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The PDN Bandini Mountain and Other Things I Didn’t Know I Didn’t Know

In engineering, it's what you don't know you don't know that can ruin your day and keep you awake at nights. This is especially true after you get your prototypes in the lab, or worse, field returns from the customer. This is one reason why I have been going to DesignCon for the last few years, and this year has been no exception.

One of the sessions I attended was the Power Integrity Boot Camp, hosted by Heidi Barnes of Keysight Technologies, and Steve Sandler of Picotest. At this boot camp, I learned that I didn’t know I didn’t know how important it was to match the voltage regulator module (VRM) output impedance to the power distribution network (PDN) input impedance. Steve and Heidi recently presented a webcast which offered a condensed version of the DesignCon boot camp session. If you are involved in PDN design, this webcast will provide you with an introduction to power integrity and give some insight into the latest tips and techniques to achieve flat impedance designs.

Of course, I always try to attend some of Eric Bogatin's presentations, because I always come away with something I didn’t know I didn’t know. Eric is an adjunct professor at the University of Colorado and the dean of Teledyne LeCroy’s SI Academy. He was honored at this year’s DesignCon with a well-deserved Engineer of the Year award.

The speed training event he hosted with Larry Smith from Qualcomm was on the top of my list to attend. During the session, Eric explained that the most critical feature of PDN design was controlling the “Bandini Mountain.”

The Bandini Mountain expression has often been used to describe a tall pile of manure. Originally, it referred to a 100-foot tall mound of fertilizer built by the Bandini Fertilizer Company in California prior to the 1984 Los Angeles summer Olympics for advertisement purposes. When the company went bankrupt, this large mound of smelly fertilizer was left behind and everyone wished it would go away.

A little background: Because of this little bit of trivia, the late, great Steve Weir coined this term to describe the large resonant frequency peak formed by the parallel combination of the on-die capacitance and the package lead inductance, as seen from the die looking into the PDN. This peak is inherent in all PDN networks, and it is almost impossible to get rid of. And like the Bandini Mountain, it was something PDN designers wish could go away.

Steve was a regular icon at past DesignCons until his sudden passing in August 2015. Steve was one of the smartest guys I knew, and I always looked forward to catching up with him when I visited DesignCon. If you knew Steve, like many of us did, you know that he often had very humorous analogies to describe empirical or simulated results. This example is no exception. He will be sorely missed, and we all appreciate his contributions to the engineering community. 

So, what I learned I didn’t know I didn’t know from Eric and Larry’s presentation was that every PDN design will have a Bandini Mountain, and unless you know its frequency and take steps to try to mitigate its peak, it could ruin your day! Even if the system seems to work in the lab, that doesn’t mean that it’s robust enough and won’t fail under certain field operating conditions that affect the transient currents.

Eric has made available the speed training slides and the associated video on his SI Academy website. If you look under Video Recordings, Presentations and Webinars (VRPW) and scroll down to the bottom you will find the slides titled, “VRPW-60-35 DesignCon 2016 PDN speed training.” If you watch the whole presentation you will learn all about the “PDN Bandini Mountain” and techniques to mitigate its effects. And while you are there, have a look at the many other videos and presentations available for free and by paid subscription.

Eric and Larry have also co-authored a new book scheduled for release in June 2016 titled, “Principles of Power Integrity for PDN Design.” I can’t wait to buy this book to add to my library so that I can find out more of what I don’t know I don’t know about PDN design. If it’s anything like Eric’s other books, I won’t be disappointed.