ncp1282 ON Semiconductor, ncp1282 Datasheet - Page 15

• Manufacturer Part Number: ncp1282
• Description: High-performance Active Clamp/reset Pwm Controller Featuring Hv Start-up
• Manufacturer: ON Semiconductor

immediately discharged and the outputs are disabled. V
should not be pulled up or down externally.
CURRENT LIMIT
cycle-by-cycle and cycle skip, providing the best
protection during momentary and continuous overcurrent
conditions.
Cycle-by-Cycle
voltage on the CS pin reaches the current limit voltage
threshold (V
re-start) the NCP1282 after a cycle skip event is detected.
This can be easily achieved by adding an external latch.
V
If the voltage on the V
The
In cycle-by-cycle, the conduction period ends once the
V
In some instances it may be desired to latch (instead of auto
CSKIP(valley)
CSKIP(peak)
Figure 36. Soft-Stop Behavior After Soft-Start
V
V
V
EA
SS
CSKIP
V
NCP1282
V
ILIM
CS
SS
ILIM
(V
is Complete and V
EA
). The NCP1282 has a V
is not controlling the duty cycle)
has
AUX
pin reaches V
two
EA
FF Ramp
overcurrent
is Open.
ILIM
AUX(off2)
Figure 37. Cycle Skip Waveforms
of 0.5 V.
, C
modes,
http://onsemi.com
SS
NCP1282
SS
is
15
Cycle Skip
been used to trigger a cycle skip fault. Unfortunately, the
fast response time of modern controllers makes it hard to
reach a voltage on the CS pin higher than V
cycle skip fault, the NCP1282 uses a timer. It monitors the
current
cycle-by-cycle current limit condition exists the converter
is disabled. The time to disable the converter and the time
the converter is disabled are programmed by the capacitor
on the CSKIP pin, C
continuous 100 mA current once cycle-by-cycle current
limit fault is detected. If the current limit fault persists,
C
upper threshold (V
converter enters the soft–stop mode and C
discharged with a constant 10 mA current. A new soft-start
sequence commences once C
skip threshold (V
condition is still present, the capacitor starts charging on
the next current limit event. Otherwise, C
discharged down to 0 V.
remembering previous overcurrent conditions. If a
continuous overcurrent condition is removed before
reaching
discharge. If the continuous overcurrent fault is once again
detected before C
charges from its existing voltage level, taking less time to
reach V
during a continuous overcurrent condition. For optimal
operation, the cycle skip discharge time should be longer
than the soft-stop period.
Figures 35 and 36 show an implementation of an integrated
and a discrete latch, respectively. In general the circuits work
by pulling CSKIP to V
CSKIP
Traditionally, a voltage on the CS higher than V
Instead of using a higher voltage threshold to detect a
The cycle skip detection circuit charges C
The cycle skip capacitor provides a means of
CSKIP(peak)
continues to charge until reaching the cycle skip
limit
V
CSKIP(peak)
CSKIP(valley)
comparator
CSKIP
. Figure 37 shows operating waveforms
CSKIP(peak)
CSKIP
, C
is completely discharged, C
REF
.
, preventing it from reaching
CSKIP
CSKIP
) of 0.5 V. If the overcurrent
) of 3.0 V. Once reached, the
and
reaches the lower cycle
starts a
if
a
ILIM
CSKIP
continuous
controlled
.
CSKIP
CSKIP
ILIM
with a
CSKIP
has
is
is