REF is the voltage reference for the error amplifier and also for many other functions on the IC. REF is also used as the logic power supply for high speed switching logic on the IC. The UCCx813-[0,1,2,4] have a 5-V reference and the UCCx813-[3,5] have a 4-V reference. Both have ±1.5% accuracy at 25°C versus ±2% in the UC3842 family. The REF output short-circuit current is lower at 5 mA, compared to 30 mA in the UC3842 family.
For reference stability and to prevent noise problems with high speed switching transients, it is important to bypass REF to GND with a ceramic capacitor as close to the pins as possible. A minimum of 0.1-µF ceramic is required. Additional REF bypassing is required for external loads greater than 2.5 mA on the reference. An electrolytic capacitor can also be used in addition to the ceramic capacitor.
When VCC is greater than 1 V and less than the UVLO on-threshold, REF is internally pulled to ground through a 5-kΩ resistor which means that REF can be used as a logic output indicating power-system status.