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LT3743
APPLICATIONS INFORMATION
I O =
( 0.05V ) 2
R S
V IN ? ( V F ) – ( V F )
ProgrammingInductorCurrent
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:
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 =
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
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:
2
L =
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
5
10
25
10
5
2
0.25
0.5
1.25
Coilcraft
Sumida
Vishay
www.coilcraft.com
www.sumida.com
www.vishay.com
Würth Electronics
www.we-online.com
30
NEC-Tokin
www.nec-tokin.com
25
1.4
20
15
10
5
1.2
1.0
0.8
0.6
0
0
2
4
6
8 10 12 14 16 18 20
R S (m )
0.4
0.2
3743 F02
Figure 2. R S Value Selection for LED Current
0
0
2
4
6
8 10 12
R S (m )
14 16 18 20
3743 F03
Figure 3. Power Dissipation in R S
3743fd
13
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