The LM5181 is a flyback dc/dc converter with integrated 100-V, 0.75-A N-channel power MOSFET. During the MOSFET on-time, the transformer primary current increases from zero with a slope of V_IN / L_MAG (where L_MAG is the transformer primary-referred magnetizing inductance) while the output capacitor supplies the load current. When the MOSFET is turned off by the control logic, the SW voltage V_SW swings up to approximately V_IN + (N_PS × V_OUT), where N_PS = N_P / N_S is the primary-to-secondary turns ratio of the transformer. The magnetizing current flows in the secondary side through the flyback diode, charging the output capacitor and supplying current to the load. Duty cycle D is defined as t_ON / t_SW, where t_ON is the MOSFET conduction time and t_SW is the switching period.

Figure 8-1 shows a typical schematic of the LM5181 PSR flyback circuit. Components denoted in red are optional depending on the application requirements.

Figure 8-1 LM5181 Flyback Converter Schematic (Optional Components in Red)