Embedded system designs are becoming increasingly complex. The end-user expects a feature-rich product, typically powered by a small battery. Many designs require a compact form factor and perhaps most importantly low cost. The scope of an embedded product now extends to include power-hogging interfaces such as wireless LAN (WLAN), a hard disc drive, and larger LCDs. As a consequence, apart from the constraint for processing power, the performance of the power supply for portable systems needs to be improved on a continual basis.
This article presents a generic power supply design for an embedded system and examines the function of each module in the design. It also provides guidelines for selection of topologies and components, thermal and packaging aspects, battery management, and hooks that interface the circuit to intelligent power management software. The principles can prove to be useful for the power supply design of any embedded system with robust feature sets and must operate battery power. Portable media players and video-conferencing units are examples of this product space. Based on the building blocks described in this paper, the reader can choose appropriate components available from different IC manufactures for a specific design.
Power management units
Specifying the exact functions and building blocks of the power circuit is usually a non-trivial job, especially because it directly affects the playtime of the battery-powered system. The architecture may vary with various classes of embedded products and usage scenarios (consumer, industrial, military, etc.). The figure below shows the blocks of a power solution for a typical consumer application.
Block diagram of a power management solution for a typical embedded system
Click on image to enlarge.
Referring to the figure, we'll define the requirements for each of the blocks in the diagram. The product is expected to work from a battery pack or from an external wall adapter. A power path controller performs the switching function to the correct power source when more than one sources are present. Emerging alternate sources of power such as USB and power over Ethernet (PoE) may need to be accommodated in some designs.