When designing with the LM6x4xx-Q1, 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 8.3.13.
- Recovery from shutdown due to overtemperature protection
- Power is applied to the VIN of the IC or the VCC UVLO is released.
Once 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
- Hiccup is disabled for the duration of soft start; see Section 8.3.13.
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 8-13.
The left curves show soft start
from 0 V. The right curves show soft starting behavior from a pre-biased or
non-zero voltage. In either case, the output voltage reaches within 10% of the
desired setpoint tSS time after soft start is initiated. During soft
start, FPWM and hiccup are disabled. Both hiccup and FPWM are enabled once
output reaches regulation or tSS2, whichever happens first.Figure 8-13 Soft-Start Operation
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 8.3.13.
- FPWM mode is allowed during recovery from dropout. If output voltage were to
suddenly be pulled up by an external supply, the LM6x4xx-Q1 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
- Duty factor is controlled by minimum 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 22.214.171.124.
- Overcurrent that is not severe enough to trigger hiccup or if the duration is
too short to trigger hiccup. See Section 8.3.13.
Whether output voltage falls due
to high load or low input voltage, once the condition that causes output to fall
below its setpoint is removed, output climbs at the same speed as during
start-up. Even though hiccup does not trigger due to dropout, it can, in
principal, be triggered during recovery if output voltage is below 0.4 times
output the setpoint for more than 128 clock cycles during recovery.Figure 8-14 Recovery From Dropout