2-4. Inrush current reduction function of LDO regulators

When the input voltage rises from 0 V and reaches the minimum operating voltage, an LDO regulator turns on to supply a regulated output voltage. At this time, the internal error amplifier drives the output transistor between the V_IN and V_OUT pins with the maximum drive capability to raise the output voltage to the programmed level. Once the output voltage reaches the programmed level, the LDO regulator tries to maintain it at that level. However, the internal circuit delay, for example, might cause the output voltage to overshoot, exceeding the rated voltage of the load IC or circuit.
In addition, a large inrush current flows into the LDO regulator to charge the output smoothing capacitor connected to the V_OUT pin. Inrush current occurs during the period from the time when the LDO regulator turns on to the time when the output voltage stabilizes. If the V_IN pin is connected with a board trace with large impedance, the input voltage might sag because of a voltage drop that occurs across the board trace impedance due to inrush current. Consequently, the LDO regulator is unable to produce an output voltage at a normal level.
The inrush current limiting function prevents these conditions.
After the LDO regulator turns on, it raises the output current slowly while limiting inrush current, thereby suppressing the output voltage overshoot and the input voltage drop. Therefore, inrush current limiting helps improve system stability.
Inrush current limiting is also called slew rate control or soft start.