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This paper presents a CMOS integrated switching buck converter with a dual-mode control scheme. A pulse-train (PT) control is employed for fast and smooth transient response, where four different pulse patterns are proposed to achieve optimal regulations at different transient operation scenarios. In addition, a high-frequency pulse-width modulation (PWM) control is adopted to ensure low output ripples in the steady state. The two controls are seamlessly incorporated in a single controller. The converter was designed and fabricated with a standard 0.35-mum digital CMOS N-well process, with the entire active die area of 1.31 mm2. Experimental results show that, in the steady state, the output voltage is well regulated at 1.5 V with plusmn12.5-mV voltage ripples in the PWM mode. The nominal input supply voltage is 3.3 V and the maximum load power is 517 mW. Measured maximum efficiency is 91%, while the converter keeps the efficiency above 70% from 20 mW to 517 mW. In transient measurements, with a 100% load step change from 50 to 100 mA, the output of the converter settles within 350 ns due to effectiveness of the PT control, with a maximum voltage variation of 140 mV. The converter functions well when the supply voltage frequently varies between 2.2 and 3.3 V, with a line regulation of 29.1 mV/V. The research provides an effective solution to high- performance integrated power converter designs.