LT3466 Linear Technology, LT3466 Datasheet - Page 12

• Manufacturer Part Number: LT3466
• Description: Dual Full Function White LED Step-Up Converter with Built-In Schottky Diodes
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT3466
Table 3. Inrush Peak Current
Typically peak inrush current will be less than the value
calculated above. This is due to the fact that the DC
resistance in the inductor provides some damping result-
ing in a lower peak inrush current.
PROGRAMMING LED CURRENT
The LED current of each LED string can set independently
by the choice of resistors R
(Figure 2). The feedback reference is 200mV. In order to
have accurate LED current, precision resistors are pre-
ferred (1% is recommended).
Table 4. R
Most White LEDs are driven at maximum currents of
15mA to 20mA.
DIMMING CONTROL
There are two different types of dimming control circuits.
The LED current in the two drivers can be set indepen-
dently by modulating the CTRL1 and CTRL2 pins
respectively.
12
R
R
FB
FB
V
IN
12
1
5
5
5
5
9
2
(V)
FB1,2
I
LED1,2
200
200
I
I
LED
LED
10
15
20
25
Value Selection
5
(mA)
mV
mV
1
2
L ( H)
U
15
33
47
68
47
33
U
FB1
C
OUT
0.47
1.00
1.00
0.47
0.22
2.2
and R
W
( F)
R
FB1,2
FB2
40.2
20.0
13.3
10.0
8.06
( )
respectively
U
I
0.78
0.77
0.95
0.53
0.84
0.93
P
(A)
Using a DC Voltage
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
The CTRL1, CTRL2 pin voltage can be modulated to set the
dimming of the respective LED string. As the voltage on
the CTRL1, CTRL2 pin increases from 0V to 1.6V, the LED
current increases from 0 to I
pin voltage increases beyond 1.6V, it has no effect on the
LED current.
The LED current can be set by:
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics graphs.
Using a Filtered PWM Signal
A variable duty cycle PWM can be used to control the
brightness of the LED string. The PWM signal is filtered
(Figure 8) 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 100k .
LOW INPUT VOLTAGE APPLICATIONS
The LT3466 can be used in low input voltage applications.
The input supply voltage to LT3466 must be 2.7V or
higher. However, the inductors can be run off a lower
battery voltage. This technique allows the LEDs to be
powered off two alkaline cells. Most portable devices have
a 3.3V logic supply voltage which can be used to power the
LT3466. The LEDs can be driven straight from the battery,
resulting in higher efficiency.
I
I
LED1,2
LED1,2
Figure 8. Dimming Control Using a Filtered PWM Signal
(200mV/R
(V
CTRL1,2
10kHz TYP
PWM
/5 • R
FB1,2
10k
R1
FB1,2
), when V
LED1,2
C1
1 F
), when V
. As the CTRL1, CTRL2
CTRL1,2
CTRL1,2
LT3466
3466 F08
CTRL1,2
> 1.6V
< 1V
3466f