8.3.1.16 VIN

The UCC2897A controller is equipped with a high-voltage N-channel-JFET startup device to initiate operation from the input-power source of the converter in applications where the input voltage does not exceed the 110-V maximum rating of the startup transistor. In these applications, the VIN pin connects directly to the positive terminal of the input-power source. The internal-JFET startup transistor provides charge-current for the energy-storage capacitor (C_BIAS) connected across the VDD (pin 14) and PGND (pin 11) terminals. Note that the startup device turns off immediately when the voltage on the VDD pin exceeds approximately 12.7 V, the undervoltage-lockout threshold of the controller for turn-on. The JFET is also disabled at all times when the high-current gate drivers are switching to protect against excessive-power dissipation and current through the device. For dependable start-up, VDD must not be loaded by more than 4 mA.

For more information on biasing the UCC2897A, refer to the and sections of this datasheet.