This paper presents a global tracking passivity-based proportional-integral (PI) control for output voltage regulation of a dc-dc Buck converter. The proposed controller is based on passivity formulation since dc-dc Buck converter has a passive structure in open-loop. Additionally, the controller takes advantage of the PI actions to design a control law that guarantees asymptotically stability in the Lyapunov's sense under closed-loop operation. The proposed controller does not depend on the parameters, which makes it a robust controller. The robustness of the proposed controller is checked by comparing its dynamical performance in front of a conventional PID controller. All simulation results were fulfilled via MATLAB software.