NCP1237 ONSEMI [ON Semiconductor], NCP1237 Datasheet - Page 28

• Manufacturer Part Number: NCP1237
• Description: Fixed Frequency Current Mode Controller for Flyback Converters
• Manufacturer: ONSEMI [ON Semiconductor]

Latch−off Input
includes two thresholds that define a working window,
between a high latch and a low latch. Within these 2
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, with the internal current source
I
high threshold if it is only pulled up by the I
reach the high threshold, the pullup current has to be higher
than the pulldown capability of the clamp (typically 1.5 mA
at V
t
longer events can actually latch the controller.
Latch(OVP)
NTC
The Latch pin is dedicated to the latch−off function. It
An active clamp prevents the voltage from reaching the
To avoid any false triggering, noise spikes shorter than
OVP
providing the necessary bias current.
Latch
).
or t
Latch(OTP)
V
1 kW
I
clamp
NTC
respectively are blanked, and only
VDD
Soft−start
I
NTC
end
Figure 26. Latch Detection Schematic
Latch
+
current. To
http://onsemi.com
V
+
−
OTP
+
28
V
OVP
+
−
the V
application can only happen if the power supply is
unplugged from the AC line.
before reaching their nominal values; and one of the
comparators could toggle inadvertantly. Therefore the
internal logic ignores the latch signal before the controller is
ready to start. Once V
latch state is enabled and the DRV switching starts only if it
is allowed; whereas the Low latch (typically sensing an
overtemperature) is taken into account only after the
soft−start is finished. In addition, the NTC current is doubled
during the soft−start period, to speed up the charging of the
Latch pin capacitor.
Reset occurs when a brown−out condition is detected or
Upon startup, the internal references take some time
Brown−out
CC
blanking
t
blanking
t
Latch(OVP)
Latch(OTP)
is cycled down to a V
Reset
CC
reaches V
CC(reset)
CC(on)
S
R
Q
, the latch pin High
, which in a real
Latch