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| NCL30105 Datasheet, PDF (12/22 Pages) ON Semiconductor – Constant Off Time PWM Current-Mode Controller | |||
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VCC
VCC(on)
VCC(off)
Internal
timer
VDRV
NCL30105
tstart(delay)
reset
reset
VSSTART
VCS
SoftâStart
Sequence
VSSTART(open)
VSSTART = VILIM * Iratio
User changes
the setpoint
VILIM
SoftâStart
Sequence
Constant Peak Variable Peak
Figure 22. A Typical Startup Sequence
SoftâStart Pin
The softâstart internal section is shown in Figure 23. The
softâstart sequence is implemented using a current source
that charges an external capacitor. The relationship between
the capacitor voltage and the peak current voltage set point
is Iratio. The maximum peak current set point is
VSSTART/Iratio. For luminosity balancing purposes, it is
possible to force the voltage on the SSTART pin from an
external source. When forcing VSSTART with an external
source, the current into the SSTART pin must be limited to
ensure that the maximum current rating is not exceeded. It
is recommended to set VSSTART by connecting a diode as
shown in Figure 23. Using this configuration, the SSTART
capacitor value to set a 15 ms softâstart duration (tSSTART)
is calculated using Equation 1:
(eq. 1)
CSSTART
+
ISSTART @ tSSTART
Iratio
+
20
m
@ 15
3
m
+
0.1
mF
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