FAN6248HC/HD/LC/LD
www.onsemi.com
13
Light Load Detection (LLD)
To guarantee stable operation under light load condition,
FAN6248 adopts a light load detection function. The
modulation current I
OFFSET
is mainly used for the adaptive
dead time control. When the output load is heavy,
I
OFFSET_STEP
declines due to large di/dt in the secondary
side current to maintain 280 ns of t
DEAD
in FAN6248HC(D).
On the contrary, I
OFFSET_STEP
increases at light load
condition by small di/dt of SR current. FAN6248 can detect
light load condition by using this I
OFFSET_STEP
as shown in
Figure 27. When SR turn-off threshold voltage is V
TH_OFF1
and the modulation current is higher than I
OFFEST_STEP8
, the
light load detection is triggered. In this mode, dead time
target becomes to 320 ns of t
DEAD_LIGHT
in
FAN6248HC(D) and 360 ns in FAN6248LC(D) version.
Green Mode
When the power supply system operates at very light load
condition, FAN6248 disables SR operation and enters into
green mode operation. Once FAN6248 is in the green mode,
all the major blocks are disabled to minimize the operating
current. When V
DS_SR
has no switching operation longer
than t
GRN_ENT
during the burst mode of the primary side
LLC controller, the green mode is enabled after
t
GRN_ENT_DBNC
of debounce time. After then, FAN6248
exits the green mode when the non−switching time in the
burst mode is less than t
GRN_EXT_H
or 7 consecutive
switching cycles are detected as shown in Figure 28.
Figure 27. Light Load Detection
V
TH_OFF2
V
TH_OFF2
−R
OFFSET
x I
OFFSET_STEP1
V
TH_OFF2
−R
OFFSET
x I
OFFSET_STEP2
V
TH_OFF1
V
TH_OFF2
−R
OFFSET
x I
OFFSET_STEP15
V
TH_OFF2
−R
OFFSET
x I
OFFSET_STEP13
V
TH_OFF1
−R
OFFSET
x I
OFFSET_STEP2
V
TH_OFF1
−R
OFFSET
x I
OFFSET_STEP14
V
TH_OFF1
−R
OFFSET
x I
OFFSET_STEP8
V
TH_OFF1
−R
OFFSET
x I
OFFSET_STEP15
Virtual V
TH_OFF
Heavy
Light
V
TH_OFF2
Range
V
TH_OFF1
Range
Load
Load
LDD Trigger
Figure 28. Green Mode Exit
V
DS_SR1
V
GATE1
I
SD_SR1
Green Exit
h
CSW_EXT
= 7 Cycles
FAN6248HC/HD/LC/LD
www.onsemi.com
14
PACKAGE DIMENSIONS
SOIC8
CASE 751EB
ISSUE A
FAN6248HC/HD/LC/LD
www.onsemi.com
15
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