Soft-Start Circuit for Step-Down Switching Regulator

There are very many switching regulator ICs for reducing a dc voltage. In the ‘step-down’ operating mode, an internal switch connects the input voltage to the charging coil L1. The increasing magnetic field stores energy. When the switch opens, coil L1 attempts to force the current to continue to flow. For this purpose, the flyback diode D1 must be present to allow continued current flow. Output capacitor C1 smoothes the output voltage, while the voltage divider R1/R2 feeds back a portion of the output voltage to the sense input (FB) of the IC.