Power Integrity: It's not just decoupling caps
Gone are the days when simple decoupling-cap rules-of-thumb were effective. Even lower speed circuitry needs more care than ever before as IC processes reach gigahertz levels. According to Steve Sandler, one of PI's leading researchers and practitioners, the field is about 15 years behind SI (signal integrity). Wow – talk about opportunity.
Power integrity encompasses the entire power system, from VRM to PCB planes to capacitors to the chips themselves. I learned so many things at DesignCon (though mostly, I learned just how much I don't know), such as:
- Impedance-match the PDN (power distribution network) just as you would a transmission line (except we're talking milliohms, not 50Ω).
- The PDN impedance should be as flat as possible. When multiple anti-resonant peaks are excited, rogue waves can result, pushing supply voltage way out of spec.
- Ceramic caps can be TOO good; consider using ESR-controlled parts.
- IC manufacturers sometimes sabotage you with poor package design, such that it's impossible to decouple adequately.
- Evaluate regulators based on output inductance. It's one of the key specs leading to a good PDN.
Intrigued by the opportunities in PI? Read (start with the links below). Read. Read some more. Simulate. Design. Practise. Go to conferences. Network. Teach.
- Target impedance, rogue waves: tales from the experts
- Rogue waves can ruin your power
- Reducing noise in PDN at DesignCon
- The inductive nature of voltage-control loops
- Book Review: Power Integrity
- Target impedance based solutions for PDN may not provide realistic assessment
- PCB characteristics affect PDN performance
- Design a VRM with perfectly flat output impedance in 5 seconds or less