In this paper, a novel state ACP (adaptive current position) is presented, which allows Buck converter to achieve fast load transient response. The PWM (pulse width modulation) and PFM (pulse frequency modulation) modes increase light-load efficiency and maintain good regulation over a wide load range. When the load rapidly changes from light to heavy, ACP can enhance the transient response, to minimize the settling time and suppresses under/overshoot voltage. Unlike other system, the proposed controller can operate without current-sensor. ACP can force the inductor current to catch up with load current quickly. Switch on high-side power MOS a period of time then change to PWM state. By using the information from pseudo current roof, the exact on-time and off-time at different Vin can be calculated. Also, we can speculate the time to execute ACP without current-sensor. Simulation and experimental results demonstrate the superior dynamic response over that of a conventional digital Buck converter.