PCB characteristics affect PDN performance

Focus:

For readers familiar with the design of power distribution networks (PDNs), the statement in the title should be obvious. But this article appears to be aimed more at designers of board-level power converters such as point-of-load regulators (POLs) and VRMs who may not understand why the output impedance of these voltage regulators needs to match the impedance of the application’s PCB to enable good PDN performance. This article explains how differences between the voltage regulator output impedance and the PCB trace impedance will affect the overall impedance profile of the PDN, with those differences introducing various amounts and types of peaking (resonances) in the PDN’s impedance curve. The relationships between these impedances are explained in terms of transmission line concepts; methods for measuring the characteristic impedance of the PCB trace are demonstrated; concepts are illustrated with simulations and measurements of cables and PCBs. Additional simulations illustrate the impact of voltage regulator-to-PCB impedance matching on transient response.

What you’ll learn:

• How to understand the impact of impedance mismatches between voltage regulators and PCBs on the performance of the PDNs implemented on those boards
• How to determine the characteristic impedance of a printed circuit board at the point where the voltage regulator connects to it

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Author & Publication:

Steve Sandler, Picotest, EDN, Apr 01 2014

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