Datasheet

LT3590
9
3590f
Programming LED Current
The feedback resistor (R1 in Figure 1) and the sense volt-
age (V
IN
-V
LED
) control the LED current.
I
VV
R
LED
IN LED
=
−
1
The CTRL pin controls the sense reference voltage as
shown in the Typical Performance Characteristics. For
CTRL higher than 1.5V, the sense reference is 200mV,
which results in full LED current. In order to have accu-
rate LED current, precision resistors are preferred (1%
is recommended). The formula and table for R1 selection
are shown below.
R
mV
I
LED
1
200
=
Table 3. R1 Theoretical Value for 200mV Sense
I
LED
(mA) R1 (Ω)
10 20
20 10
30 6.8
40 5.0
50 4.0
Dimming Control
There are three different types of dimming control circuits.
The LED current can be set by modulating the CTRL pin
with a DC voltage, a fi ltered PWM signal or directly with
a PWM signal.
APPLICATIONS INFORMATION
Using a DC Voltage
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 dim-
ming of the LED string. As the voltage on the CTRL pin
increases from 0V to 1.5V, the LED current increases from
0 to I
LED
. As the CTRL pin voltage increases beyond 1.5V,
it has no effect on the LED current.
The LED current can be set by:
I
LED
=
200mV
R1
,when V
CTRL
> 1.5V
I
LED
=
V
CTRL
6.25 •R1
,when V
CTRL
< 1.25V
Feedback voltage variation versus control voltage is
shown in Figure 4.
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 fi ltered
(Figure 5) by a RC network and fed to the CTRL 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 100kΩ.
Figure 4. Dimming and Shutdown Using CTRL Pin
V
CTRL
(V)
0
V
IN
-V
LED
(V)
0.10
0.15
2.0
3590 F04
0.05
0
0.5
1.0
1.5
0.25
0.20
Figure 5. Dimming Control Using a Filtered PWM Signal
PWM
kHz TYP
CTRL
C1
1
μF
R1
10k
3590 F05
LT3590