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LT3743 Datasheet(PDF) 13 Page - Linear Technology

Part # LT3743
Description  High Current Synchronous Step-Down LED Driver with Three-State Control
PDF  28 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LT3743 Datasheet(HTML) 13 Page - Linear Technology

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LT3743
13
3743f
APPLICATIONS INFORMATION
Programming Inductor Current
The analog voltage at the CTRL_L and CTRL_H pins is
buffered and produces a reference voltage, VCTRL, across
an internal resistor. The regulated average inductor current
is determined by:
I
V
R
O
CTRL
S
=
30 •
where RS is the external sense resistor and IO is the aver-
age inductor current, which is equal to the LED current.
Figure 2 shows the LED current vs RS. The maximum
power dissipation in the resistor will be:
P
V
R
RS
S
= ()
005
2
.
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 RS.
Table 1. Sense Resistor Values
MAXIMUM LED
CURRENT (A)
RESISTOR, RS (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
VV
V
fI
V
IN
F
F
SO
IN
=
() ()
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
•–
.•
•
•
2
03
where VFistheLEDforwardvoltagedrop,IOisthemaximum
regulated current in the inductor and fS is the switching
frequency. Using this equation, the inductor will have ap-
proximately 15% 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
RS (mΩ)
0
0
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
RS (mΩ)
0
10
20
30
5
15
25
4
8
12
16
3743 F02
20
2
0
6
10
14
18
Figure 2. RS Value Selection for LED Current
Figure 3. Power Dissipation in RS



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