Datasheet

LT3486
13
3486fe
APPLICATIONS INFORMATION
The LED current can be set by:
I
LED
≈ (200mV/R
FB
), when V
CTRL
> 1.8V
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.
(b) Using a Filtered PWM Signal
A variable duty cycle PWM can be used to control the bright-
ness of the LED string. The PWM signal is filtered (Figure
7) by an RC network and fed to the CTRL1, CTRL2 pins.
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 100kΩ.
3486 F07
C1
1µF
PWM
10kHz TYP
R1
10k
LT3486
CTRL1,2
Figure 7. Dimming Control Using a Filtered PWM Signal
3486 TA10a
12V (TYP)
9V TO 15V
C
OUT1
2.2µF
C
OUT2
2.2µF
100mA
PWM
FREQ?
1kHz
R
FB1
2Ω
R
FB2
2Ω
100mA
OFF ON
D1
Q1
100k 100k
3.65k
3.65k
21.5k
Q2
5V
D2
C
IN
10µF
C1 1µF
L1
10µH
L2
10µH
V
IN
V
IN
DIMMING
INPUT 1
DIMMING
INPUT 2
2.2nF
22pF
2.2nF
LUXEON
LEDs
LXCL-PWF1
LUXEON
LEDs
LXCL-PWF1
C
OUT1
, C
OUT2
: 35V, X5R OR X7R
C
IN
: 25V, X5R OR X7R
C1: 10V, X5R OR X7R
C
REF
: 6.3V, X5R OR X7R
C
REF
0.1µF
V
C1
V
C2
R
T
SW1 SW2
V
IN
LT3486
OVP2
CTRL2
REF
PWM2
FB2
OVP1
CTRL1
SHDN
PWM1
FB1
PWM
FREQ
1kHz
D1, D2: ZETEX ZLLS1000
L1, L2: TOKO D53LC (TYPE A)
Q1, Q2: FAIRCHILD FDN5630
Figure 8a. 12V to 8/8 White LEDs
Pulse-Width Modulation (PWM)
Adjusting the forward current flowing in the LEDs changes
the intensity of the LEDs, as explained in the previous sec-
tion. However, a change in forward current also changes the
color of the LEDs. The chromaticity of the LEDs changes
with the change in forward current. Many applications
cannot tolerate any shift in the color of the LEDs. Control-
ling the intensity of the LEDs via applying a PWM signal
allows dimming of the LEDs without changing the color.
Dimming the LEDs via a PWM signal essentially involves
turning the LEDs on and off at the PWM frequency. The
human eye has a limit of 60 frames per second. By in-
creasing the PWM frequency to say, 80Hz, the eye can
be deceived into believing that the pulsed light source is
continously on. Additionally by modulating the duty cycle
(amount of “on-time”), the intensity of the LEDs can be
controlled. The color of the LEDs remains unchanged in
this scheme since the LED current value is either zero or
a constant value.
Figure 8(a) shows a 12V to 8/8 white LED driver. The PWM
dimming control method requires an external NMOS tied
to the cathode of the lowest LED in the string, as shown in
I
L
500mA/DIV
I
LED
200mA/DIV
PWM
5V/DIV
0.2ms/DIV
3486 G18
V
IN
= 12V
8/8 LEDs
PWM FREQ = 1kHz
Figure 8b. PWM Dimming Waveforms