A composite backstepping integral sliding mode controller (BISMC) is proposed in this work for a DC-DC boost converter feeding a constant power load (CPL) in a simple DC distribution system. Here, the main objective is to ensure the large-signal stability with wide variations in operating regions. The proposed BISMC is designed based on the feedback linearized model of the DC-DC boost converter with a CPL where the average state-space model is exactly linearized. The use of exact feedback linearization overcomes the non-minimum phase problem of a DC-DC boost converter due to the presence of the CPL that exhibits negative impedance characteristics. The control law is obtained using the combination of backstepping and integral sliding mode control schemes where both approaches ensure the faster convergence of errors for all states and the integral action on the sliding surface is designed in such a fashion that will ensure the constant frequency operation of the power converter feeding the CPL while eliminating the variable frequency associated problem of the existing controllers. In addition, the stability of the whole system is analyzed through the Lyapunov stability theory. Numerical simulations are conducted to verify the performance of the designed composite nonlinear controller whose performance is also compared with a double-loop proportional integral (PI) controller to verify the superiority of the designed controller.