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LT3466
APPLICATIO S I FOR ATIO
Table 3. Inrush Peak Current
Using a DC Voltage
V IN (V)
5
5
5
5
9
12
L ( μ H)
15
33
47
68
47
33
C OUT ( μ F)
0.47
1.00
2.2
1.00
0.47
0.22
I P (A)
0.78
0.77
0.95
0.53
0.84
0.93
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
The CTRL pin voltage can be modulated to set the dimming
of the respective LED string. As the voltage on the CTRL
pin increases from 0V to 1.6V, the LED current increases
from 0 to I LED . As the CTRL pin voltage increases beyond
1.6V, it has no effect on the LED current.
Typically peak inrush current will be less than the value
calculated above. This is due to the fact that the DC
resistance in the inductor provides some damping result-
ing in a lower peak inrush current.
PROGRAMMING LED CURRENT
The LED current of each LED string can be set indepen-
dently by the choice of resistors R FB1 and R FB2 respec-
tively (Figure 2). The feedback reference is 200mV. In
order to have accurate LED current, precision resistors are
preferred (1% is recommended).
The LED current can be set by:
I LED ≈ (200mV/R FB ), when V CTRL > 1.6V
I LED ≈ (V CTRL /5 ? R FB ), when V CTRL < 1V
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics graphs.
Using a Filtered PWM Signal
A variable duty cycle PWM can be used to control the
brightness of the LED string. The PWM signal is filtered
(Figure 8) by an RC network and fed to the CTRL1, CTRL2
pins.
R FB 1 =
R FB 2 =
200 mV
I LED 1
200 mV
I LED 2
The corner frequency of R1, C1 should be much lower than
the frequency of the PWM signal. R1 needs to be much
smaller than the internal impedance in the CTRL pins,
which is 100k ? .
Table 4. R FB Value Selection
I LED (mA)
5
R FB ( ? )
40.2
PWM
10kHz TYP
R1
10k
C1
1 μ F
LT3466
CTRL1,2
3466 F08
10
20.0
15
20
25
13.3
10.0
8.06
Figure 8. Dimming Control Using a Filtered PWM Signal
LOW INPUT VOLTAGE APPLICATIONS
Most White LEDs are driven at maximum currents of
15mA to 20mA.
DIMMING CONTROL
There are two different types of dimming control circuits.
The LED current in the two drivers can be set indepen-
dently by modulating the CTRL1 and CTRL2 pins
The LT3466 can be used in low input voltage applications.
The input supply voltage to the LT3466 must be 2.7V or
higher. However, the inductors can be run off a lower
battery voltage. This technique allows the LEDs to be
powered off two alkaline cells. Most portable devices have
a 3.3V logic supply voltage which can be used to power the
LT3466. The LEDs can be driven straight from the battery,
resulting in higher efficiency.
respectively.
3466fa
12
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