This paper analyses the control of a DC-DC boost converter using both instantaneous and averaged models. The control objectives are discussed and the main focus is the load disturbance rejection. The transient performance is shown to be limited by saturation constraints on the control variable. In particular, it is shown that the transient behaviour of the output voltage in the presence of load disturbances cannot be minimized beyond a certain limit by the control strategy. Simulation results obtained using two different control laws put in evidence these limitations.