The LM61440-Q1 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 tSS 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 9-8.
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-Q1 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