LT3591
APPLICATIONS INFORMATION
Table 4. R
SENSE
Value Selection for 200mV Sense
I
LED
(mA)
5
10
15
20
R
SENSE
(Ω)
40
20
13.3
10
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 CTRL
pin which is 10MΩ (typ).
Direct PWM Dimming
Changing the forward current flowing 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 driver for ten white LEDs.
Direct PWM dimming method requires an external NMOS
tied between the cathode of the lowest LED in the string
V
IN
3V TO
5V
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 filtered 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:
200mV
, when V
CTRL
>
1.5V
I
LED
≈
R
SENSE
I
LED
≈
V
CTRL
, when V
CTRL
<
1.25V
6.25 • R
SENSE
2
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
Using a Filtered PWM Signal
A filtered PWM signal 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.
L1
22µH
C1
1µF
V
IN
SW LT3591
GND
CTRL
CAP
R
SENSE
10Ω
LED
C2
2.2µF
PWM
10kHz TYP
C1
0.1µF
CTRL
3591 F05
0V
100k
Figure 5. Dimming Control Using a Filtered PWM Signal
Figure 6. Li-Ion to Ten White LEDs with Direct PWM Dimming
3591f
3591 F06
R1
100k
LT3591
5V
PWM
FREQ
Q1
Si2308
9
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
