A theoretical analysis of two-phase fully-on-chip step-down switched-capacitor DC-DC converters is presented. It is based on a universal model of calculating the output voltage and the efficiency of converters with conversion ratios (N-l)/N. Both the top- and bottom-plate parasitic capacitors of each flying capacitors are included. Simulation results show that the errors of the computed output voltage and maximum efficiency of the model are less than 0.7% and 3%, when N is 2, 3, 4 and 5, respectively.