LT3465 LINER [Linear Technology], LT3465 Datasheet - Page 7

• Manufacturer Part Number: LT3465
• Description: 1.2MHz/2.4MHz White LED Step-Up Converters with Built-In Schottky in ThinSOT
• Manufacturer: LINER [Linear Technology]

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
inductors, which is usually the case for this application,
the peak inrush current can be simplified as follows:
I
V
P
FB
I
CTRL 5V/DIV
IN
V
I
100mV/DIV
OUT
P
50mA/DIV
r
V
5V/DIV
2
IN
L
L C
•
V
1 5
– .
•
1
•
L
IN
.
L
0 6
V
4 LEDs, 20mA
L = 22 H
C = 0.22 F
IN
– .
•
–
= 3.6V
0 6
• exp –
r
Figure 5. Start-Up Waveforms
4
• exp –
•
1 5
L
U
.
2
IN
2
and V
• arctan
200 s/DIV
U
OUT
•
2
generates inrush current
W
IN
• sin arctan
IN
pin, the voltage
. The maximum
3465 F05
U
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 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-
ure 6. 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 .
I
R
V
LED
IN
Figure 6. Dimming Control Using a Filtered PWM Signal
3.6
FB
5
5
5
(V)
= 200mV/I
= V
FULL I
FB
FB
Resistor Value Selection
/R
10
15
20
LED
5
FB
r ( )
(mA)
PWM
0.5
0.5
0.5
0.5
LED-Full
FB
R1
5k
selection are shown below.
LT3465/LT3465A
L ( H)
22
22
22
33
C1
100nF
(1)
CTRL
LT3465A
LT3465/
3465A F06
C ( F)
0.22
0.22
R1 ( )
1
1
40.0
20.0
13.3
10.0
I
0.38
0.70
0.26
0.60
P
(A)
3465af
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