When designing with the The LMQ644xx, 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 7.3.14.
- Recovery from shutdown due to overtemperature protection
- Power is applied to the VIN of the IC or the VCC UVLO is released.
After 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 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 the
output.
- Hiccup is disabled for the duration of soft start; see Section 7.3.14.
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 7-12.
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 7.3.14.
- FPWM mode is allowed during recovery from dropout. If output voltage were to
suddenly be pulled up by an external supply, the The LMQ644xx 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 maximum on-time or
- When the part is operating in current limit.
This primarily occurs under the following conditions:
- Dropout: When there is insufficient input voltage for the desired output
voltage to be generated. See Section 7.4.3.5.
- Overcurrent that is not severe enough to trigger hiccup or if the duration is
too short to trigger hiccup. See Section 7.3.14.