6

UVLO, SOFT START AND FAULT MANAGEMENT

Soft start is programmed by a capacitor on the SS pin. At

power up, SS is discharged. When SS is low, the error amp

output is also forced low. As the internal 9

µA source

charges the SS pin, the error amp output follows until

closed loop regulation takes over.

Anytime ILIM exceeds 1.2V, the fault latch will be set and

the output pins will be driven low. The soft start cap is then

discharged by a 250

µA current sink. No more output

pulses are allowed until soft start is fully discharged, and

ILIM is below 1.2V. At this point the fault latch will be re-

set and the chip will execute a soft start.

Should the fault latch be set during soft start, the outputs

will be immediately terminated, but the soft start cap will

not be discharged until it has been fully charged. This re-

sults in a controlled hiccup interval for continuous fault con-

ditions.

UC1823A,B/1825A,B

UC2823A,B/2825A,B

UC3823A,B/3825A,B

APPLICATIONS INFORMATION (cont.)

Soft Start and Fault Waveforms

UDG-95106

ACTIVE LOW OUTPUTS DURING UVLO

The UVLO function forces the outputs to be low and con-

siders both VCC and VREF before allowing the chip to

operate.

Output V and I During UVLO

Simplified Schematic

PWM APPLICATIONS

Current Mode

Voltage Mode

UDG-95107

UDG-95108

UDG-95109

UDG-95110