A buck converter with digital adaptive on-time (AOT) control that achieves a fast load transient response is proposed in this paper. The proposed digital inductor current ramp estimator circuit can not only allow the switching regulator to achieve a fast transient response, but also operate stably without a high cost output voltage sampling analog-to-digital converter (ADC). The digital AOT controller which adjusting the on-time according to the supply voltage and different reference voltage conditions can achieve pseudo-fixed switching frequency in the continuous conduction mode (CCM). And the conversion efficiency under ultralight-load conditions can be improved by working in the discontinuous conduction mode (DCM). The measurement results show that the buck converter can operate under load current between 10 mA and 600 mA for the supply voltage from 2.7V to 4.2 V and the output voltage range from 0.8 V to 1.2 V. The recovery time is 30 μs when the load current varies from 10 mA to 600 mA. The peak efficiency is 94.66 % at 50 mA.