lt3465a Linear Technology Corporation, lt3465a Datasheet - Page 9

• Manufacturer Part Number: lt3465a
• Description: 2.4mhz White Led Step-up Converters With Built-in Schottky In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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.
Inrush Current
The LT3465 and LT3465A have a built-in Schottky diode.
When supply voltage is applied to the V
difference between V
flowing from input through the inductor and the Schottky
diode to charge the output capacitor to V
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:
where L is the inductance, r is the resistance of the
inductor and C is the output capacitance. For low DCR
V
I
α
ω
P
FB
I
CTRL 5V/DIV
IN
V
100mV/DIV
=
=
OUT
=
50mA/DIV
r
V
5V/DIV
2
IN
+
L C
L
•
1 5
– .
•
1
•
L
.
ω
V
4 LEDs, 20mA
L = 22µH
C = 0.22µF
0 6
IN
–
= 3.6V
(
• exp –
r
Figure 5. Start-Up Waveforms
4
+
•
1 5
U
L
⎡
⎢
⎢
⎣
.
2
IN
)
2
ω
α
and V
200µs/DIV
• arctan
U
OUT
generates inrush current
⎛
⎜
⎝
ω
W
α
⎞
⎟
⎠
⎤
⎥
⎥
⎦
IN
• sin arctan
IN
. The maximum
pin, the voltage
3465 F05
⎡
⎢
⎢
⎣
U
⎛
⎜
⎝
ω
α
⎞
⎟
⎠
⎤
⎥
⎥
⎦
inductors, which is usually the case for this application,
the peak inrush current can be simplified as follows:
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Current
LED Current and Dimming Control
The LED current is controlled by the feedback resistor (R1
in Figure 1) and the feedback reference voltage.
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
Table 4. R
The filtered PWM signal can be considered to be an
adjustable DC voltage. It can be used to adjust the CTRL
voltage source in dimming control. The circuit is shown in
Figure 6. The corner frequency of R1 and C1 should be
I
R
I
V
LED
P
IN
3.6
FB
5
5
5
=
(V)
= 200mV/I
= V
FULL I
V
IN
FB
L
FB
Resistor Value Selection
– .
•
/R
10
15
20
LED
ω
5
0 6
FB
r (Ω)
(mA)
0.5
0.5
0.5
0.5
• exp –
LED-Full
FB
⎛
⎜
⎝
selection are shown below.
LT3465/LT3465A
α
ω
L (µH)
•
22
22
22
33
π
2
⎞
⎟
⎠
(1)
C (µF)
0.22
0.22
R1 (Ω)
1
1
40.0
20.0
13.3
10.0
I
0.38
0.70
0.26
0.60
P
(A)
3465afa
9