The converter uses a reference-based soft start that prevents output voltage overshoot and large inrush current during start-up. Soft start is triggered by any of the following conditions:

• Power is applied to the VIN pins of the IC, releasing UVLO.
• EN/SYNC goes high to turn on the device.
• Recovery from a hiccup-waiting period
• Recovery from thermal shutdown protection

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

• The reference used by the IC to regulate the output voltage is slowly ramped. The net result is that the output voltage takes t_SS to reach 90% of its desired value.
• The operating mode is set to AUTO, activating diode emulation. This action allows a pre-biased start-up without pulling the output voltage low if there is a voltage already present on the output.

Together, these actions provide start-up with limited inrush currents and also facilitate the use of high output capacitance and higher loading conditions that cause the peak inductor current to border on current limit during start-up without triggering hiccup. See Figure 8-6.

Soft start functions with the output voltage starting from 0 V in (a), or if there is already a prebiased output as shown in (b). In either case, the output voltage must reach within 10% of the setpoint within t_SS after soft start initiates. FPWM and hiccup are disabled during soft start, with both FPWM and hiccup enabled after the output voltage reaches regulation or after the t_SS2 time interval expires, whichever happens first.

Figure 8-6 Soft-Start Operation

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

• The reference voltage is set to approximately 1% above what is needed to achieve the preset output voltage setpoint.
• Hiccup is allowed if the output voltage is less than 40% of the nominal setpoint. Note that during dropout regulation itself, hiccup is inhibited.
• FPWM mode is allowed during recovery from dropout. If the output voltage were to suddenly be pulled up by an external supply, the converter can pull down on the output.

Despite being called recovery from dropout, this feature is active whenever the output voltage drops to a few percent lower than the setpoint. This action primarily occurs under the following conditions:

• Dropout: When there is insufficient input voltage to maintain the desired output voltage
• Overcurrent: When there is an overcurrent event that is not severe enough to trigger hiccup

Whether the output voltage falls due to high load current or low input voltage, after the condition that causes the output to fall below its setpoint is removed, the output recovers at the same rate as during start-up. Even though hiccup does not trigger due to dropout, it can, in principle, be triggered during recovery if output voltage is below 40% of the output voltage setpoint for more than 128 clock cycles.

Figure 8-7 Recovery From Dropout