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LT1937

1937f

White LED

Step-Up Converter in

SC70 and ThinSOT

, LTC and LT are registered trademarks of Linear Technology Corporation.

The LT

designed to drive white LEDs with a constant current. The

device can drive two, three or four LEDs in series from a

Li-Ion cell. Series connection of the LEDs provides iden-

tical LED currents resulting in uniform brightness and

eliminating the need for ballast resistors. The LT1937

switches at 1.2MHz, allowing the use of tiny external

components. The output capacitor can be as small as

0.22

µF, saving space and cost versus alternative solu-

tions. A low 95mV feedback voltage minimizes power loss

in the current setting resistor for better efficiency.

The LT1937 is available in low profile SC70 and ThinSOT

packages.

s

Inherently Matched LED Current

s

High Efficiency: 84% Typical

s

Drives Up to Four LEDs from a 3.2V Supply

s

Drives Up to Six LEDs from a 5V Supply

s

36V Rugged Bipolar Switch

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Fast 1.2MHz Switching Frequency

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Uses Tiny 1mm Tall Inductors

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Requires Only 0.22

µF Output Capacitor

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Low Profile SC70 and ThinSOT

s

Cellular Phones

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PDAs, Handheld Computers

s

Digital Cameras

s

MP3 Players

s

GPS Receivers

ThinSOT is a trademark of Linear Technology Corporation.

LED CURRENT (mA)

0

60

65

70

75

80

85

90

510

15

20

1937 TA01b

Figure 1. Li-Ion Powered Driver for Three White LEDs

Conversion Efficiency

SW

LT1937

L1

22

µH

C1

1

µF

C2

0.22

µF

15mA

1937 F01a

3V TO 5V

OFF ON

SHDN

R1

6.34

Ω

C1, C2: X5R OR X7R DIELECTRIC

D1: CENTRAL SEMICONDUCTOR CMDSH-3

L1: MURATA LQH3C-220 OR EQUIVALENT

FB

LED 3

LED 2

LED 1

D1

GND

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO