Establishing Confidence in PDN Simulation

Bogatin Enterprises, LLC Setting the Standard for Signal Integrity Training Multimedia SI training: read it, watch it, hear it, do it, answer it, ask it, experience it!
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Establishing Confidence in PDN Simulation Dr. Eric Bogatin, eric@beTheSignal.com Bogatin Enterprises, www.beTheSignal.com March 13, 2009
Abstract: A powerful signal integrity analysis tool must be flexibility, easy to use and integrated into an existing EDA framework and design flow. In addition, it is important for the tool to be accurate enough. This report reviews a validation study for the Mentor Graphics HyperLynx 8.0 PI tool to establish confidence in using it for power integrity analysis.
Executive Summary The goal in any product design is to meet performance and schedule goals at the lowest practical cost. In our globally competitive environment, if your product doesn't meet all three goals, your competitor's product will. To find the optimum design, it is not cost effective to build a bunch of alternative designs and measure which is the lowest cost version that still works. A more efficient process is to use a simulation tool to explore design space and to evaluate the final performance of a design before you commit to hardware. It is always going to be less expensive to "build" virtual prototypes than real prototypes. In addition to all the important features a tool must have, like flexibility, ease of use and integration into your existing EDA framework and design flow, it's important to use a tool that is accurate enough. This report reviews a validation study for the Mentor Graphics HyperLynx 8.0 PI tool to establish confidence in using it for power integrity analysis. For this validation study, we compare the results of the HyperLynx 8.0 PI tool to an analytical example and to a test vehicle with a series of plane cavities having 0603 capacitors integrated to the board.
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In each case, the predicted impedance profiles of HyperLynx closely match the measured performance. HyperLynx is able to predict the capacitor and plane interactions, spreading inductance in the planes, the effect of cavity thickness and height below the top surface and even saturation of the spreading inductance when closely grouped low inductance capacitors are used. The agreement between the self and parallel resonances is between 1% and 10% for a wide variety of cases. The variation may be dominated by a real frequency dependence to the mounting inductance of a capacitor, which could be related to the capacitor internal structure. A significant limitation to better accuracy for the impedance profile is also set by the uncertainty in the capacitor's C and ESR values. This is inherent in the use of commercially available capacitors and sets a fundamental limit to how well an impedance profile can be predicted. The figure below shows an example of the measured and predicted transfer impedance profile for a capacitor mounted to a board with a buried power and ground plane. It is critically important, when precision impedance measurements are performed, to always use the 2-port technique described in this report. Measured Simulated Example: Thin cavity, close to the surface. Comparing the measured transfer impedance of the board with a capacitor and the simulated transfer impedance. Note that the parallel resonance peak at about 300 MHz is very accurately predicted to better than 1%. This is a combination of the total ESL and the cavity's capacitance. The self resonant frequency is predicted to within about 10% of the measured value. Bogatin Enterprises, LLC Setting the Standard for Signal Integrity Training www.BeTheSignal.com Page 3 of 36
Introduction: the Role of Simulation Tools The purpose of the Power Distribution Network, or PDN, is to provide a low impedance path from the voltage regulator module (VRM) to the pads on the chip. Though there are a few general design guidelines to follow which will help to achieve this, rarely can these guidelines be followed in their entirety. This means the PDN is never as low an impedance as it could be. The important question to answer is, is it low enough for the application, or are there less expensive ways of achieving comparable performance? In our globally competitive environment, it is not cost ef... [download for more]