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參數(shù)資料
| 型號： | LT3465A |
| 廠商： | Linear Technology Corporation |
| 元件分類： | LED驅動器 |
| 英文描述： | Single Cell 350mA LED Driver |
| 中文描述： | 單細胞350mA的LED驅動器 |
| 文件頁數(shù)： | 7/12頁 |
| 文件大小： | 375K |
| 代理商： | LT3465A |
LT3465/LT3465A
7
3465af
Soft-Start (LT3465)
The LT3465 has an internal soft-start circuit to limit the
input current during circuit start-up. The circuit start-up
waveforms are shown in Figure 5.
APPLICATIOU
W
U
U
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Current
V
IN
(V)
5
5
3.6
5
r (
)
0.5
0.5
0.5
0.5
L (
μ
H)
22
22
22
33
C (
μ
F)
0.22
1
0.22
1
I
P
(A)
0.38
0.70
0.26
0.60
LED Current and Dimming Control
The LED current is controlled by the feedback resistor (R1
in Figure 1) and the feedback reference voltage.
I
LED
= V
FB
/R
FB
The CTRL pin controls the feedback reference voltage as
shown in the Typical Performance Characteristics. For
CTRL higher than 1.8V, the feedback reference is 200mV,
which results in full LED current. CTRL pin can be used as
dimming control when CTRL voltage is between 200mV to
1.5V. In order to have accurate LED current, precision
resistors are preferred (1% is recommended). The for-
mula and table for R
FB
selection are shown below.
R
FB
= 200mV/I
LED-Full
(1)
Table 4. R
FB
Resistor Value Selection
FULL I
LED
(mA)
5
10
15
20
R1 (
)
40.0
20.0
13.3
10.0
The filtered PWM signal can be considered as an adjust-
able DC voltage. It can be used to adjust the CTRL voltage
source in dimming control. The circuit is shown in Fig-
ure6. The corner frequency of R1 C1 should be lower than
the freqency of the PWM signal. R1 needs to be much
smaller than the internal impedance in the CTRL pin, which
is 50k
.
LT3465/
LT3465A
CTRL
3465A F06
PWM
C1
100nF
R1
5k
Figure 6. Dimming Control Using a Filtered PWM Signal
I
IN
50mA/DIV
V
= 3.6V
4 LEDs, 20mA
L = 22
μ
H
C = 0.22
μ
F
200
μ
s/DIV
3465 F05
V
OUT
5V/DIV
V
FB
100mV/DIV
CTRL 5V/DIV
Figure 5. Start-Up Waveforms
Inrush Current
The LT3465 and LT3465A have a built-in Schottky diode.
When supply voltage is applied to the V
IN
pin, the voltage
difference between V
IN
and V
OUT
generates inrush current
flowing from input through the inductor and the Schottky
diode to charge the output capacitor to V
IN
. The maximum
current the Schottky diode in the LT3465 and LT3465A
can sustain is 1A. The selection of inductor and capacitor
value should ensure the peak of the inrush current to be
below 1A. The peak inrush current can be calculated as
follows:
I
V
L
+
2
r
L
L C
r
L
P
IN
=
=
=
+
4
(
)
–0 6
.
1 5
exp –
arctan
sin arctan
–
.
ω
1
1 5
2
2
α
ω
ω
α
ω
α
α
ω
where L is the inductance, r is the resistance of the
inductor and C is the output capacitance. For low DCR
inductors, which is usually the case for this application,
the peak inrush current can be simplified as follows:
I
V
L
P
IN
=
–0 6
ω
exp –
2
α
ω
π
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