Datasheet

8
LT3491
3491fa
For low DCR inductors, which is usually the case for this
application, the peak inrush current can be simplified as
follows:
I
V
L
r
L
LC
PK
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
=
=
–.
•
• exp – •
•
•
–
06
2
2
1
ω
α
ω
π
α
ω
rr
L
2
2
4•
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.
Table 3. Inrush Peak Currents
V
IN
(V) r (Ω)L (µH) C
OUT
(µF) I
P
(A)
4.2 0.3 10 1.0 1.06
4.2 1.2 10 1.0 0.86
4.2 0.58 15 1.0 0.83
4.2 1.6 15 1.0 0.68
PROGRAMMING LED CURRENT
The feedback resistor (R
SENSE
) and the sense voltage
(V
CAP
– V
LED
) control the LED current.
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.
R
mV
I
SENSE
LED
=
200
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 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 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.
The LED current can be set by:
I
mV
R
when V V
I
V
LED
SENSE
CTRL
LED
CTRL
≈ >
≈
200
15
6
,.
.
225
125
•
,.
R
when V V
SENSE
CTRL
<
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
APPLICATIO S I FOR ATIO
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