This study investigates the problem of overlarge current protection for a DC–DC synchronous buck converter with the existence of uncertainties and disturbances. Aiming to deal with the hardware damage in the electric circuit of a DC–DC buck that may be caused by overlarge transient current, a new nonsmooth current-constrained control (NCC) algorithm is proposed to replace the traditional ones, which use conservative coefficients to satisfy current constraint, leading to a sacrifice of dynamic performance. Based on the homogeneous system technique, a nonsmooth state feedback controller is improved by adding a penalty term that prompts the adaptive gain of the controller according to the inductor current and current constraint. Then by using two finite-time extended state observers (FTESO), the unmatched disturbances and matched disturbances can be compensated to enhance the robustness of the DC–DC synchronous buck converter. The effect of proposed scheme has been verified by experimental results.