Datasheet

LT3743
13
3743fd
APPLICATIONS INFORMATION
Programming Inductor Current
The analog voltage at the CTRL_L and CTRL_H pins is
buffered and produces a reference voltage, V
CTRL
, across
an internal resistor. The regulated average inductor current
is determined by:
I
O
=
V
CTRL
30 •R
S
where R
S
is the external sense resistor and I
O
is the aver-
age inductor current, which is equal to the LED current.
Figure 2 shows the LED current vs R
S
. The maximum
power dissipation in the resistor will be:
P
RS
=
0.05V
(
)
2
R
S
Table 1 contains several resistors values, the correspond-
ing maximum current and power dissipation in the sense
resistor. Figure 3 shows the power dissipation in R
S
.
Table 1. Sense Resistor Values
MAXIMUM LED
CURRENT (A) RESISTOR, R
S
(mΩ) POWER DISSIPATION (W)
1 50 0.05
5 10 0.25
10 5 0.5
25 2 1.25
Inductor Selection
The recovery time between regulated states is critical
to maintaining accurate control of the LED current. For
this reason, sizing the inductor to have no less than 30%
peak-to-peak ripple will provide excellent recovery time
with reasonable ripple. The overcurrent set point is equal
to
the high level regulated current level set by the CTRL_H
pin with an additional 23mV offset between the SENSE
+
and SENSE
–
pins. The saturation current for the inductor
should be at least 20% higher than the maximum regu-
lated current. The following equation sizes the inductor
to achieve a reasonable recovery time while minimizing
the inductor ripple:
L =
V
IN
• V
F
( )
– V
F
( )
2
0.2 • f
S
•I
O
• V
IN
where V
F
is the LED forward voltage drop, I
O
is the maximum
regulated current in the inductor and f
S
is the switching
frequency. Using this equation, the inductor will have
approximately 10% ripple at maximum regulated current.
Table 2. Recommended Inductor Manufacturers
VENDOR WEBSITE
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Vishay www.vishay.com
Würth Electronics www.we-online.com
NEC-Tokin www.nec-tokin.com
R
S
(mΩ)
0
0
POWER DISSIPATION (W)
0.2
0.6
0.8
1.0
1.4
2
10
14
3743 F03
0.4
1.2
8
18
20
4
6
12 16
R
S
(mΩ)
0
LED CURRENT (A)
10
20
30
5
15
25
4 8 12 16
3743 F02
2020 6 10 14 18
Figure 2. R
S
Value Selection for LED Current
Figure 3. Power Dissipation in R
S