NCP1271P100G Datasheet, PDF (12/21 Pages) ON Semiconductor – Soft-Skip Mode Standby PWM Controller with Adjustable Skip Level and External Latch

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.
During this mode, V_CC 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.,
V_bulk discharges).
Figure 25 shows a timing diagram of the V_CC double
hiccup operation. Note that at each restart attempt, a soft
start is issued to minimize stress.
Supply voltage, VCC
12.6 V
9.1 V
5.8 V
Drain current, ID ^tstartup
time
time
Switching is missing in
every two V_CC hiccup cycles
featuring a “double−hiccup”
Figure 25. V_CC Double Hiccup Operation in a Fault
Condition
V_CC Capacitor
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_CC drops below V_CC(off) (9.1 V typical).
Therefore, to take advantage of these features, the V_CC
capacitor needs to be sized so that operation can be
maintained in the absence of the auxiliary winding for at
least 130 ms.
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.
tstartup
+
CVCCDV
ICC1
+
85
mF
·
(12.6 V−9.1
2.3 mA
V)
+
130
ms
(eq. 1)
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
can supply V_CC. Figure 26 describes different startup
scenarios with different V_CC capacitor values. If the V_CC
cap is too small, the application fails to start because the
bias supply voltage cannot be established before V_CC is
reduced to the V_CC(off) level.
V_out
12.6 V
V_CC
9.1 V
0.6 V
t _startup
time
Output waveforms with a large enough V_CC capacitor
Desired level of V_out
12.6 V
9.1 V
V_CC
5.8 V
0.6 V
V_out
time
Output waveforms with too small of a V_CC capacitor
Figure 26. Different Startup Scenarios of the
Circuits with Different V_CC Capacitors
It is highly recommended that the V_CC capacitor be as
close as possible to the V_CC 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
enough, it could potentially cause V_CC to go below V_CC(off)
and force a restart of the controller.
It is also recommended to have a margin between the
winding bias voltage and V_CC(off) so that all possible
transient swings of the auxiliary winding are allowed. In
standby mode, the V_CC voltage swing can be higher due to
the low−frequency skip−cycle operation. The V_CC
capacitor also affects this swing. Figure 27 illustrates the
possible swings.
Supply voltage, VCC
Feedback pin voltage, VFB
Drain current, ID
9.1 V
time
Vskip
time
time
Figure 27. Timing Diagram of Standby Condition
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