LT3743FE(RevB) 데이터 시트보기 (PDF) - Linear Technology

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LT3743FE
(Rev.:RevB)

High Current Synchronous Step-Down LED Driver with Three-State Control

Linear Technology 

LT3743

APPLICATIONS INFORMATION

Programming Inductor Current

The analog voltage at the CTRL_L and CTRL_H pins is

buffered and produces a reference voltage, VCTRL, across

an internal resistor. The regulated average inductor current

is determined by:

IO

=

VCTRL

30 •RS

where RS is the external sense resistor and IO is the aver-

age inductor current, which is equal to the LED current.

Figure 2 shows the LED current vs RS. The maximum

power dissipation in the resistor will be:

PRS

=

(0.05V

RS

)2

Table 1 contains several resistors values, the correspond-

ing maximum current and power dissipation in the sense

resistor. Figure 3 shows the power dissipation in RS.

Table 1. Sense Resistor Values

MAXIMUM LED

CURRENT (A)

1

RESISTOR, RS (mΩ)

50

5

10

10

5

25

2

POWER DISSIPATION (W)

0.05

0.25

0.5

1.25

30

25

20

15

10

5

0

0 2 4 6 8 10 12 14 16 18 20

RS (mΩ)

3743 F02

Figure 2. RS Value Selection for LED Current

Inductor Selection

The recovery time between regulated states is critical

to maintaining accurate control of the LED current. For

this reason, sizing the inductor to have no less than 30%

peak-to-peak ripple will provide excellent recovery time

with reasonable ripple. The overcurrent set point is equal

to the high level regulated current level set by the CTRL_H

pin with an additional 23mV offset between the SENSE+

and SENSE– pins. The saturation current for the inductor

should be at least 20% higher than the maximum regu-

lated current. The following equation sizes the inductor

to achieve a reasonable recovery time while minimizing

the inductor ripple:

L = VIN • (VF ) – (VF )2

0.2 • fS •IO • VIN

where VF is the LED forward voltage drop, IO is the maximum

regulated current in the inductor and fS is the switching

frequency. Using this equation, the inductor will have ap-

proximately 10% ripple at maximum regulated current.

Table 2. Recommended Inductor Manufacturers

VENDOR

WEBSITE

Coilcraft

www.coilcraft.com

Sumida

www.sumida.com

Vishay

www.vishay.com

Würth Electronics

www.we-online.com

NEC-Tokin

www.nec-tokin.com

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

0 2 4 6 8 10 12 14 16 18 20

RS (mΩ)

3743 F03

Figure 3. Power Dissipation in RS

3743fb

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