An Integrated Battery-Hardware Model for Portable Electronics

We describe an integrated model of the hardware and the battery sub-systems in batterypowered VLSI systems. We demonstrate that, under this model and for a fixed operating voltage, the battery life decreases super-linearly as the average current dissipation increases. With the aid of analyses and empirical studies, we then show that the implications of this phenomenon are farreaching and change our perceptions about low power design techniques targeted toward batterypowered VLSI circuits. 1. Integrated battery-hardware model With the rapid progress in the semiconductor technology, the chip density and clock frequency have increased significantly, making power consumption in digital CMOS circuits a major concern for VLSI designers. High power consumption reduces the battery service life in portable microelectronic devices. Minimizing the power consumption and maximizing the battery life are however not the same. This is mainly because the battery capacity changes as a function of the average discharge current. Low power design techniques can be divided into two categories: 1. Techniques that minimize the switched capacitance under a fixed voltage level and circuit speed requirement. In digital circuits, this is equivalent to minimizing the average current per clock cycle [1]. 2. Techniques that lower the supply voltage level to reduce the energy dissipation, but make up for loss in the circuit speed by a combination of architectural transformations and circuit optimizations. These techniques can be divided into static voltage scaling [2] and dynamic voltage scaling [3]. The effectiveness of these techniques can be evaluated by using appropriate metrics, such as power, energy, and delay. These metrics have been used in different applications to guide optimization toward the best solution. It has been recently shown in [2] that the energy-delay product is an effective criterion for comparing various low power design methodologies and techniques. Figure 1 An integrated model of a batterypowered system As shown in Figure 1, a battery-powered digital system consists of the VLSI circuit, the DC/DC converter, and the battery cell. Although low-power design for portable electronics targets at extending the battery life, discussions of low-power-design metrics and methodologies have entirely focused on the VLSI circuit itself, assuming that the battery subsystem is an ideal source that outputs a constant voltage and stores/delivers a fixed amount of energy. In reality however, the energy stored in a battery cannot be extracted/used to the full extent. In some situations, even 50% energy delivery is not possible. An Integrated Battery-Hardware Model for Portable Electronics