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LT3465

White LED Step-Up Converter with Built-In Schottky in ThinSOT

Linear Technology 

LT3465

APPLICATIO S I FOR ATIO

Inrush Current

The LT3465 has a built-in Schottky diode. When supply

voltage is applied to the VIN pin, the voltage difference

between VIN and VOUT generates inrush current flowing

from input through the inductor and the Schottky diode to

charge the output capacitor to VIN. The maximum current

the Schottky diode in the LT3465 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:

IP

=

VIN

L

– 0.6

•ω

•

exp–

α

ω

•

tg–1

ω

α







•

sintg–1

•



ω

α







α

=

r

+ 1.5

2•L

ω=

L

1

•C

–

(r

+

4

1.5)2

• L2

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:

IP

=

VIN

L

– 0.6

•ω

•

exp –

α

ω

•

π

2



Table 3 gives inrush peak currents for some component

selections.

Table 3. Inrush Peak Current

VIN (V)

5

r (Ω)

0.5

5

0.5

3.6

0.5

5

0.5

L (µH)

22

22

22

33

C (µF)

0.22

1

0.22

1

IP (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.

ILED = VFB/RFB

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 RFB selection are shown below.

RFB = 200mV/ILED-Full

(1)

Table 4. RFB Resistor Value Selection

FULL ILED (mA)

5

10

15

20

R1 (Ω)

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-

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 50kΩ.

R1

5k

PWM

LT3465

CTRL

C1

100nF

3465 F06

Figure 6. Dimming Control Using a Filtered PWM Signal

sn3465 3465is

7

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