Photovoltaic systems are increasingly used as renewable energy sources, thus it is necessary to implement control algorithms that strive to obtain maxim power output. Consequently, these algorithms imply that a DC-DC converter is used as a subsystem in controlling the output voltage. In this context, the paper proposes a mathematical model of a DC-DC BOOST converter for a PV panel that can be used in designing conventional control algorithms. The model is developed with the State Space Averaging Method and also the parameters for the electronic components are chosen accordingly to be compatible with the PV module. Also, a digital PI controller is designed for the subsystem considering the developed linear approximation and the voltage quality requirements. The control structure, as well as the BOOST converter were modelled and validated using Matlab/Simulink. The digital controller performances respects the imposed energy/voltage quality limits for a variable voltage reference.