LT3491EDC#TRPBF Linear Technology, LT3491EDC#TRPBF Datasheet - Page 9

• Manufacturer Part Number: LT3491EDC#TRPBF
• Description: IC LED DRIVER WHITE BCKLGT 6-DFN
• Manufacturer: Linear Technology
• Type: Backlight, White LEDr

Specifications of LT3491EDC#TRPBF

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 1
• Internal Driver: Yes
• Type - Primary: Backlight
• Type - Secondary: White LED
• Frequency: 2.3MHz
• Voltage - Supply: 2.5 V ~ 12 V
• Mounting Type: Surface Mount
• Package / Case: 6-DFN
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 200mA
• Internal Switch(s): Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Efficiency: -

APPLICATIO S I FOR ATIO
Using a Filtered PWM Signal
A filtered PWM signal can be used to control the bright-
ness of the LED string. The PWM signal is filtered (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 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 continuously 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 four white
LEDs. 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. A simple logic
Figure 5. Dimming Control Using a Filtered PWM Signal
10kHz TYP
PWM
U
100k
R1
U
C1
0.1µF
W
CTRL
LT3491
3491 F05
U
level Si2302 MOSFET can be used since its source is
connected to ground. The PWM signal is applied to the
CTRL pin of the LT3491 and the gate of the MOSFET. The
PWM signal should traverse between 0V to 2.5V, to ensure
proper turn on and off of the driver and the NMOS
transistor Q1. When the PWM signal goes high, the LEDs
are connected to ground and a current of I
R
goes low, the LEDs are disconnected and turn off. The
MOSFET ensures that the LEDs quickly turn off without
discharging the output capacitor which in turn allows the
LEDs to turn on faster. Figure 7 shows the PWM dimming
waveforms for the circuit in Figure 6.
200mA/DIV
20mA/DIV
Figure 6. Li-Ion to Four White LEDs with Direct PWM Dimming
SENSE
5V/DIV
PWM
I
3V TO 5V
LED
I
L
flows through the LEDs. When the PWM signal
V
4 LEDs
V
IN
IN
Figure 7. Direct PWM Dimming Waveforms
= 3V
10µH
2.5V
C1
1µF
L1
0V
PWM
FREQ
SW
GND
2ms/DIV
LT3491
CTRL
V
IN
100k
CAP
LED
R
10Ω
Q1
Si2302
SENSE
LED
3491 F06
LT3491
3491 F07
= 200mV/
C2
1µF
3491fa
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