| Nowadays the portable devices have been developing rapidly. They become smaller, but integrate more functions and provide more services to consumers. While the system gets more complicated, the power management unit is required to power every module flexibly and efficiently. Switching inductor converter plays a key role in DC-DC conversion, but too much inductors cause increment of size and cost. As the monolithic inductor on present process is still not competent for efficient energy storing, driving two or more loads with only one inductor is undoubtly an appealing solution. Moreover, since the output voltage of battery varies during usage, adaptive step-up or step-down conversion helps enhance the battery life. In addition, portable devices like mobile phones spend more time in idle mode, which usually means extremely ligh load condition, so the optimization of coversion efficiency and other performances at light load is very important.A single-inductor dual-output buck-boost converter is investigated and implemented in this thesis, espetially on the stability, conversion efficiency and output voltage ripple at light load. The teady-state relations and small-signal transfer functions are analyzed, while various modulation and controlling methods are compared. From the analysis of four sorts of open-loop condition with three loops, the system stability is generally guaranteed even at light load, with the novel extended pulse-width modulation and VLX-filter-based compensation. Meanwhile, the system would switch to pulse-frequency modulation when driving light load according to the zero-inductor-current duration in discontinuous conduction mode. Then, with appropriate scheduling arithmetic and constant-voltage-ripple modulation, the light-load characterizations including conversion efficiency and voltage ripple are optimized very much.The chip is designed and implemented on TSMC 2.5V/5V 0.25um mixed-signal CMOS process. With 2.5V-5V supply, the converter generates two outputs of 1.2V-5V and 2.5V-5V. The system keeps stable even when the load currents are low to mA orders. About 80mA overshoot and undershoot are observed during load response of one output. And if the loads of both outputs drop to light levels, the converter will swith to pulse-frequency modulation. More than three times of improvement on conversion efficiency is observed at extremely light load, while the output voltage ripples are kept 70mV or less under various conversion and load conditions. The measurement results meet with the design specification, and demonstrate the validity of our analysis and innovations in design. |
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