NCP13992
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19
Figure 16. Soft Start Behavior
The Controller Operation during Soft−start Sequence
Evolves as Follows:
The Soft−Start counter is loaded by last Mupper on−time
value at the end of the dedicated startup sequence. The
ON−time counter is released and starts count−up from zero
until the value that is equal to the actual Soft−Start counter
state. The Mupper switch is active during the time when
ON−time counter counts−up. The Maximum ON−time
comparator then changes counting mode of the ON−time
comparator from count−up to count−down. A dead−time is
placed and the Mlower switch is activated till the ON−time
counter reaches zero value. The Soft−Start counter is
incremented by selected increment during corresponding
Mlower on−time period so that the following Mupper switch
on−time is prolonged automatically – the frequency thus
drops naturally. Because the operating frequency of the
controller drops and Mlower DRV signal is used as a clock
source for the Soft−start counter, the soft−start speed starts
to decrease on each (or on each N−th) Mlower driver pulse
(where N is defined by K
SS_INCREMENT
) of switching cycle.
So we have non−linear soft−start that helps to speed up
output charging in the beginning of the soft−start operation
and reduces the output voltage slope when the output is close
to the regulation level. The output bus of the Soft−Start
counter addresses the D/A converter that defines the
ON−time comparator reference voltage. This reference
voltage thus also increases non−linearly from initial zero
level until the level at which the current mode regulation
starts to work. The on−time of the Mupper and Mlower
switch is then defined by the ON−time comparator action
instead of the Maximum ON−time comparator. The
soft−start then continues until the regulation loop is closed
and the on−time is fully controlled by the secondary
regulator. The Soft−Start counter then continues in counting
and saturates at its maximum possible value which
corresponds to IC minimum operating frequency. The
maximum on−time fault detection system is enabled when
Soft−Start counter value is equal to t
TON_MAX
value.
The previous on−time repetition feature, described above
in the ON−time modulation and feedback loop chapter, is
disabled in the beginning of soft start period. This is because
the ON−time comparator output stays high for several cycles
of soft start period – until the current mode regulation takes
over. The previous on−time repetition feature is enabled
once the current modulation starts to work fully, i.e. in the
time when the ON−time comparator output periodically
drops to low state within actual Mupper switch on−time
period. Typical startup waveform of the LLC application
driven by NCP13992 controller can be seen in Figure 17.
Figure 17. Application Startup with NCP13992 −
Primary Current − Green, V
out
− Magenta
