The PCB Design Magazine

PCBD-Apr2015

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52 The PCB Design Magazine • April 2015 Power distribution networks (PDN) are be- coming an important topic. Many engineers are finding that properly designing the power sup- plies and providing adequate decoupling for de- vices 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 sys- tem is the PCB, and how it contributes to the power distribution system's ability to decouple the switching devices. The following experi- ment 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 regula- tor model (VRM), the discrete decoupling ca- pacitors, 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; specifi- cally 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 oth- ers. 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 includ- ing the local on-die decoupling capacitance, the PCB, the discrete capacitors, and finally the VRM. [1] The edge rate of this switching current is ex- tremely 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 dis- tributed RLC network. Capacitance is formed by the copper layers and the dielectric between them. Inductance is formed by the loop area be- tween the layers, and the resistance is formed by article Effective Decoupling Radius by kirk Fabbri KsPT eNGiNeeRiNG CoNsulTiNG

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