Our design is slightly exceeding the published transient undershoot limit voltage on our OMAP-L138. Spec is 20% of Vcc for up to 20% of duty cycle, with a separate limit of 20mA clamping current. We have about 800mV for about 3nS below ground, on inputs that have have about 300nS periods or more. Our IBIS sim (using TI model and a vendor model for the source-terminated 3V buffer that drives the inputs along a long trace) shows about 10mA peak current.
Questions:
1. why would Vcc affect the undershoot limit, which is ground not Vcc relative? oris this just a shorthand way of saying "20% of 3.6V"?
2. why the duty cycle? we have a very low duty cycle. if our duty cycle is << 20%, can we get extra credit for that?
3. how do voltage and current limits interact? i'm assuming it's a diode...true or no? if so, then wouldn't the limit really be the clamping current? or is the situation more complicated?
Thanks for any insight!