A 40-mA buck regulator operating in the inherently stable Discontinuous Conduction Mode (DCM) for the entire load range is presented. A pulse frequency modulation control scheme is implemented using a proposed hysteretic-assisted adaptive minimum-on-time controller to automatically adapt the regulator to a wide range of operating scenarios in terms of input, output, and passive component values while ensuring compensationless DCM operation with minimized inductor peak current. Thus, compact silicon area, low quiescent current, high efficiency, and robust performance across all possible scenarios can be achieved without any calibration. Moreover, power gating is employed in the analog circuits of the proposed controller to further improve efficiency at sub-mA loads. The regulator is integrated within a low-power microcontroller in 90-nm CMOS to power its digital core while allowing maximum flexibility in the powering options of the microcontroller and the choice of the passive components. It occupies 0.1 mm 2 and achieves 92% peak efficiency, and 78.5% and 86% efficiency at 200-μA and 40-mA loads, respectively. It handles an input in the range of 1.8-4.2 V, an output in the range of 0.9-1.4 V, an inductor in the range of 4.7-10 μH, and an output capacitor in the range of 2.2-10 μF without any calibration or reoptimization.