Datasheet

LTC3783
21
3783fb
OPERATION
Similar to the boost converter, which can be dimmed via
the digital PWMIN input or the analog FBP pin, the buck-
boost can be dimmed via the PWMIN pin or the analog
I
LIM
pin, which adjusts the offset voltage to which the loop
will drive (V
FBP
– V
FBN
). In the case of the buck-boost,
however, the dimming ratio cannot be as high as in the
boost converter, since there is no load switch to preserve
the V
OUT
level while PWMIN is low.
Using the LTC3783 for Buck Applications
As shown in Figure 12, high side current sensing also al-
lows the LTC3783 to control a functional buck converter
when load voltage is always sufficiently less than V
IN
. In
this scheme the input voltage to the inductor is lowered
by the load voltage. The boost converter now sees a
V
IN
’ = V
IN
– V
LOAD
, meaning the controller is now boosting
from (V
IN
– V
LOAD
) to V
IN
.
LTC3783
RUN
PWMIN
I
TH
SS
V
REF
FBP
FBN
FREQ
SYNC
V
IN
OV/FB
PWMOUT
I
LIM
GATE
SENSE
INTV
CC
GND
V
IN
6V TO 36V
LED STRING
GND
3783 F12
Figure 12. LED Buck Application