When designing with the LM6x4xx, slowed rise in output voltage due to recovery from dropout and soft start must be considered separate phenomena. Soft start is triggered by any of the following conditions:

• EN is used to turn on the device.
• Recovery from a hiccup waiting period; see Section 8.3.13.
• Recovery from shutdown due to overtemperature protection
• Power is applied to the VIN of the IC or the VCC UVLO is released.

Once soft start is triggered, the IC takes the following actions:

• The reference used by the IC to regulate output voltage is slowly ramped from zero. The net result is that output voltage, if previously 0 V, takes t_SS to reach 90% of its desired value.
• Operating mode is set to auto, activating diode emulation. This allows start-up without pulling output low if there is a voltage already present on the output.
• Hiccup is disabled for the duration of soft start; see Section 8.3.13.

All of these actions together provide start-up with limited inrush currents. They also allow the use of output capacitors and loading conditions that cause current to border on current limit during start-up without triggering hiccup. In addition, if output voltage is already present, output is not pulled down. See Figure 8-13.

The left curves show soft start from 0 V. The right curves show soft starting behavior from a pre-biased or non-zero voltage. In either case, the output voltage reaches within 10% of the desired setpoint t_SS time after soft start is initiated. During soft start, FPWM and hiccup are disabled. Both hiccup and FPWM are enabled once output reaches regulation or t_SS2, whichever happens first.

Figure 8-13 Soft-Start Operation

Any time output voltage is more than a few percent low for any reason, output voltage ramps up slowly. This condition, called recovery from dropout, differs from soft start in three important ways:

• Hiccup is allowed only if output voltage is less than 0.4 times its set point. Note that during dropout regulation itself, hiccup is inhibited. See Section 8.3.13.
• FPWM mode is allowed during recovery from dropout. If output voltage were to suddenly be pulled up by an external supply, the LM6x4xx can pull down on the output. Note that all the protections that are present during normal operation are in place, protecting the device if output is shorted to a high voltage or ground.
• The reference voltage is set to approximately 1% above that needed to achieve the current output voltage. It is not started from zero.

Despite the name, recovery from dropout is active whenever output voltage is more than a few percent lower than the setpoint for long enough that:

• Duty factor is controlled by minimum on time or
• When the part is operating in current limit.

• Dropout: When there is insufficient input voltage for the desired output voltage to be generated. See Section 8.4.3.5.
• Overcurrent that is not severe enough to trigger hiccup or if the duration is too short to trigger hiccup. See Section 8.3.13.

Whether output voltage falls due to high load or low input voltage, once the condition that causes output to fall below its setpoint is removed, output climbs at the same speed as during start-up. Even though hiccup does not trigger due to dropout, it can, in principal, be triggered during recovery if output voltage is below 0.4 times output the setpoint for more than 128 clock cycles during recovery.

Figure 8-14 Recovery From Dropout