An integrated low-output-ripple step-down DC-DC converter, which cascades a Buck converter with a low-dropout regulator (LDO), is presented. The output of the Buck converter can be used to supply digital circuits, while the LDO output is used to power analog circuits. The designed Buck converter can adaptively and automatically switch between the pulse-width modulation (PWM) and pulse-frequency modulation (PFM) to improve the conversion efficiency over the whole load current range. The chip was implemented by TSMC 0.35μm 2P4M polycide CMOS process. The measured maximal load current of the Buck converter was 280 mA, and the measured LDO maximal load current was 100 mA.