NCP1236
LOW LOAD OPERATION
Frequency Foldback
V
, and is complete before V reaches V
,
FB(foldS)
FB
skip(in)
In order to improve the efficiency in light load conditions,
the frequency of the internal oscillator is linearly reduced
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.
from its nominal value down to f . This frequency
OSC(min)
foldback starts when the voltage on FB pin goes below
fOSC
Nominal fOSC
Skip
fOSC(min)
FB
Vskip(in)
VFB(foldE)
VFB(foldS)
Figure 47. Frequency Foldback when the FB Voltage Decreases
Skip Cycle Mode
−
+
+
V
skip
S
R
Q
DRV stage
K
FB
CS
FB
+
blanking
−
t
LEB
Figure 48. Skip Cycle Schematic
When the FB voltage reaches V
while decreasing,
below V , the controller remains in this state; but as
skip(out)
skip(in)
skip mode is activated: the driver stops, and the internal
consumption of the controller is decreased. While V is
soon as V crosses the skip out threshold, the DRV pin
starts to pulse again.
FB
FB
http://onsemi.com
27
ONSEMI [ ONSEMI ]
