5 - POWER DISTRIBUTION

Summary

Digital logic requires a stable, quiet, DC supply voltage while drawing a large AC current with very high-frequency components comparable to signal rise times. It is not unusual for a high-performance digital circuit board to draw over 200 A from the primary power supply with the derivative at the point of use over 200 GA/s. However, with careful design, a power supply network can tolerate large variations in current draw while holding the supply voltage within a specified range (typically ±10% or less).

In this chapter we discuss the characteristics of power supply networks and their loads and explore several methods for providing quiet supplies for high-performance digital systems. We begin by examining the primarily inductive, off-chip power supply network in Section 5.1. The supply and ground are distributed over a network with inductive and resistive components. The current causes IR drops across the resistive components, and the derivative of the current causes Ldi/dt drops across the inductive components. In this section we look at the most commonly used passive method to provide load regulation: bypass capacitors, which supply the high-frequency components of the current demand and thus smooth the current load carried by the inductive components of the distribution network.

Section 5.2 examines active methods to control the power supply. Clamps and shunt regulators smooth the current profile of the load by adding an additional current in parallel with the load current.