NCP1234 ONSEMI [ON Semiconductor], NCP1234 Datasheet - Page 27

• Manufacturer Part Number: NCP1234
• Description: Fixed Frequency Current Mode Controller for Flyback Converters
• Manufacturer: ONSEMI [ON Semiconductor]

Latch−off Input
includes two levels of detection that define a working
window, between a high latch and a low latch: within these
two thresholds, the controller is allowed to run; but as soon
as either the low or the high threshold is crossed, the
controller is latched off. The lower threshold is intended to
be used with an NTC thermistor, thanks to an internal current
source I
high threshold if it is only pulled up by the I
reach the high threshold, the pull−up current has to be higher
than the pull−down capability of the clamp (typically
1.5 mA at V
The Latch pin is dedicated to the latch−off function: it
An active clamp prevents the voltage from reaching the
V
V
V
skip(out)
FB(fold)
skip(in)
Latch
NTC
V
DRV
FB
.
OVP
).
V
1 kW
I
Enters
clamp
NTC
skip
VDD
Soft−start
I
NTC
end
Figure 45. Skip Cycle Timing Diagram
Figure 46. Latch Detection Schematic
NTC
+
current. To
V
http://onsemi.com
Exits
skip
+
OTP
−
+
27
V
OVP
+
−
Enters
(for the high latch and 65 kHz version) or 350 ms (for the low
latch) are blanked and only longer signals can actually latch
the controller.
which in a real application can only happen if the power
supply is unplugged from the AC line.
before being at their nominal values; so one of the
comparators could toggle even if it should not. Therefore the
internal logic does not take the latch signal into account
before the controller is ready to start: once V
V
skip
CC(on)
To avoid any false triggering, spikes shorter than 50 ms
Reset occurs when V
Upon start−up, the internal references take some time
blanking
t
, the latch pin High latch state is taken into account
blanking
Latch(OVP)
t
Latch(OTP)
Reset
CC
is cycled down to a reset voltage,
S
R
Q
Exits
skip
Latch
CC
reaches
Time
Time