In this paper, a fully integrated low-dropout regulator (LDO) using voltage-to-time conversion (VTC) technique is presented for under-1 V supply voltage application. A synchronous VTC technique is proposed using constant-current (CC) charging and discharging to achieve high loop gain. A high-gain charge pump (CP) is proposed to improve power-supply-rejection (PSR). Furthermore, an asynchronous step detection recovery technique is proposed to achieve fast transient response. A frequency-adaptive oscillator is proposed to remove the noise of the clock signal. The proposed LDO is designed in 28-nm process to achieve a droop voltage of 104 mV at load current transient of 90 mA. The proposed LDO achieves PSR of -77 dB at I LOAD =100 mA and PSR of -65 dB at I LOAD =10 mA for 1-kHz supply ripple frequency. The quiescent current is 32 µA and the peak current efficiency is 99.98%.