In some applications, input voltage can drop below the desired output voltage then recover to a higher value suddenly. With most regulators, the sudden increase in input voltage results in output voltage rising at a rate limited only by current limit until regulation is achieved. As input voltage reaches the desired output voltage, there is overshoot due to wind up in the control loop. This overshoot can be large in applications that have small output capacitors and light loads. Also, large inrush currents can cause large fluctuations on the input line after the regulator starts regulating the output voltage. This action typically requires less current than during this initial inrush.

The LM644A2-Q1 greatly reduces inrush current and overshoot. This reduction is done by engaging the soft-start circuit whenever the input voltage suddenly rises, after dipping low enough to cause the output voltage to droop. To prevent this feature from accidentally engaging, output voltage must fall more than 1% to engage this feature. Also, this feature engages only if operating in dropout or current limit, preventing interference with normal transient response but allowing several percent overshoot while engaging. If output voltage is very close to the desired level, overshoot is reduced by inductor current not having time to rise to a high level before regulation starts.

Figure 7-23 When Output Voltage Falls, Output Voltage Recovers Slowly, Preventing Overshoot and Large Inrush Currents