NCP1288
http://onsemi.com
32
LOW LOAD OPERATION
Frequency Foldback
In order to improve the efficiency in light load conditions,
the frequency of the internal oscillator is linearly reduced
from its nominal value down to f
OSC(min)
. This frequency
foldback starts when the voltage on FB pin goes below
V
FB(fold)
, and is complete before V
FB
reaches V
skip(in)
,
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.
Note that the frequency foldback is disabled if the
controller runs at its maximum duty cycle.
Figure 60. Frequency Foldback when the FB Voltage Decreases
FB
f
OSC
Nominal f
OSC
V
skip(in)
V
FB(fold)
f
OSC(min)
Skip
Skip Cycle Mode with Soft−Skip
Figure 61. Skip Cycle with Soft−Skip Schematic
−
+
DRV stage
CS
S
R
Q
tSSKIP
Soft−skip ramp
FB
Reset
tLEB
blanking
Oscillator
DCMAX
−
+
KFB
+
Vskip
Sawtooth
−
+
When V
FB
reaches V
skip(in)
while decreasing, the skip
mode is activated: the driver stops, and the internal
consumption of the controller is decreased. While V
FB
is
below V
skip(out)
, the controller remains in this state. When
V
FB
crosses V
skip(out)
, the DRV pin starts to pulse again, and
the controller restarts with a short Soft−Skip duration
(t
SSKIP
). The soft−skip imposes the peak current from nearly
0, in a voltage−mode manner: it doesn’t have the same
behavior as the startup soft−start which is current−mode
driven.
