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LT3498
APPLICATIONS INFORMATION — LED DRIVER
Programming LED Current
The LED current can be set by:
The feedback resistor (R SENSE1 ) and the sense voltage
(V CAP1 – V LED1 ) control the LED current. The CTRL1 pin
controls the sense reference voltage as shown in the
I LED ≈
200mV
R SENSE1
, when V CTRL1 > 1.5V
I LED ≈
, when V CTRL1 < 1.25V
Typical Performance Characteristics. For CTRL1 higher
than 1.5V, the sense reference is 200mV, which results in
full LED current. To have accurate LED current, precision
resistors are preferred (1% is recommended). The formula
and table for R SENSE selection are shown below.
V CTRL1
6.25 ? R SENSE1
Feedback voltage variation versus control voltage is
given in the Typical Performance Characteristics.
R SENSE 1 =
200mV
I LED
Using a Filtered PWM Signal
A ?ltered PWM signal can be used to control the bright-
Table 4: R SENSE1 Value Selection for 200mV Sense
ness of the LED string. The PWM signal is ?ltered (Figure 4)
I LED (mA)
5
10
15
20
Dimming Control
R SENSE1 ( Ω )
40
20
13.3
10
by a 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 of the CTRL1
pin which is 10M Ω (typ).
There are three different types of dimming control circuits.
The LED current can be set by modulating the CTRL1 pin
with a DC voltage, a ?ltered PWM signal or directly with
a PWM signal.
PWM
10kHz TYP
R1
100k Ω
C1
0.1μF
LT3498
CTRL1
3498 F04
Using a DC Voltage
For some applications, the preferred method of brightness
Figure 4. Dimming Control Using a Filtered PWM Signal
control is a variable DC voltage to adjust the LED current.
The CTRL1 pin voltage can be modulated to set the dim-
ming of the LED string. As the voltage on the CTRL1 pin
increases from 0V to 1.5V, the LED current increases from
0 to I LED . As the CTRL1 pin voltage increases beyond 1.5V,
it has no effect on the LED current.
3498fa
11
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