UDG−95106

1.2 V

5 V

1.2 V

0.2 V

ON

OFF

FAULT

PWM

UC2825A-EP

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SGLS305D – JULY 2005 – REVISED SEPTEMBER 2010

Undervoltage Lockout (UVLO), Soft-Start, and Fault Management

Soft-start is programmed by a capacitor on the soft-start (SS) pin. At power up, SS is discharged. When SS is

low, the error-amplifier output also is forced low. While the internal 9-mA source charges SS, the error-amplifier

output follows until closed-loop regulation takes over.

Anytime ILIM exceeds 1.2 V, the fault latch is set and the output pins are driven low. The soft-start cap then is

discharged by a 250-mA current sink. No more output pulses are allowed until soft-start is fully discharged and

ILIM is below 1.2 V. At this point, the fault latch resets and the chip executes a soft-start.

Should the fault latch get set during soft-start, the outputs are terminated immediately, but the soft-start capacitor

does not discharge until it has been fully charged first. This results in a controlled hiccup interval for continuous

fault conditions.

Figure 5. Soft-Start and Fault Waveforms

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