NCP1246_15 Datasheet, PDF (19/39 Pages) ON Semiconductor – Fixed Frequency Current Mode Controller

NCP1246
For safety reasons, the start−up current is lowered when
V_CC is below V_CC(inhibit), to reduce the power dissipation in
case the V_CC pin is shorted to GND (in case of V_CC capacitor
failure, or external pull−down on V_CC to disable the
controller). There is only one condition for which the current
source doesn’t turn on when V_CC reaches V_CC(inhibit): the
voltage on HV pin is too low (below V_HV(min)).
HV Sensing of Rectified AC Voltage
The NCP1246 features on its HV pin a true ac line
monitoring circuitry. It includes a minimum start−up
threshold and an autorecovery brown−out protection; both
of them independent of the ripple on the input voltage. It is
allowed only to work with an unfiltered, rectified ac input to
ensure the X2 capacitor discharge function as well, which is
described in following. The brown−out protection
thresholds are fixed, but they are designed to fit most of the
standard ac−dc conversion applications.
When the input voltage goes below V_HV(stop), a
brown−out condition is detected, and the controller stops.
The HV current source maintains V_CC at V_CC(min) level until
the input voltage is back above V_HV(start).
VHV
HV timer elapsed
VHV(start)
VHV(stop)
HV stop
VCC
V_CC(on)
V_CC(min)
Brown-out
t_HV
detected
Waits next
VccON before
starting
time
time
DRV
Brown-out
condition
resets the
Internal Latch
time
Figure 49. Ac Line Drop−out Timing Diagram
time
When V_HV crosses the V_HV(start) threshold, the controller
can start immediately. When it crosses V_HV(stop), it triggers
a timer of duration t_HV, this ensures that the controller
doesn’t stop in case of line cycle drop−out.
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