A novel integrated digital controller for buck converter based on the direct system-variable calculation is presented. Compared with linear compensators, the discrete-time direct controller is conceived to generate an additional control variable during each switching period that increases the convergence rate when system states change. An all-digital current estimator is adopted to eliminate the use of current sensor. Experimental results verify closed-loop operation at switching frequency up to 4MHz. The dynamic response time for a load current step-up from 0.3A to 0.45A is 23.8μs, and 21.4μs for a step-down from 0.45A to 0.3A. An efficiency of over 75% is maintained over a power range from 29.3mW to 1.12W in which the maximum efficiency of 93.2% is achieved at 1.88V output.