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The growth of IoT applications requires DC-DC converters to have small printed circuit board (PCB) areas and high efficiency over a wide load range. A multi-mode operation of DC-DC converters, therefore, is often necessary to achieve this high efficiency over a wide load range. This paper introduces a tri-mode DC-DC buck converter for IoT applications. The tri-mode operation with PWM, PFM, and multiple-sawtooth PWM (MSPWM) modes helps to retain the high efficiency over the wide load range. The MSPWM mode also enhances a noise margin of the converter, ensuring a regular switching operation at an ultra-light load condition. With a load current prediction scheme, the proposed tri-mode converter requires neither additional load current sensor nor multiple mode controllers. In addition, a soft-start operation is employed to prevent an in-rush current. The proposed tri-mode buck converter has been fabricated through a 0.18- $\mu \text{m}$ CMOS process with a chip area of 0.97 mm $\times0.88$ mm. A load-dependent seamless mode transition is realized, and a maximum converter efficiency of 91% has been measured with a 7.4-MHz switching frequency. [ABSTRACT FROM AUTHOR]