7.3.7 Soft Start

The soft-start feature allows the regulator to gradually reach the initial steady-state operating point, thus reducing start-up stresses and surges. The internal 10-µA soft-start current source gradually charges an external soft-start capacitor connected to the SS pin. The SS pin is connected to the positive input of the internal error amplifier. The error amplifier controls the pulse-width modulator such that the FB pin approximately equals the SS pin as the SS capacitor is charged. Once the SS pin voltage exceeds the internal 1.23-V reference voltage, the error amp is controlled by the reference instead of the SS pin. The SS pin voltage is clamped by an internal amplifier at a level of 150 mV above the FB pin voltage. This feature provides a soft-start controlled recovery in the event a severe overload pulls the output voltage (and FB pin) well below normal regulation but doesn’t persist for 256 clock cycles.

Various sequencing and tracking schemes can be implemented using external circuits that limit or clamp the voltage level of the SS pin. The SS pin acts as a noninverting input to the error amplifier anytime SS voltage is less than the 1.23-V reference. In the event a fault is detected (overtemperature, VCC undervoltage, hiccup current limit), the soft-start capacitor will be discharged. When the fault condition is no longer present, a new soft-start sequence will begin.