The soft-start circuit increases the output voltage of the converter gradually until the steady-state programmed output is reached. During soft start, the error amplifier uses the voltage on the soft-start pin as its reference until the SS pin voltage rises above V_REF. Once the voltage at SS pin is above V_REF, the soft-start period is complete. Note that the voltage at SS pin will continue to rise and once it reaches 1 V, the synchronous rectifier outputs of the controller will become active.

A capacitor between the SS pin and AVSS controls the soft-start time of the PWM controller. The following equation can be used to select the capacitor for the desired soft-start time:

where:

• t_SS is the desired soft-start time
• V_REF is voltage reference of 0.613 V (typical)
• I_SS is the soft-start charging current of 2.7 µA (typical)