Compact Li-Ion Battery Charger and Fully Integrated Switched-Capacitor DC-DC Converter for Mobile Applications

Abstract

Wearable and portable devices such as smart watches and smart phones are mostly powered by batteries, which have limited energy resources. In general, the lithium-ion (Li-ion) batteries have been broadly adopted for these mobile devices because of their higher volumetric and gravimetric energy density compared to nickel metal-hydride or nickel-cadmium batteries. A single-cell Li-ion battery pack system generally includes a Li-ion cell and charge protection circuits, and thus the parasitic resistances, called a built-in resistance (BIR), are generated in the battery charging path, resulting in an increase in charging time. On the other hand, the printed circuit board (PCB) size and/or the number of external components are required to be reduced for lightweight and thin portable devices. To solve the aforementioned problems, this dissertation presents a compact Li-ion battery charger and a fully integrated switched-capacitor DC-DC converter for mobile applications. The proposed Li-ion battery charger employs a compensation method to minimize an IR-drop across the BIR (VBIR) in the charging path. It also uses a successive BIR detection (SBIRD) with the constant average current (CAC) mode to achieve a fast charging time by accurately compensating for the VBIR. In addition, the LDO-based structure is adopted to realize a compact-sized charger without using any extra external components such as inductors. The proposed battery charger is fabricated in a 0.13 μm BCD process technology with 6 V high-voltage CMOS devices. The measurement results show that the overall charging time of the proposed battery charger is reduced by 0.5 h when a 750 mAh Li-ion battery is used. The switched capacitor (SC) DC-DC converter, which includes the flying and storage capacitors, is proposed to eliminate the necessity of external components. The hybrid output regulation method adopted in the proposed SC DC-DC converter, which is based on the digital capacitance modulation, realizes a fine regulation and an automatic frequency scaling for a coarse regulation. The on-chip current sensor and automatic frequency scaler are implemented to adjust the switching frequency at one of the frequencies generated by the binary frequency divider with change in the load current. Thus, the proposed SC DC–DC converter can predict the switching noise spectrum over the entire load range. The proposed SC DC–DC converter was implemented in a 0.13 μm CMOS process with 1.5 V devices, and its measurement results show that the peak efficiency and the efficiency at light load condition are 69.2% and higher than 45%, respectively, while maintaining a predictable switching noise spectrum.