The UC284xL-Q1 device is featured with undervoltage lockout protection circuits for controlled operation during power-up and power-down sequences.

During UVLO the IC draws typically 0.25mA of supply current. This lower VCC current than the UC284xAQ/UC284xA-Q1 families results in lower power drawn from the line. The reduced start-up current is of particular concern in off-line supplies where the IC is powered-up from the high-voltage DC rail, then bootstrapped to an auxiliary winding on the main transformer. Power is then dissipated in the start-up resistor which is sized by the IC’s start-up current. Once crossing the turnon threshold the IC supply current increases typically to about 11mA, During undervoltage lockout, the UC284xL-Q1 prevents the power MOSFET from parasitically turning on due to the Miller effect at power-up. This improved design to the lower totem-pole transistor’s operation during undervoltage lockout allows the IC to sink higher currents, up to 10mA, at saturation voltages as low as 0.7V.

Figure 7-3 Undervoltage Lockout