Larry Smith and I address this question in the preface to our new book, Principles of Power Integrity for PDN Design--Simplified, published in April 2017 by Prentice Hall. While we are both experts in the field, we have our own blinders, preconceptions, narrowed focus and personal preferences about what we mean when we discuss power integrity. So, we asked around and paid attention to what was being presented at conferences and in publications.
We found this question to be a little like the story of the five blind people and the elephant. They each are asked, what is an elephant? Depending on what part they were facing, imagined the elephant as that feature: a wall, a rope, a tree trunk, etc.
The answer to this question of what is power integrity is important. We think this general topic of power integrity is confusing in the electronics industry partly because it is not well-defined and can encompass a wide range of problems, each with their own set of root causes and solutions. There is universal agreement that the field of power integrity includes everything from the voltage regulator module (VRM) to the on-die core power rails and on-die capacitance.
Between the VRM and die are interconnects on the package and board, which often carry discrete capacitors with their associated mounting inductance. The power distribution network (PDN) refers to all interconnects (usually inductive), the intentional energy storage devices (usually capacitive), and loss mechanisms (damping) between the VRM and the on-die Vdd-Vss power rails.
Power integrity is all about the quality of the power seen by the circuits on the die. What about noise created on the board power and ground planes by signals passing through cavities? Is this a signal integrity problem or a power integrity problem? Is the voltage noise generated by I/O switching currents and seen by the on-die Vcc and Vss rails a power integrity or signal integrity problem? Current that comes in through the common package lead inductance, which is ultimately connected to the VRM, generates this noise, which is sometimes referred to as switching noise or ground bounce.
This gray area between signal and power integrity has a profound impact on solutions that are offered for “power integrity” problems. Adding decoupling capacitors on the board often provides a solution for reducing Vdd core noise but seldom improves the cavity noise induced by high bandwidth signals. In general, board-level capacitors offer little or no improvement to return-plane bounce noise. In some cases, the parallel resonances they create can actually increase the cavity-to-signal cross talk.
The first step to solving a problem is to clearly identify the problem and then correctly identify its root cause. A well-defined problem is often only a few steps away from a solution. Efficient solutions to problems are developed based on the actual root cause. Rather than generalizing all PDN related problems under the general category of power integrity, it is more useful to look at the specific problems, their root cause, and the best design practices to eliminate them for a design.
For more on this topic, download a free copy of the Preface to Principles of Power Integrity for PDN Design--Simplified. We would love to hear your answers to What is Power Integrity? Feel free to hit the comments section below to add your thoughts.