NCP1250
www.onsemi.com
17
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.
Frequency Foldback
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
fold
, set
around 1.5 V. At this point, the oscillator enters frequency
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
fold
/4.2 (250 mV or 31% of the maximum 0.8 V setpoint)
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 43 depicts the
adopted scheme for the part.
F
sw
V
FB
V
CS
V
FB
65 kHz
26 kHz
350 mV
V
fold
3.4 V
V
fold
3.4 V
0.8 V
0.36 V
FB
V
freeze
[
0.25 V
1.05 V 1.5 V
1.5 V
max
min
max
min
V
fold,end
Frequency Peak current setpoint
V
FB
min
Figure 43. 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
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
and the V
CC
pin slowly goes down to around 7 V. At this
point, the controller wakes−up and the V
CC
builds up again
due to the resistive starting network. When V
CC
reaches
VCC
ON
, the controller attempts to re−start, checking for the
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.
