Effective Decoupling Radius


Reading time ( words)

Power distribution networks (PDN) are becoming an important topic. Many engineers are finding that properly designing the power supplies and providing adequate decoupling for devices is a challenge, especially since devices are switching faster and dimensions are shrinking. Engineers often focus on discrete decoupling capacitors placed local to switching devices in hopes of providing the required capacitance for these high current demands. One of the more overlooked items of the power distribution system is the PCB, and how it contributes to the power distribution system’s ability to decouple the switching devices. The following experiment will outline a basic principle that should be in mind when designing a stack-up and PDN.

Basic PDN Model

A basic PDN includes the voltage regulator model (VRM), the discrete decoupling capacitors, the PCB, and any on-die capacitance formed on the IC or device. Each one of these components could be written about separately, but it is the PCB that will be focused on; specifically the effective decoupling radius.[2]

When a device is active, it will require current. The type of device (process size), load on the I/O drivers, and how the device is operated, all have an effect on the current required, among others. When the device demands current, it flows through the complex impedance of the PDN and causes a ripple voltage to appear. This transient current is drawn from a variety of sources including the local on-die decoupling capacitance, the PCB, the discrete capacitors, and finally the VRM.[1] The edge rate of this switching current is extremely important when trying to calculate how effective the PDN will be in suppressing the ripple voltage. The switching edge can be dissected into a variety of harmonic sine waves at decreasing amplitude described by a Fourier series equation. It is here that we discover the importance of the PCB, and its role in the PDN.

The simplest way to represent a PCB is a distributed RLC network. Capacitance is formed by the copper layers and the dielectric between them. Inductance is formed by the loop area between the layers, and the resistance is formed by the cross sectional area and length of the copper planes.

To read this article from the April 2015 issue of The PCB Design Magazine, click here.

Share


Suggested Items

Fadi Deek Discusses Mentor’s New Power Integrity eBook

04/22/2018 | Andy Shaughnessy, Design007 Magazine
At DesignCon 2018, I ran into Mentor’s Fadi Deek, the author of both of Mentor’s I-Connect007 eBooks: the newest, "The Printed Circuit Designer’s Guide to Power Integrity by Example," and their first book, "The Printed Circuit Designer’s Guide to Signal Integrity by Example." We sat down and discussed how the idea for the books came about, as well as some of the power integrity challenges facing PCB designers and engineers.

Cadence’s Sigrity Automates Power Integrity Simulation Earlier in Design Cycle

04/04/2018 | Kelly Dack, CID+, EPTAC
DesignCon is always a great place to check out the latest PCB layout and simulation software tools. During DesignCon 2018, Guest Editor Kelly Dack met with Sam Chitwood, a product engineer with Cadence. Sam explained how the Cadence Sigrity simulation software now allows users to make decisions early in the design process, and how this can help optimize the design of the power delivery network and ensure signal integrity in complex PCBs.

AltiumLive Summit—Munich, Germany, Part 1

11/07/2017 | Pete Starkey, I-Connect007
Altium held a very successful AltiumLive PCB Design Summit in San Diego, California at the beginning of October for the benefit of their North American design community, and followed it three weeks later with a counterpart European event in Munich. And what an eye-opener it proved to be—literally hundreds of delegates, a superbly organised and managed programme, billed as a completely immersive two-day interactive design experience on a theme of learning, connecting and getting inspired.



Copyright © 2018 I-Connect007. All rights reserved.