Supplier eBooks

22 When putting together a design, several considerations affect PDN impedance: • Power supply. When choosing a power source, it is critical that you understand the static and dynamic load of your IC. • Capacitors. Bulk capacitors are forgiving in their placement. Because their package size is typically too large to fit inside the confines of an FPGA's pin field, they are usually placed near the power supplies and around the outer edge of the FPGA. High-frequency caps (small-package, low-capacitance capacitors) are typically found inside the pin field of the FPGA and affect the impedance curve in the megahertz range. • Printed circuit board. The PCB affects the impedance curve typically in the 10- to 300-megahertz range. This is where thin dielectric material and placing your copper power layers close to the surface of your stack-up are critical in helping reduce the overall loop inductance of the plane. • IC package and die capacitance. The high-frequency component of the impedance curve is dominated by package and die capacitance. Most high-end FPGAs' package and die capacitance begin to reduce the impedance curve in the 30-megahertz range. At these high frequencies, we are simply at the mercy of the FPGA manufacturers to provide adequate decoupling. Both the power and the serial data want to use the same area on the PCB. The serial data needs the layers directly below the FPGA to provide access to inner layers through microvias and to reduce stubs and trace length. At the same time, the PDN wants these layers directly under the FPGAs to reduce loop inductance and keep the overall plane impedance low. Using 1-mil cores and carefully planning the routing of power planes and the placement of the bypass capacitance as well as understanding the overall load step on the core voltage go a long way toward achieving both a low-impedance plane and serial data lines that meet their respective specifications. '' Simulation software helps reduce board spins by simulating all aspects of the boards, power density, voltage drops, plane impedance, signal integrity, and other things."

• Cover