Volume 31 Issue 8 - April 27, 2018 PDF
A Four-Phase Buck Converter With Capacitor-Current-Sensor Calibration for Load-Transient-Response Optimization That Reduces Undershoot/Overshoot and Shortens Settling Time to Near Their Theoretical Limits
Yi-Wei Huang, Tai-Haur Kuo*, Szu-Yu Huang, and Kuan-Yu Fang
Department of Electrical Engineering, National Cheng Kung University
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【106 MOST Outstanding Research Award】Special Issue

Next-generation mobile electronic devices such as smart phones and tablets will keep increasing processing speed while saving power. Hence, switching DC-DC converters with ultra-fast load-transient responses will be in great demand. This paper presents a four-phase buck converter with capacitor-current-sensor (CCS) calibration for load transient-response optimization that targets the theoretically minimal output-voltage undershoot ΔVUS, overshoot ΔVOS, and settling time ts when large and rapid load-current transients ΔIload occur. The proposed CCS calibration calibrates the CCS’ equivalent impedance to emulate a scaled replica of the output capacitor’s impedance ZCo. Thus, the CCS can accurately sense the output-capacitor current ICo despite ZCo variations due to different output voltages, fabrication variations, and printed circuit-board parasitics. Moreover, a load-transient optimizer is proposed to utilize the accurately sensed ICo to instantly detect the large and rapid ΔIload, and synchronously control the charging and discharging durations of the output inductors in all four phases, resulting in small ΔVUS/ΔVOS and short ts. The converter is implemented in a 0.18-μm CMOS process with 1.93-mm2 chip area. For a 1.8-A/5-ns step-up (step-down) ΔIload, the measured ΔVUS (ΔVOS) and ts are 92 mV (75 mV) and 133 ns (110 ns), respectively. Compared with other state-of-the-arts, both the measured ΔVUS (ΔVOS) and ts in this paper are the closest to their respective theoretical limits, i.e., the fastest load-transient response with the smallest ΔVUS (ΔVOS) and the shortest ts under the same input voltage, output voltage, output inductance, and output capacitance.
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