NCP1237 ONSEMI [ON Semiconductor], NCP1237 Datasheet - Page 9

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

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Normal operation: As long as the feedback voltage is
within the regulation range, the NCP1237 runs at a
fixed frequency (with jittering) in current−mode
control, where the peak current (sensed on the CS pin)
is set by the voltage on the FB pin. A fixed ramp
compensation is applied internally to prevent
sub−harmonic oscillations from occurring. The V
must be supplied by an external source (such as an
auxiliary winding), as the startup current source cannot
permanently supply the controller without overheating.
Light load operation: When the FB voltage decreases
below V
20% (DCM design only) to 30% (CCM / DCM design)
of the maximum load, the switching frequency starts to
decrease down to f
losses, this feature helps to improve the efficiency in
light load conditions. The frequency jittering is disabled
in light load operation.
No load operation: When the FB voltage decreases
below V
of the maximum load, the controller enters Skip mode.
By completely stopping the switching while the
feedback voltage is below V
further reduced, allowing to minimize the power
dissipation under extremely low load conditions. In
order to avoid audible noise, the peak current is
gradually increased during the t
exiting the skip mode (Soft−Skip function). In case of
abrupt load increase during Soft−Skip mode, the
soft−skip portion is bypassed and the peak current
FB(fold)
skip(in)
, typically corresponding to a load of 1%
, typically corresponding to a load of
OSC(min)
. By lowering the switching
skip(out)
SSKIP
, the losses are
duration while
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CC
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needed for regulation is directly applied. V
maintained between V
Overload: The NCP1237 features a timer−based
dual−level overload detection, solely dependent on the
feedback information: as soon as the internal peak
current setpoint goes above the V
first internal timer starts to count, but the controller is
still able to regulate up to V
V
When either timer times out, the controller stops and
enters the protection mode, autorecovery for the B
version (the controller initiates a new start−up after
t
is released if a brown−out event occurs or V
Brown−out: The NCP1237 features on its HV pin a
true AC line monitoring circuitry which includes a
minimum startup threshold, brown−out protection, and
overvoltage protection. All of these circuits are
autorecovery and operate independently of any ripple
on the input voltage. They can even work with an
unfiltered, rectified AC input. All thresholds are fixed,
but they are designed to fit most of the standard ac−dc
conversion applications.
Latch−off: When the Latch input is pulled up (typically
by an overvoltage condition), or pulled low (typically
by an overtemperature condition, using the provided
current source with an NTC), the controller latches off.
The latch is released when a brown−out condition
occurs, or when V
autorec
ILIM
clamp, the internal overload timer starts to count.
elapses), or latched for the A version (the latch
CC
decreases below V
CC(on)
ILIM
and V
. Once it reaches the
CS(tran)
CC(min)
CC(reset)
threshold, a
by the DSS.
CC
CC
can be
is reset).
.