SLUSBQ5D November 2013 – July 2016 UCC28180
The UCC28180 operates from an external bias supply. It is recommended that the device be powered from a regulated auxiliary supply. (This device is not intended to be used from a bootstrap bias supply. A bootstrap bias supply is fed from the input high voltage through a resistor with sufficient capacitance on VCC to hold up the voltage on VCC until current can be supplied from a bias winding on the boost inductor. For that reason, the minimal hysteresis on VCC would require an unreasonable value of hold-up capacitance.)
During normal operation, when the output is regulated, current drawn by the device includes the nominal run current plus the current supplied to the gate of the external boost switch. Decoupling of the bias supply must take switching current into account in order to keep ripple voltage on VCC to a minimum. A ceramic capacitor of 0.1-µF minimum value from VCC to GND with short, wide traces is recommended.
The device's bias operates in several states. During startup, VCC Under-Voltage LockOut (UVLO) sets the minimum operational DC input voltage of the controller. There are two UVLO thresholds. When the UVLO turn-on threshold is exceeded, the PFC controller turns ON. If the VCC voltage falls below the UVLO turn-off threshold, the PFC controller turns off. During UVLO, current drawn by the device is minimal. After the device turns on, Soft Start (SS) is initiated and the boost inductor current is ramped up in a controlled manner to reduce the stress on the external components and avoids output voltage overshoot. During soft start and after the output is in regulation, the device draws its normal run current. If any of several fault conditions are encountered or if the device is put in standby with an external signal, the device draws a reduced standby current.