Efficient on-chip power management using fully integrated DC-DC converters

Device scaling has resulted in the implementation of entire systems on a single chip. The increased level of integration has reduced system costs and has proved advantageous from a signal integrity point of view by reducing the number of high-speed signals that need to be routed off-chip. Unfortunately, this higher level of integration has resulted in increased power dissipation in both mobile and stationary devices. Battery life of mobile devices and increased package and cooling costs for stationary devices are the driving forces behind the new focus on methods to reign power dissipation [1]. The supply voltage is one of the primary levers available to control power dissipation [2]. As shown in Equation 1.4, power dissipation in a digital system is approximately proportional to third power of the supply voltage (V 3 dd). We can consider this by viewing Equations 1.1 through 1.4. The active power dissipation of a digital circuit is given by Equation 1.1, where Ctot is the total digital capacitor switched, V dd is the supply voltage, and f is the frequency of switching. On-chip power conversion is a complex process especially in battery-operated devices. Figure 1.1a shows a typical teardown of a smartphone that indicates the requirement of different power domains, making it a complete system. It should be noted that the majority of the blocks are of digital or mixed signal type. The demand for small form factor and more features resulted in stacked integrated circuits (ICs) and double stacked printed circuit boards (PCBs). Chemical energy-based batteries are the primary power source in handheld devices. State-of-the-art batteries occupy 20% by volume and 25% by weight. Typical volume density of Li-ion batteries is 400 J/mL and a typical weight energy density of 900 J/g. According to Ref. [3], one word on 3G SMS will cost about 1 J and talking on 3G for 1 minute will cost about 30 J, which means, a best state-of-the-art system solution can provide 30 minutes of 3G talk per gram of lithium battery. These numbers throw light on the problem of energy density of such batteries. Another important aspect of Li-ion is the variation of its voltage with time as depicted in Figure 1.1b. Given a time-variant input voltage profile, we need to provide fairly time-constant voltages involving both buck and boost conversions. So the power conversion system not only requires good static performances (steady-state efficiencies) but excellent dynamic behavior (fast feedback at input and output side).