LT3491ESC8 datasheet(8/16 Pages) LINER

Part #	LT3491ESC8

Description	White LED Driver with Integrated Schottky in SC70 and 2mm 횞 2mm DFN

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Manufacturer	LINER [Linear Technology]

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8

LT3491

3491fa

For low DCR inductors, which is usually the case for this

application, the peak inrush current can be simplified as

follows:

I

V

L

r

L

LC

PK

IN

=

⎛

⎝⎜

⎞

⎠⎟

=

–.

•

• exp –

•

–

06

2

1

ω

α

ω

π

α

ω

rr

L

2

4•

where L is the inductance, r is the DCR of the inductor and

C is the output capacitance.

Table 3 gives inrush peak currents for some component

selections.

Table 3. Inrush Peak Currents

VIN (V)

r (

Ω)L (µH)

COUT (µF)

IP (A)

4.2

0.3

10

1.0

1.06

4.2

1.2

10

1.0

0.86

4.2

0.58

15

1.0

0.83

4.2

1.6

15

1.0

0.68

PROGRAMMING LED CURRENT

The feedback resistor (RSENSE) and the sense voltage

(VCAP – VLED) control the LED current.

The CTRL pin controls the sense reference voltage as

shown in the Typical Performance Characteristics. For

CTRL higher than 1.5V, the sense reference is 200mV,

which results in full LED current. In order to have accurate

LED current, precision resistors are preferred (1% is

recommended). The formula and table for RSENSE selec-

tion are shown below.

R

mV

I

SENSE

LED

=

200

Table 4. RSENSE Value Selection for 200mV Sense

ILED (mA)

RSENSE (Ω)

540

10

20

15

13.3

20

10

DIMMING CONTROL

There are three different types of dimming control circuits.

The LED current can be set by modulating the CTRL pin

with a DC voltage, a filtered PWM signal or directly with a

PWM signal.

Using a DC Voltage

For some applications, the preferred method of brightness

control is a variable DC voltage to adjust the LED current.

The CTRL pin voltage can be modulated to set the dimming

of the LED string. As the voltage on the CTRL pin increases

from 0V to 1.5V, the LED current increases from 0 to ILED.

As the CTRL pin voltage increases beyond 1.5V, it has no

effect on the LED current.

The LED current can be set by:

I

mV

R

when V

V

I

V

LED

SENSE

CTRL

LED

CTRL

≈

>

≈

200

15

6

,.

.2

25

125

•

,.

R

when V

V

SENSE

CTRL <

Feedback voltage variation versus control voltage is given

in the Typical Performance Characteristics.

APPLICATIO S I FOR ATIO

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