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A fast mode-switching technique between pulse width modulation (PWM) and pulse frequency modulation (PFM) modes is proposed to keep high efficiency over wide load range. Load dependent zero-current detector (ZCD) can effectively speed up the transition from PWM to PFM mode. Especially, the overshoot voltage can be minimized by using the reversed current provided by the ZCD circuit. Furthermore, the undershoot voltage of the transition from PFM to PWM mode can be reduced because the recovery time can be reduced by restoring the output voltage of the error amplifier. Thus, the efficiency of current mode DC-DC buck converter is higher than 88% at light loads and the recovery time is smaller than 35 mus when load current changes from 500mA to 10mA suddenly.