A step-down multi-mode switching DC-DC converter for battery-powered portable IoT products is proposed, and it was implemented by using a 0.35-μm CMOS mixed-signal 2P4M polycide 5V/3.3V process. The load capacitor at converter output can be removed from the dynamic circuit which helps during transients. The output stage is designed with both the static and dynamic controls to suppress the possible large overshoot/undershoot voltage during load change. Moreover, a novel adaptive switching frequency (ASF) technique is also proposed in this work to optimize the converter performance at different input voltages. There are three operational modes in the proposed regulator, and, thus, a wide input voltage range, 2.1-5 V, is achieved. The output voltage is set to 1.8 V, and the maximal load current is 100mA. The measured conversion efficiency may go up to 87.6%.