Altera Power Delivery Network User Manual

Page 15

Chapter 1: Power Delivery Network (PDN) Tool User Guide

1–11

Setting Up the PDN Tool

Altera Corporation

Power Delivery Network (PDN) Tool User Guide

Spreading R, L Parasitics

The spreading R, L library provides various options for the default effective spreading
inductance values that the decoupling capacitors see with respect to the FPGA based
on the quality of the PDN design.

You can choose a Low value of effective spreading inductance if you have optimally
designed your PDN. Optimum PDN design involves implementing the following
design rules:

■

PCB stackup that provides a wide solid power/ground sandwich for a given
supply with a thin dielectric between the planes. This minimizes the current loop,
which reduces the spreading inductance. The thickness of the dielectric material
between the power/ground pair directly influences the amount of spreading/loop
inductance that a decoupling cap can see with respect to the FPGA.

■

Placing Capacitors closer to the FPGA from an electrical standpoint.

■

Minimizing via perforations in the power/ground sandwich in the current path
from the decoupling caps to the FPGA device.

Due to layout and design constraints, the PDN design might not be optimum; thus,
you can choose either a Medium or High value of spreading R and L. You also can
change the default values or use the Custom field listed in the library specific to the
design.

Dielectric Material Library

The dielectric materials section lists the dielectric constant values for the various
dielectric materials. These values are used in the plane capacitance calculations listed
under the Plane Cap tab. You can change the values listed in this section.

If you change the default values listed in the various sections in the Library tab, you
can save the changes by clicking Save Custom. You can restore the default library by
clicking Restore Default located at the top right corner of the Library page. You also
can restore the saved custom library by clicking Restore Custom.