NCP1271
external components during a fault event. After the second
cycle, the controller tries to restart the application. If the
restart is not successful, then the process is repeated.
V
out
During this mode, V never drops below the 4 V latch
reset level. Therefore, latched faults will not be cleared
unless the application is unplugged from the AC line (i.e.,
CC
V
CC
12.6 V
9.1 V
V
bulk
discharges).
t
startup
0.6 V
Figure 25 shows a timing diagram of the V double
CC
time
hiccup operation. Note that at each restart attempt, a soft
start is issued to minimize stress.
Output waveforms with a large enough V capacitor
CC
Desired level of V
out
Supply voltage, V
CC
12.6 V
9.1 V
12.6 V
9.1 V
V
CC
5.8 V
0.6 V
V
out
5.8 V
time
Output waveforms with too small of a V capacitor
time
time
CC
Figure 26. Different Startup Scenarios of the
Circuits with Different VCC Capacitors
t
startup
Drain current, I
D
It is highly recommended that the V capacitor be as
CC
close as possible to the V and ground pins of the product
to reduce switching noise. A small bypass capacitor on this
pin is also recommended. If the switching noise is large
CC
Switching is missing in
every two V hiccup cycles
CC
featuring a “double−hiccup”
Figure 25. VCC Double Hiccup Operation in a Fault
Condition
enough, it could potentially cause V to go below V
and force a restart of the controller.
CC
CC(off)
It is also recommended to have a margin between the
winding bias voltage and V so that all possible
transient swings of the auxiliary winding are allowed. In
V
Capacitor
CC
CC(off)
As stated earlier, the NCP1271 enters a fault condition
when the feedback pin is open (i.e. FB is greater than 3 V)
for 130 ms or V drops below V (9.1 V typical).
standby mode, the V voltage swing can be higher due to
CC
CC
CC(off)
the low−frequency skip−cycle operation. The V
CC
Therefore, to take advantage of these features, the V
CC
capacitor also affects this swing. Figure 27 illustrates the
possible swings.
capacitor needs to be sized so that operation can be
maintained in the absence of the auxiliary winding for at
least 130 ms.
Supply voltage, V
CC
The controller typically consumes 2.3 mA at a 65 kHz
frequency with a 1 nF switch gate capacitance. Therefore,
to ensure at least 130 ms of operation, equation 1 can be
used to calculate that at least an 85 mF capacitor would be
necessary.
9.1 V
time
85 mF · (12.6 V−9.1 V)
C
DV
VCC
I
t
+
+
+ 130 ms
Feedback pin voltage, V
FB
startup
2.3 mA
CC1
(eq. 1)
V
skip
If the 130 ms timer feature will not be used, then the
capacitance value needs to at least be large enough for the
output to charge up to a point where the auxiliary winding
time
time
can supply V . Figure 26 describes different startup
CC
Drain current, I
scenarios with different V capacitor values. If the V
CC
CC
D
cap is too small, the application fails to start because the
bias supply voltage cannot be established before V is
CC
reduced to the V
level.
CC(off)
Figure 27. Timing Diagram of Standby Condition
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