4.2 Soft-Start

Soft-start reduces the power supply input surge current at start-up by controlling the output voltage rise time. The input surge appears while the output capacitor is charged up. A slower output rise time will draw a lower input surge current.

The MIC26400 implements an internal digital soft-start by making the 0.8V reference voltage, V_REF, ramp from 0% to 100% in about 6ms with 9.7mV steps. Therefore, the output voltage is controlled to increase slowly by a stair-case V_FB ramp. Once the soft-start cycle ends, the related circuitry is disabled to reduce current consumption. VDD must be powered up at the same time or after VIN to make the soft-start function correctly.