The increasing prominence of portable systems and the need to limit power consumption (and hence, heat dissipation) in very-high density ULSI chips have led to rapid and innovative developments in low-power design during the recent years. The driving forces behind these developments are portable applications requiring low power dissipation and high throughput, such as notebook computers, portable communication devices and personal digital assistants (PDAs). In most of these cases, the requirements of low power consumption must be met along with equally demanding goals of high chip density and high throughput. Hence, low-power design of digital integrated circuits has emerged as a very active and rapidly developing field of CMOS design.
The limited battery lifetime typically imposes very strict demands on the overall power consumption of the portable system. Although new rechargeable battery types such as Nickel-Metal Hydride (NiMH) are being developed with higher energy capacity than that of the conventional Nickel-Cadmium (NiCd) batteries, revolutionary increase of the energy capacity is not expected in the near future. The energy density (amount of energy stored per unit weight) offered by the new battery technologies (e.g., NiMH) is about 30 Watt-hour/pound, which is still low in view of the expanding applications of portable systems. Therefore, reducing the power dissipation of integrated circuits through design improvements is a major challenge in portable systems design.
The need for low-power design is also becoming a major issue in high-performance digital systems, such as microprocessors, digital signal processors (DSPs) and other applications. Increasing chip density and higher operating speed lead to the design of very complex chips with high clock frequencies. Typically, the power dissipation of the chip, and thus, the temperature, increase linearly with the clock frequency. Since the dissipated heat must be removed effectively to keep the chip temperature at an acceptable level, the cost of packaging, cooling and heat removal becomes a significant factor. Several high-performance microprocessor chips designed in the early 1990s (e.g., Intel Pentium, DEC Alpha, PowerPC) operate at clock frequencies in the range of 100 to 300 MHz, and their typical power consumption is between 20 and 50 W.
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