This work applies unequal duty cycle to switched-capacitor (SC) dc-dc converters to improve its output ripple as well as the power conversion efficiency. By adjusting the duty cycle, the charge and discharge states of the capacitor are reasonably balanced. Therefore, the output ripple is effectively reduced, and the charge redistribution loss is reduced, improving the efficiency. This scheme is verified with an SC converter with voltage conversion ratios (VCRs) of 1/4 and 1/3, and is simulated in a 65nm low-power process. Operating with a 33MHz switching frequency, an input voltage of 2.4V, an output range of 0.525mA, and a VCR of 1/4, the peak efficiency is increased by 1.0% and the output ripple is reduced by 43.6% compared to the equal duty cycle mode.