When designing with the LMR436x0-Q1, slow rise in
output voltage due to recovery from dropout and soft start should be considered as a
two separate operating conditions, as shown in Figure 8-10 and Figure 8-11. Soft start is triggered by any of the following conditions:
- Power is applied to the VIN pin of the device, releasing undervoltage lockout.
- EN is used to turn on the device.
- 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 up. The net result is that output voltage, if previously 0 V, takes tSS to reach 90% of the desired value.
- Operating mode is set to auto mode of operation, activating the diode emulation mode for the low-side MOSFET. This allows start-up without pulling the output low. This is true even when there is a voltage already present at the output during a pre-bias start-up.
Figure 8-10 Soft Start With and Without Pre-bias Voltage