltage depend on chip vendors?

Best Regards,
J.J. Leu

*--------------(REPLY FROM DR. JOHNSON)--------------*

Thanks for your interest in High-Speed Digital Design.

Overshoot can in severe cases cause CMOS latch-up and destruction of the receiver. Many CMOS receivers include built-in protection diodes on every input to preclude this possibility. Some don't. In particular, watch out for 3.3-V parts with 5-V "tolerant" inputs. There are often no protection diodes to Vcc on these parts.

When the protection diodes are present, overshoot, if it over-stresses the protection diodes, can cause aluminum migration in the diode contacts and eventual failure of the diodes. Nobody seems to know (or will say) under what general conditions this will happen. The maximum voltage (and maximum current) ratings for inputs remain peculiar to each vendor.

On another subject, milder amounts of overshoot, and the consequent ring-back will affect your timing margin. Basically, you have to wait for the ringing to settle down before it's safe to sample the received signal. This is the most common problem caused by overshoot.

Best regards,
Dr. Howard Johnson

To Subscribe to this HSDD Newsletter, send an email to hsdd-request@freelists.org with 'subscribe' in the subject field.

All Publications by Dr. Howard Johnson except as noted.
Signal Integrity Training Classes taught exclusively by Dr. Howard Johnson - for full schedule, see www.sigcon.com
© 1998 Signal Consulting, Inc., Dr. Howard Johnson. All rights reserved. • site map