LT3590
APPLICATIONS INFORMATION
Programming LED Current
Using a DC Voltage
Forsomeapplications,thepreferredmethodofbrightness
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
The feedback resistor (R1 in Figure 1) and the sense volt-
age (V - V ) control the LED current.
IN LED
V − VLED
IN
ILED
=
R1
0 to I . As the CTRL pin voltage increases beyond 1.5V,
LED
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 (1ꢀ
is recommended). The formula and table for R1 selection
are shown below.
it has no effect on the LED current.
The LED current can be set by:
200mV
ILED
=
,when VCTRL > 1.5V
R1
VCTRL
6.25 •R1
ILED
=
,when VCTRL < 1.25V
200mV
ILED
R1=
Feedback voltage variation versus control voltage is
shown in Figure 4.
Table 3. R1 Theoretical Value for 200mV Sense
I
(mA)
R1 (Ω)
20
LED
Using a Filtered PWM Signal
10
20
30
40
50
10
A variable duty cycle PWM can be used to control the
brightness of the LED string. The PWM signal is filtered
(Figure 5) by a RC network and fed to the CTRL pin.
6.8
5.0
4.0
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.
Dimming Control
Therearethreedifferenttypesofdimmingcontrolcircuits.
The LED current can be set by modulating the CTRL pin
with a DC voltage, a filtered PWM signal or directly with
a PWM signal.
LT3590
R1
10k
PWM
kHz TYP
CTRL
C1
1μF
0.25
0.20
0.15
0.10
0.05
0
3590 F05
Figure 5. Dimming Control Using a Filtered PWM Signal
0.5
1.0
1.5
0
2.0
V
(V)
CTRL
3590 F04
Figure 4. Dimming and Shutdown Using CTRL Pin
3590f
9
Linear [ Linear ]
