The UCx84x devices feature undervoltage lockout protection circuits for controlled operation during power-up and power-down sequences. The UVLO circuit insures that VCC is adequate to make the UCx84x fully operational before enabling the output stage. Undervoltage lockout thresholds for the UCx842, UCx843, UCx844, and UCx845 devices are optimized for two groups of applications: off-line power supplies and DC-DC converters. The 6-V hysteresis in the UCx842 and UCx844 devices prevents VCC oscillations during power sequencing. This wider VCC_ON to VCC_OFF range, make these devices ideally suited to off-line AC input applications. The UCx843 and UCx845 controllers have a much narrower VCC_ON to VCC_OFF hysteresis and can be used in DC to DC applications where the input is considered regulated.

Start-up current is less than 1 mA for efficient bootstrapping from the rectified input of an off-line converter, as illustrated by Figure 7-7. During normal circuit operation, VCC is developed from auxiliary winding N_A with D_BIAS and C_VCC. At start-up, however, C_VCC must be charged to 16 V through R_START. With a start-up current of 1 mA, R_START can be as large as 100 kΩ and still charge C_VCC when V_AC = 90 V RMS (low line). Power dissipation in R_START is then be less than 350 mW even under high line (V_AC= 130 V RMS) conditions.

During UVLO the IC draws less than 1 mA of supply current. Once crossing the turnon threshold the IC supply current increases to a maximum of 17 mA, typically 11 mA, During undervoltage lockout, the output driver is biased to a high impedance state and sinks minor amounts of current. A bleeder resistor, placed between the gate and the source of the MOSFET can be used to prevent activating the power switch with extraneous leakage currents during undervoltage lockout.

Figure 7-5 UVLO Threshold

Figure 7-6 UVLO ON and OFF Profile

Figure 7-7 Providing Power to UCx84x