This paper focuses on the design of a P-δ I controller for the stabilization of a buck dc/dc converter. The basis of this work is a geometric approach which allows to partition the parameters space of the controllers gain into regions in which the characteristic equation of the closed-loop system has a constant number of unstable roots. The main contribution of the paper is that it provides an explicit tool to find P-δ I gains ensuring the stability of the closed-loop system. In addition, the proposed methodology allows the design of a controller with a desired exponential decay rate σ. In order to illustrate the effectiveness of the proposed controller, several numerical examples are presented. © 2019 IEEE.