The UCx84xA devices feature undervoltage lockout protection circuits for controlled operation during power-up and power-down sequences. Undervoltage lockout thresholds for the UCx842A, UCx843A, UCx844A, and UCx845A devices are optimized for two groups of applications: off-line power supplies and DC-DC converters. With a wider VCC_ON to VCC_OFF range, the UCx842A and UCx844A devices are ideally suited to off-line AC input applications. The UCx843A and UCx845A controllers have a much narrower VCC_ON to VCC_OFF hysteresis and may be used in DC to DC applications where the input is considered regulated.

During UVLO the IC draws typically 0.3 mA of supply current. This VCC current is considerable less than the UCx84x family and 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. Lowering this by 50% in the UCx84xA version family, as compared to the UCx84x family, reduces the resistors power loss by the same percentage. Once crossing the turnon threshold the IC supply current increases typically to about 11 mA, During undervoltage lockout, the UCx84xA series of devices prevent 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 10 mA, at saturation voltages as low as 0.7 V, compared to the UCx84x devices which would only sink up to 0.2 mA under the same conditions.

Figure 7-3 Undervoltage Lockout