SLUS829G August 2008 – February 2020 UCC2897A
This input monitors the incoming-power source to provide an accurate undervoltage-lockout function with user-programmable hysteresis for the power supply controlled by the UCC2897A. The unique property of the UCC2897A is to use only one pin to implement these functions without sacrificing on performance. The input voltage of the power supply is scaled to the precise 1.27-V threshold of the undervoltage-lockout comparator by an external-resistor divider (RIN1, RIN2 in ). Once the input threshold of the line monitor is exceeded, an internal-current source gets connected to the LINEUV pin. The current generator is programmed by the RDEL resistor connected to pin 1 of the controller. The actual current level is given in Equation 8.
As this current flows through RIN2 of the input divider, the undervoltage-lockout hysteresis is a function of IHYST and RIN2 allowing accurate programming of the hysteresis of the line-monitoring circuit. When LINEUV is detected, PWM follows VSS capacitor discharge and soft-stop function is provided. The soft-start capacitor starts discharging when the soft-start capacitor voltage reaches 2.5 V. Both OUT and AUX stop switching while the soft-start capacitor continues discharging until the voltage reaches 0.5 V when the soft start is resumed on the assumption of all other soft-start conditions are met.
For more information on how to program the line-monitoring function refer to the of this datasheet.