LT3497
11
3497f
APPLICATIONS INFORMATION
Table 4: R
SENSE
Value Selection for 200mV Sense
I
LED
(mA) R
SENSE
(Ω)
540
10 20
15 13.3
20 10
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.
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
mV
R
V
I
V
LED
SENSE
LED
CTRL
≈>
≈
200
15
6
.when V
CTRL
.. •
.
25
125
R
V
SENSE
when V
CTRL
<
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
Using a Filtered PWM Signal
A fi ltered 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 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 10MΩ (typ).
Direct PWM Dimming
Changing the forward current fl owing in the LEDs not only
changes the intensity of the LEDs, it also changes the color.
The chromaticity of the LEDs changes with the change in
forward current. Many applications cannot tolerate any
shift in the color of the LEDs. Controlling the intensity of
the LEDs with a direct PWM signal allows dimming of the
LEDs without changing the color. In addition, direct PWM
dimming offers a wider dimming range to the user.
Dimming the LEDs via a PWM signal essentially involves
turning the LEDs on and off at the PWM frequency. The
typical human eye has a limit of ~60 frames per second.
By increasing the PWM frequency to ~80Hz or higher,
the eye will interpret that the pulsed light source is con-
tinuously 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 6 shows a Li-ion powered 4/4 white LED driver. Direct
PWM dimming method requires an external NMOS tied
between the cathode of the lowest LED in the string and
ground as shown in Figure 6. Si2318DS MOSFETs can be
used since its sources are connected to ground. The PWM
signal is applied to the (CTRL1 and CTRL2) control pins of
the LT3497 and the gate of the MOSFET. The PWM signal
should traverse between 0V to 5V to ensure proper turn
on and off of the converters and the NMOS transistors (Q1
and Q2). When the PWM signal goes high, LEDs are con-
nected to ground and a current of I
LED
= (200mV/R
SENSE
)
fl ows through the LEDs. When the PWM signal goes low,
the LEDs are disconnected and turn off. The low PWM
input applied to the LT3497 ensures that the respective
Figure 5. Dimming Control Using a Filtered PWM Signal
LT3497
CTRL1,2
C1
0.1µF
PWM
10kHz TYP
3497 F05
R1
100k
