9.3.1.8 Pin 8 (REF)

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. When VCC is greater than 1 V and less than the UVLO threshold, REF is pulled to ground through a 5-kΩ resistor. This means that REF can be used as a logic output indicating power system status. It is important for reference stability that REF is bypassed to GND with a ceramic capacitor as close to the pin as possible. An electrolytic capacitor may also be used in addition to the ceramic capacitor. A minimum of 0.1-μF ceramic capacitor is required. Additional REF bypassing is required for external loads greater than 2.5 mA on the reference. To prevent noise problems with high-speed switching transients, bypass REF to ground with a ceramic capacitor close to the IC package.

The UCC2802-Q1 and UCC2804-Q1 have a 5-V reference. The UCC2803-Q1 and UCC2805-Q1 have a 4-V reference; both ±1.5% versus ±2% on the UC3842-Q1 family. The output short-circuit current is lower 5 mA versus 30 mA. REF must be bypassed to ground with a ceramic capacitor to prevent oscillation and noise problems. REF can be used as a logic output; as when VCC is lower than the UVLO threshold, REF is held low.