lt3743 Linear Technology Corporation, lt3743 Datasheet - Page 13

• Manufacturer Part Number: lt3743
• Description: High Current Synchronous Step-down Led Driver With Three-state Control
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Programming Inductor Current
The analog voltage at the CTRL_L and CTRL_H pins is
buffered and produces a reference voltage, V
an internal resistor. The regulated average inductor current
is determined by:
where R
age inductor current, which is equal to the LED current.
Figure 2 shows the LED current vs R
power dissipation in the resistor will be:
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
Table 1. Sense Resistor Values
MAXIMUM LED
CURRENT (A)
I
P
O
RS
=
10
25
=
1
5
30 •
V
S
(
Figure 2. R
CTRL
is the external sense resistor and I
0 05
30
25
20
15
10
.
R
5
0
R
0
S
S
V
2
)
RESISTOR, R
2
4
S
Value Selection for LED Current
6
50
10
5
2
8
R
S
10
(mΩ)
S
(mΩ)
12
14
POWER DISSIPATION (W)
16
S
18
3743 F02
. The maximum
20
0.05
0.25
1.25
O
CTRL
0.5
is the aver-
, across
S
.
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
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:
where V
regulated current in the inductor and f
frequency. Using this equation, the inductor will have ap-
proximately 15% ripple at maximum regulated current.
Table 2. Recommended Inductor Manufacturers
L
=
⎛
⎜
⎜
⎝
Würth Electronics
F
V
is the LED forward voltage drop, I
0 3
IN
NEC-Tokin
VENDOR
. • • •
Coilcraft
1.4
1.2
1.0
0.8
0.6
0.4
Sumida
0.2
–
Vishay
0
•
pins. The saturation current for the inductor
( ) ( )
0
Figure 3. Power Dissipation in R
V
f I
S O
F
2
–
4
V
V
6
F
IN
2
8
R
⎞
⎟
⎟
⎠
S
10
(mΩ)
12
14
www.we-online.com
www.nec-tokin.com
www.coilcraft.com
www.sumida.com
www.vishay.com
16
S
WEBSITE
18 20
3743 F03
O
is the switching
is the maximum
S
LT3743
13
3743f
+