Some applications have transients that place the LDO into dropout, such as slower ramps on V_IN during start-up. As with other LDOs, the output potentially overshoots on recovery from these conditions. Figure 7-1 shows that a ramping input supply causes an LDO to overshoot on start-up. This condition occurs when the slew rate and voltage levels are in the correct range. Use an enable signal to avoid this condition.

Line transients out of dropout also cause overshoot on the output of the regulator. These overshoots are caused by the error amplifier having to drive the gate capacitance of the pass transistor. Subsequently, the error amplifier then brings the gate back to the correct voltage for proper regulation. Figure 7-2 illustrates what is happening internally with the gate voltage and how overshoot is caused during operation. When the LDO is placed in dropout, the gate voltage (V_GS) is pulled to ground to give the pass transistor the lowest on-resistance possible. However, if a line transient occurs when the device is in dropout, the loop is not in regulation. This condition causes the output to overshoot until the loop responds and the output current pulls the output voltage back down into regulation. If these transients are not acceptable, then continue to add input capacitance in the system until the transient is slow enough to reduce the overshoot.