NCP1236
www.onsemi.com
27
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(foldS)
, 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. Note that the frequency foldback is
disabled if the controller runs at its maximum duty cycle.
FB
f
OSC
Nominal f
OSC
V
skip(in)
V
FB(foldS)
f
OSC(min)
Skip
Figure 47. Frequency Foldback when the FB Voltage Decreases
V
FB(foldE)
Skip Cycle Mode
Figure 48. Skip Cycle Schematic
−
+
CS
S
R
Q
FB
blanking
−
+
+
DRV stage
V
skip
K
FB
t
LEB
When the FB voltage reaches V
skip(in)
while decreasing,
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; but as
soon as V
FB
crosses the skip out threshold, the DRV pin
starts to pulse again.
