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This paper proposes a pulse width modulation-pulse skipping modulation (PWM-PSM) dual mode control and high efficiency buck dc-dc converter with 91.7% peak efficiency. A simple but steady PWM-PSM automatic switching module is proposed to improve the conversion efficiency when the load is light and synchronous rectifier is used to boost the conversion efficiency when the load is heavy. At the same time, a new saw tooth waveform generator is proposed to suit both the PWM and PSM. The dc-dc converter has been realized with a 0.18μm TSMC mixed-signal CMOS process and operates at 100 KHz with supply voltage from 4.5V to 5.5V which is the voltage range of USB interface. Measured conversion efficiency is over 80% for load current from 10mA to 600mA when the input voltage is 5V and output voltage is 3.3V.