The LM61440 uses a reference-based soft start that prevents output voltage overshoots and large inrush currents during start-up. Soft start is triggered by any of the following conditions:

• Power is applied to the VIN pin of the IC, releasing UVLO.
• EN is used to turn on the device.
• Recovery from a hiccup waiting period
• Recovery from shutdown due to overtemperature protection

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

• The reference used by the IC to regulate output voltage is slowly ramped. The net result is that output voltage 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 output.

These actions together provide start-up with limited inrush currents and also allow the use of larger output capacitors and higher loading conditions that cause current to border on current limit during start-up without triggering hiccup. See Figure 8-8.

Soft start works with both output voltages starting from 0 V on the left curves, or if there is already voltage on the output, as shown on right. In either case, output voltage must reach within 10% of the desired value t_SS 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 8-8 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 existing output voltage.
• Hiccup is allowed if output voltage is less than 0.4 times its set point. 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 LM61440 can pull down on the output.

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

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

Whether output voltage falls due to high load or low input voltage, once the condition that causes output to fall below its set point is removed, the output climbs at the same speed 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 0.4 times the output set point for more than 128 clock cycles.

Figure 8-9 Recovery From Dropout

Figure 8-10 Recovery From Dropout (V_OUT = 5 V, I_OUT = 4 A, V_IN = 13.5 V to 4 V to 13.5 V)