NCP1237 ONSEMI [ON Semiconductor], NCP1237 Datasheet - Page 26

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

Frequency Foldback
the frequency of the internal oscillator is linearly reduced
from its nominal value down to f
foldback starts when the voltage on FB pin V
V
Skip Cycle Mode with Soft−Skip
mode is activated: the driver stops, and the internal
consumption of the controller is decreased. While V
below V
V
FB(fold)
FB
In order to improve the efficiency in light load conditions,
When V
crosses V
skip(out)
, and is complete before V
FB
CS
FB
Nominal f
skip(out)
reaches V
, the controller remains in this state. When
f
OSC(min)
, the DRV pin starts to pulse again, and
OSC
skip(in)
f
+
OSC
Figure 23. Frequency Foldback when the FB Voltage Decreases
V
blanking
K
t
skip
LEB
while decreasing, the skip
FB
+
−
OSC(min)
Figure 24. Skip Cycle with Soft−Skip Schematic
Skip
FB
V
reaches V
. This frequency
skip(in)
FB
Reset
Soft−skip ramp
LOW LOAD OPERATION
goes below
+
−
skip(in)
http://onsemi.com
t
SSKIP
FB
V
is
,
FB(fold)
26
whatever the nominal switching frequency option is. The
current−mode control is still active while the oscillator
frequency decreases, but the frequency jittering is off.
controller runs at its maximum duty cycle.
the controller restarts with a short Soft−Skip duration
(t
0, in a voltage−mode manner: it doesn’t have the same
behavior as the startup soft−start which is current−mode
driven.
SSKIP
Note that the frequency foldback is disabled if the
+
−
). The soft−skip imposes the peak current from nearly
Sawtooth
DC
Oscillator
MAX
S
R
Q
DRV stage
FB