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LT3466-1
APPLICATIO S I FOR ATIO
R FB 1 =
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.
SETTING THE LED CURRENT
The current in the LED string can be set by the choice of the
resistor R FB1 (Figure 1). The feedback reference is 200mV.
In order to have accurate LED current, precision resistors
are preferred (1% is recommended).
200mV
I LED 1
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 pin.
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 pin, which
is 100k ? .
R1 LT3466-1
Table 4. R FB1 Value Selection
I LED1 (mA)
R FB1 ( ? )
PWM
10kHz TYP
10k
C1
1 μ F
CTRL1
34661 F08
5
40.2
10
15
20
25
20.0
13.3
10.0
8.06
Figure 8. Dimming Control Using a Filtered PWM Signal
SETTING THE BOOST OUTPUT VOLTAGE
The LT3466-1 regulates the voltage at the FB2 pin to 0.8V.
Most White LEDs are driven at maximum currents of
15mA to 20mA.
The output voltage of the boost converter (V OUT2 ) is set by
a resistor divider according to the formula:
V OUT2 = 0 . 8 V ? 1 +
?
DIMMING WHITE LEDS
The LED current in the driver can be set by modulating the
?
?
R 1 ?
R 2 ?
CTRL1 pin. There are two different ways to control the
intensity of white LEDs.
Using a DC Voltage
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
The CTRL1 pin voltage can be modulated to set the
dimming of the LED string. As the voltage on the CTRL1
pin increases from 0V to 1.8V, the LED current increases
from 0 to I LED1 . As the CTRL1 pin voltage increases
beyond 1.8V, it has no effect on the LED current.
The LED current can be set by:
I LED1 ≈ (V CTRL1 /5 ? R FB1 ), when V CTRL1 < 1V
I LED1 ≈ (200mV/R FB1 ), when V CTRL1 > 1.8V
Choose 1% resistors for better accuracy. The FB2 input
bias current is quite low, on the order of 10nA (typ). Large
resistor values (R1 ~ 1M ? ) can be used in the divider
network maximizing efficiency.
PROGRAMMING THE BOOST OUTPUT VOLTAGE
The output voltage of the boost converter can be modu-
lated by applying a variable DC voltage at the CTRL2 pin
The nominal voltage at the FB2 pin is 800mV. As the
voltage on the CTRL2 pin is ramped from 0V to 1V, the FB2
pin voltage ramps up to 0.8V. The feedback voltage can be
programmed as:
V FB2 ≈ V CTRL2 , when V CTRL2 < 0.8V
V FB2 ≈ 0.8V, when V CTRL2 > 1V
34661f
12
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