DC-DC Power Conversions and System Design Considerations for Battery Operated System

The demand for portable devices such as cellular phones, PDAs, MP3 players and portable DVD players has grown significantly during the last few years. The portable devices usually include a variety of loads including LCD display, memories, mini microprocessor, Universal Series Bus (USB), and a hard disk drive. These loads require different operating voltages and load currents, and are powered by the rechargeable batteries such as Lithium-ion (Li-Ion) batteries, Nickel Metal Hydride (NiMH) batteries and Silver-Zinc batteries through DC-DC converters. Among them, the lithium-ion (Li-Ion) battery is widely-adopted because of its high energy density on both a gravimetric and volumetric basis. To achieve longer system run-time and smaller size, more and more system designers are focusing on improving a system’s power conversion efficiency with advanced circuit topologies through a better understanding of the battery characteristics. This paper first reviews the typical Li-Ion battery discharge characteristics and then discusses five commonly used DC-DC converters in portable power devices. Light load efficiency improvement, output voltage regulation accuracy, battery impedance impact on the system efficiency and system stability are also analyzed in detail.