1.8 Power management  (Page 22/7)

Power-supply circuitry

Many electronics textbooks often ignore the power supply. It is simply assumed that a bias rail for an amplifier circuit or the VCC operating voltage for a microprocessor is “just there.” But in reality, engineers developing a complete system will need to create (or at least select) an appropriate solution to generate the necessary power-supply rails for the rest of the circuitry. The complexity of these circuits can vary considerably depending on the application.

The purpose of this chapter is not to provide a detailed explanation about the theory and design of voltage regulator circuits, but to allow you to understand how to implement a power-supply solution for a given system using readily available integrated solutions. For many DC/DC conversion applications, system design engineers can now select from a wide variety of readily available catalog solutions to implement in their power systems without becoming a power-supply design specialist. Figure 1 illustrates the power conversion circuitry in an electronic system.

Determining the power-supply requirements and architecture

As indicated in Figure 1, the basic purpose of a power-supply circuit is to convert the voltage available from the “bulk” input power source into one or more regulated voltages as required by different subsystems/circuits in the rest of the design. For example, a typical microprocessor may require 1.8 V for the processor core, plus other voltage levels like 2.5 V or 3.3 V for peripherals or I/O devices. If the system has a display, you will need additional higher-voltage rails to bias and/or illuminate that display.

Let's outline one approach to the process of developing a power system for a given application (see Figure 2 as well):

• What type of input source is available?
  • Understand how much bulk (raw) power you have available, and if it is sufficient to operate all of the circuitry in the overall system.
• Determine the power budget:
  • Understand the basic needs for the output power rails required – the volts and amps required for each part of the system to operate.
• Consider the accuracy (regulation) requirements for each power output rail:
  • Understand ripple and noise requirements for each power rail.
  • If high efficiency is required, consider switch-mode converters where appropriate.
  • Consider linear regulators for sensitive rails such as audio or radio circuits (including linear post-regulators after a switching supply when needed), or for low-power rails where efficiency is less important.
• Consider power-sequencing requirements for a given processor or system and how the different rails will be controlled (some microprocessors require multiple power rails, and they must be turned on/off in a specific order).
• Check thermal stress/power dissipation limits.
• Choose the regulator architecture (linear or switch-mode) for each output required.
• Assemble and evaluate prototypes – use evaluation modules (EVMs) from TI, or build a dedicated power printed circuit board (PCB) customized to your system requirements.