An ultra-low quiescent current dual-mode buck converter system is designed for IoT application, which includes a double clock time (DCT) and a pulse-width-modulation (PWM) control modes. The proposed DCT mode can reduce the conversion loss over a wide loading range from nA-to-mA and achieve seamless mode transition from DCT to PWM control. Implemented in a 0.18μm CMOS, this converter achieves a peak efficiency of 92.7%94.7% in DCT/PWM and >80% efficiency from 10μ A to 50 mA (5000x), with a wide input voltage from 2 V to 5 V. A quiescent current of 470 nA including bandgap voltage reference and internal oscillator is achieved. The DCT-to-PWM mode selection mechanism achieves an undershoot of 80 mV at 11μs recovery time when load current jumps from 6.67μ A to 50 mA.