The voltage applied to the VIN pin can vary within the range of 3.1 V to 75 V. The current into the VIN pin depends primarily on the gate charge of the power MOSFET, the switching frequency, and any external load on the VCC pin. It is recommended to use the filter shown in Figure 8-1 to suppress transients that may occur at the input supply. This is particularly important when VIN is operated close to the maximum operating rating of the LM5002.

When power is applied and the VIN voltage exceeds 2.8 V with the EN pin voltage greater than 0.45 V, the VCC regulator is enabled, supplying current into the external capacitor connected to the VCC pin. When the VIN voltage is between 2.8 V and 6.9 V, the VCC voltage is approximately equal to the VIN voltage. When the voltage on the VCC pin exceeds 6.9 V, the VCC pin voltage is regulated at 6.9 V. In typical flyback applications, an auxiliary transformer winding is connected through a diode to the VCC pin. This winding must raise the VCC voltage above 6.9 V to shut off the internal start-up regulator. The current requirements from this winding are relatively small, typically less than 20 mA. If the VIN voltage is much higher than the auxiliary voltage, the auxiliary winding significantly improves the conversion efficiency. It also reduces the power dissipation within the LM5002. The externally applied VCC voltage must never exceed 14 V. Also the applied VCC must never exceed the VIN voltage to avoid reverse current through the internal VCC to VIN diode shown in the LM5002 Section 7.2.