The converter uses a reference-based soft start
that prevents output voltage overshoot and large inrush current
during start-up. Soft start is triggered by any of the following
conditions:
- Power is applied to the VIN pins of the IC, releasing UVLO.
- EN goes high to turn on the device.
- Recovery from a hiccup-waiting period
- Recovery from thermal shutdown protection
After soft start is triggered, the IC takes the
following actions:
- The reference used by the IC to regulate the output voltage is slowly ramped.
The net result is that the output voltage takes
tSS to reach 90% of the desired
value.
- The operating mode is set to AUTO, activating diode emulation. This action
allows a pre-biased start-up without pulling the output
voltage low if there is a voltage already present on the
output.
Together, these actions provide start-up with
limited inrush currents and also facilitate the use of high output
capacitance and higher loading conditions that cause the peak
inductor current to border on current limit during start-up without
triggering hiccup. See Figure 7-14.
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 preset output voltage setpoint.
- Hiccup is allowed if the output voltage is less than 40% of the nominal
setpoint. Note that during dropout regulation, 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 converter can pull down on the
output.
Despite being called recovery from dropout, this
feature is active whenever the output voltage drops to a few percent
lower than the setpoint. This action primarily occurs under the
following conditions:
- Dropout: When there is insufficient input voltage to maintain the desired
output voltage
- Overcurrent: When there is an overcurrent event that is not severe enough to trigger hiccup