NCP1238
http://onsemi.com
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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
CC
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
FB(fold)
, typically corresponding to a load of
16% of the maximum load (for a DCM design), the
switching frequency starts to decrease down to
f
OSC(min)
. By lowering the switching 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
skip(in)
, typically corresponding to a load of 1%
of the maximum load, the controller enters Skip mode.
By completely stopping the switching while the
feedback voltage is below V
skip(out)
, the losses are
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
SSKIP
duration while
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
needed for regulation is directly applied. V
CC
can be
maintained between V
CC(on)
and V
CC(min)
by the DSS.
• Overload: The NCP1238 features a timer−based
overload detection, solely dependent on the feedback
information: as soon as the internal peak current
setpoint hits the V
ILIM
clamp, the internal overload
timer starts to count. When the timer times out, the
controller stops and enter the protection mode,
autorecovery for the B version (the controller initiates a
new start−up after t
autorec
elapses), or latched for the A
version (the latch is released if a brown−out event
occurs or V
CC
is reset).
• Brown−out: The NCP1238 features on its HV pin a
true AC line monitoring circuitry which includes
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
CC
decreases below V
CC(reset)
.
