NCP13992
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11
of the PFC stage during light load or fault mode operation.
This technique further reduces the no−load power
consumption down again since the power losses of voltage
divider are not affected by the bulk voltage at all.
Please refer to Figure 23 through Figure 25 for an
illustration of NCP13992 Brown−out protection system in
all operating conditions/modes.
The VBULK/PFC FB pin voltage is also used by Line
Feed Forward block (LFF). Please refer to ON−time
modulation and feedback loop block description for more
information about LFF function.
Over−voltage and Over−temperature Protection
The OVP/OTP pin is a dedicated input to allow for a
simple and cost effective implementation of two key
protection features that are needed in adapter applications:
over−voltage (OVP) and over−temperature (OTP)
protections. Both of these protections can be either latched
or auto−recovery– depending on the version of NCP13992.
The OVP/OTP pin has two voltage threshold levels of
detection (V
OVP
and V
OTP
) that define a no−fault window.
The controller is allowed to run when OVP/OTP input
voltage is within this working window. The controller stops
the operation, after filter time delay, when the OVP/OTP
input voltage is out of the no−fault window. The controller
then either latches−off or or starts an auto−recovery timer −
depending on the IC version − and triggered the protection
threshold (V
OTP
or V
OVP
).
The internal current source I
OTP
allows a simple OTP
implementation by using a single negative temperature
coefficient (NTC) thermistor. An active soft clamp
composed from V
clamp
and R
clamp
components prevents the
OVP/OTP pin voltage from reaching the V
OVP
threshold
when the pin is pulled up by the I
OTP
current. An external
pull*up current, higher than the pull*down capability of
the internal clamp (V
CLAMP_OVP/OTP
), has to be applied to
pull the OVP/OTP pin above V
OVP
threshold to activate the
OVP protection. The t
OVP_FILTER
and t
OTP_FILTER
filters
are implemented in the system to avoid any false triggering
of the protections due to application noise and/or poor
layout.
Figure 6. Internal Connection of OVP/OTP Input
The OTP protection could be falsely triggered during
controller startup due to the external filtering capacitor
charging current. Thus the t
BLANK_OTP
period has been
implemented in the system to overcome such behavior. The
OTP comparator output is ignored during t
BLANK_OTP
period. In order to speed up the charging of the external
filtering capacitor C
OVP_OTP
connected to OVP/OTP pin,
the I
OTP
current has been doubled to I
OTP_BOOST
. The
maximum value of filtering capacitor is 100 nF.
The OVP/OTP ON signal is set after the following events:
• the V
CC
voltage exceeds the V
CC_ON
threshold during
first start−up phase (after VCC pin voltage was below
V
CC_RESET
threshold)
• BO OK signal is received from BO block
• Auto−recovery timer elapsed and a new restart occurs
• IC returns to operation from skip−mode (V
FB_SKIP_IN
+
V
FB_SKIP_HYST
threshold was reached)
The I
OTP
current source is disabled when:
• V
CC
falls below V
CC_OFF
threshold
• BO OK signal goes to low state (i.e. Brown−out
condition occurs on the mains)
• Fault signal is activated (Auto−recovery timer starts
counting or Latch fault is present)
• IC goes into the skip−mode operation (V
FB_SKIP_IN
threshold was reached)
IC option that keeps OVP/OTP block working during skip
mode is also available. The IC consumption is increased for
this version by OVP/OTP block bias.
The latched OVP or OTP versions of NCP13992 enters
latched protection mode when V
CC
voltage cycles between
V
CC_ON
and V
CC_OFF
thresholds and no pulses are provided
by drivers. The controller VCC pin voltage has to be cycled
down below V
CC_RESET
threshold in order to restart
operation. This would happen when the power supply is
unplugged from the mains.
