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LT3491
APPLICATIO S I FOR ATIO
For low DCR inductors, which is usually the case for this
Table 4. R SENSE Value Selection for 200mV Sense
application, the peak inrush current can be simplified as
follows:
I LED (mA)
5
10
R SENSE ( ? )
40
20
? α π ?
? ω 2 ?
I PK =
V IN – 0 . 6
L ? ω
? exp ? – ? ?
15
20
13.3
10
α =
r
2 ? L
DIMMING CONTROL
ω =
1
L ? C
–
r 2
4 ? L 2
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.
where L is the inductance, r is the DCR of the inductor and
C is the output capacitance.
Table 3 gives inrush peak currents for some component
selections.
Using a DC Voltage
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
Table 3. Inrush Peak Currents
V IN (V) r ( ? )
4.2 0.3
4.2 1.2
4.2 0.58
L ( μ H)
10
10
15
C OUT ( μ F)
1.0
1.0
1.0
I P (A)
1.06
0.86
0.83
The CTRL pin voltage can be modulated to set the dimming
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.
4.2
1.6
15
1.0
0.68
The LED current can be set by:
PROGRAMMING LED CURRENT
I LED ≈
200mV
R SENSE
, when V CTRL > 1 . 5 V
The feedback resistor (R SENSE ) and the sense voltage
(V CAP – V LED ) control the LED current.
I LED ≈
V CTRL
6 .225 ? R SENSE
, when V CTRL < 1 . 25 V
The CTRL pin controls the sense reference voltage as
shown in the Typical Performance Characteristics. For
CTRL higher than 1.5V, the sense reference is 200mV,
which results in full LED current. In order to have accurate
LED current, precision resistors are preferred (1% is
recommended). The formula and table for R SENSE selec-
tion are shown below.
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
R SENSE =
200mV
I LED
3491fa
8
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