7.3.2 Soft Start

The TPS61175 has a built-in soft start circuit which significantly reduces the start-up current spike and output voltage overshoot. When the IC is enabled, an internal bias current (6 μA typically) charges a capacitor (C3) on the SS pin. The voltage at the capacitor clamps the output of the internal error amplifier that determines the duty cycle of PWM control, thereby the input inrush current is eliminated. Once the capacitor reaches 1.8 V, the soft start cycle is completed and the soft-start voltage no longer clamps the error amplifier output. Refer to Figure 16 for the soft start waveform. See Table 3 for C3 and corresponding soft start time. A 47-nF capacitor eliminates the output overshoot and reduces the peak inductor current for most applications.

When the EN is pulled low for 10 ms, the IC enters shutdown and the SS capacitor discharges through a 5-kΩ resistor for the next soft start.