LT1937
White LED
Step-Up Converter in
SC70 and ThinSOT
FEATURES
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DESCRIPTIO
Inherently Matched LED Current
High Efficiency: 84% Typical
Drives Up to Four LEDs from a 3.2V Supply
Drives Up to Six LEDs from a 5V Supply
36V Rugged Bipolar Switch
Fast 1.2MHz Switching Frequency
Uses Tiny 1mm Tall Inductors
Requires Only 0.22µF Output Capacitor
Low Profile SC70 and ThinSOT
TM
Packaging
APPLICATIO S
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The LT
®
1937 is a step-up DC/DC converter specifically
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
0.22µF, 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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
Cellular Phones
PDAs, Handheld Computers
Digital Cameras
MP3 Players
GPS Receivers
TYPICAL APPLICATIO
L1
22µH
C1
1µF
V
IN
LT1937
OFF ON
SHDN
GND
FB
SW
Conversion Efficiency
D1
C2
0.22µF
15mA
EFFICIENCY (%)
90
85
80
V
IN
= 3V
75
70
65
60
0
5
10
15
LED CURRENT (mA)
20
1937 TA01b
V
IN
3V TO 5V
LED 1
LED 2
LED 3
R1
6.34Ω
1937 F01a
C1, C2: X5R OR X7R DIELECTRIC
D1: CENTRAL SEMICONDUCTOR CMDSH-3
L1: MURATA LQH3C-220 OR EQUIVALENT
Figure 1. Li-Ion Powered Driver for Three White LEDs
U
V
IN
= 3.6V
1937f
U
U
1
LINER [ LINEAR TECHNOLOGY ]
