NCP1251
Finally, please note that another comparator internally
fixes the maximum peak current set point to 0.8 V even if the
OPP pin is inadvertently biased above 0 V.
foldback and reduces its switching frequency. The peak
current setpoint follows the feedback pin until its level
reaches 1.05 V. Below this value, the peak current freezes to
V
/4.2 (250 mV or 31% of the maximum 0.8 V setpoint)
fold
Frequency Foldback
and the only way to further reduce the transmitted power is
to reduce the operating frequency down to 26 kHz. This
value is reached at a voltage feedback level of 350 mV
typically. Below this point, if the output power continues to
decrease, the part enters skip cycle for the best noise−free
performance in no−load conditions. Figure 44 depicts the
adopted scheme for the part.
The reduction of no−load standby power associated with
the need for improving the efficiency, requires a change to
the traditional fixed−frequency type of operation. This
controller implements a switching frequency foldback when
the feedback voltage passes below a certain level, V , set
around 1.5 V. At this point, the oscillator enters frequency
fold
Frequency
Peak current setpoint
Fsw
VCS
FB
max
65 kHz
max
0.8 V
[ 0.36 V
min
26 kHz
min
[0.25 V
VFB
VFB
350 mV
Vfold,end
1.5 V
Vfreeze Vfold
3.4 V
3.4 V
V
fold
1.05 V 1.5 V
Figure 44. By Observing the Voltage on the Feedback Pin, the Controller Reduces its Switching Frequency for an
Improved Performance at Light Load
Auto−Recovery Short−Circuit Protection
due to the resistive starting network. When V reaches
CC
In case of output short−circuit or if the power supply
experiences a severe overloading situation, an internal error
flag is raised and starts a countdown timer. If the flag is
asserted longer than 100 ms, the driving pulses are stopped
VCC , the controller attempts to re−start, checking for the
ON
absence of the fault. If the fault is still there, the supply enters
another cycle of so−called hiccup mode. If the fault has
cleared, the power supply resumes normal operation. Please
note that the soft−start is activated during each of the re−start
sequence.
and the V pin slowly goes down to around 7 V. At this
CC
point, the controller wakes−up and the V builds up again
CC
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