This paper presents design and harmonic distortion analysis using Volterra series for a 12 V to 1.2 V buck converter designed in the 180 nm BCD8 technology of STMicroelectronics. The series determines the closed-form equations for fundamental, second and third harmonics. From the analysis, the results are 96 %, 97 % and 90 % matched with the simulation results when the amplitude of the input ripple varies from 0 V to 1.8 V.