SNVS480J January 2007 – July 2020 LM5022
The LM5022 contains an internal high-voltage start-up regulator that allows the VIN pin to be connected directly to line voltages as high as 60 V. The regulator output is internally current limited to 35 mA (typical). When power is applied, the regulator is enabled and sources current into an external capacitor, CF, connected to the VCC pin. The recommended capacitance range for CF is 0.1 µF to 100 µF. When the voltage on the VCC pin reaches the rising threshold of 5 V, the controller output is enabled. The controller remains enabled until VCC falls below 4.7 V. In applications using a transformer, an auxiliary winding can be connected through a diode to the VCC pin. This winding must raise the VCC pin voltage to above 7.5 V to shut off the internal start-up regulator. Powering VCC from an auxiliary winding improves conversion efficiency while reducing the power dissipated in the controller. The capacitance of CF must be high enough that it maintains the VCC voltage greater than the VCC UVLO falling threshold (4.7 V) during the initial start-up. During a fault condition when the converter auxiliary winding is inactive, external current draw on the VCC line must be limited such that the power dissipated in the start-up regulator does not exceed the maximum power dissipation capability of the controller.
An external start-up or other bias rail can be used instead of the internal start-up regulator by connecting the VCC and the VIN pins together and feeding the external bias voltage (7.5 V to 14 V) to the two pins.