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LT3465/LT3465A

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.

APPLICATIO S I FOR ATIO

Table 3 gives inrush peak currents for some component

selections.

Table 3. Inrush Peak Current

VIN (V)

r (

Ω)L (µH)

C (

µF)

IP (A)

0.5

0.22

0.38

0.5

0.70

3.6

0.5

0.22

0.26

0.5

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)

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 50k

Ω.

LT3465/

LT3465A

CTRL

3465A F06

PWM

100nF

Figure 6. Dimming Control Using a Filtered PWM Signal

IIN 50mA/DIV

VIN = 3.6V

200

µs/DIV

3465 F05

4 LEDs, 20mA

L = 22

µH

C = 0.22

µF

VOUT 5V/DIV

VFB 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 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 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:









































= +

()

–.

•

• exp –

• arctan

• sin arctan

•

–

•

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:









–.

•

• exp –

•

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