20.5 A 2-/3-phase fully integrated switched-capacitor DC-DC converter in bulk CMOS for energy-efficient digital circuits with 14% efficiency improvement

Reducing the supply voltage of digital circuits to the sub- or near-threshold regions minimizes dynamic power consumption and achieves better efficiency [1]. This technique is widely used in energy-efficient applications, and is especially beneficial for wirelessly powered devices such as wearable electronics, biomedicai implants and smart sensor networks. Such devices have long standby times and battery-less operation is highly desirable. As shown in Fig. 20.5.1, for a typical wireless power transmission system, there is a gap between the rectified V_IN (>2V) and the low supply voltage V_OUT (<;700mV) for powering up energy-efficient digital circuits. To bridge this voltage gap without sacrificing compact size, fully integrated power converters with a low voltage conversion ratio (M=V_out/V_IN) and high efficiency are needed. However, a low M results in low efficiency for linear regulators and fully integrated buck converters. On the other hand, fully integrated switched-capacitor power converters (SCPCs) are good alternatives that can achieve high efficiency at low M in low power applications [2-5].