Datasheet
LT3796/LT3796-1
15
3796fa
For more information www.linear.com/3796
longer varies with CTRL. The typical V
(ISP-ISN)
threshold
vs CTRL is listed in the Table 1.
Table 1. V
(ISP-ISN)
Threshold vs CTRL
V
CTRL
(V) V
(ISP-ISN)
(mV)
1 225
1.05 236
1.1 244.5
1.15 248.5
1.2 250
When CTRL is higher than 1.2V, the LED current is regu-
lated to:
I
LED
=
250mV
R
LED
The CTRL pin should not be left open (tie to V
REF
if not
used). The CTRL pin can also be used in conjunction with
a thermistor to provide overtemperature protection for the
LED load, or with a resistor divider to V
IN
to reduce output
power and switching current when V
IN
is low. The presence
of a time varying differential voltage signal (ripple) across
ISP and ISN at the switching frequency is expected. The
amplitude of this signal is increased by high LED load
current, low switching frequency and/or a smaller value
output filter capacitor.
Programming Output Voltage (Constant-Voltage
Regulation) or Open LED/Overvoltage Threshold
The LT3796/LT3796-1 have two voltage feedback pins,
FB1 and FB2. Either one can be used for a boost or SEPIC
application. The difference between these two pins is
FB1 has a comparator that senses when FB1 exceeds
V
FB
– 60mV (VMODE threshold) and asserts the VMODE
output if V
(ISP-ISN)
is less than 25mV. This indicates that
the output is in voltage regulation mode and not current
Figure 2. Feedback Resistor Connections for Boost and SEPIC
Applications
Figure 3a. Feedback Resistor Connection for Buck Mode or
Buck-Boost Mode LED Driver
APPLICATIONS INFORMATION
LT3796/
LT3796-1
37961 F03a
R8
R5
LED
STRING
CSP
V
OUT
CSN
V
S
CSOUT
FB1
R
LED
R6
R7
–
+
regulation. FB2 does not have this extra comparator. The
output voltage can be set by selecting the values of R3
and R4 (see Figure 2) according to the following equation:
V
OUT
=1.25 •
R3+R4
R4
For a boost type LED driver, set the resistor from the output
to the FB1 pin such that the expected V
FB1
during normal
operation does not exceed 1.15V. For an LED driver of buck
mode or a buck-boost mode configuration, the FB voltage
is typically level shifted to a signal with respect to GND as
illustrated in Figure 3. The output can be expressed as:
V
OUT
= 1.25 •
R5
R8
•
R6+R7
R6
for Figure 3a
or V
OUT
= 1.25 •
R9
R10
+ V
BE(Q1)
for Figure 3b
LT3796/LT3796-1
37961 F02
FB1
R3
R4
V
OUT