UCC28180
SLUSBQ5A –NOVEMBER 2013–REVISED NOVEMBER 2013
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Soft Start
Soft-Start controls the rate of rise of VCOMP in order to obtain a linear control of the increasing duty cycle as a
function of time. VCOMP, the output of the voltage loop transconductance amplifier, is pulled low during UVLO,
ICOMPP, ISOP and OLP (Open-Loop Protection)/STANDBY. Once the fault condition is released, an initial pre-
charge source rapidly charges VCOMP to 1.5 V. After that point, a constant 40 µA of current is sourced into the
compensation components causing the voltage on this pin to ramp linearly until the output voltage reaches 85%
of its final value. At this point, the sourcing current decreases until the output voltage reaches its final rated
voltage. The soft-start time is controlled by the voltage error amplifier compensation capacitor values selected,
and is user programmable based on desired loop crossover frequency. Once the output voltage exceeds 98% of
rated voltage, soft start is over, the initial pre-charge source is disconnected, and EDR is no longer inhibited.
Soft-Start
5V
+
gmv
VCOMP
FAULT
VSENSE
VCOMP
ISS = -40uA
for VSENSE < 4.25V
during Soft-Start
FAULT
END OF SS
(LATCHED)
1.5V source for
rapid pre-charge
of VCOMP prior
to Soft-Start
+
Figure 24. Soft Start
System Protection
System-level protection features help keep the system within safe operating limits.
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