LT1937 Datasheet, PDF (1/12 Pages) Linear Technology – White LED Step-Up Converter in SC70 and ThinSOT

FEATURES
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}
s _{Fast 1.2MHz Switching Frequency}
s _{Uses Tiny 1mm Tall Inductors}
s _{Requires Only 0.22µF Output Capacitor}
s _{Low Profile SC70 and ThinSOT}^TM _Packaging
U
APPLICATIO S
s _{Cellular Phones}
s _{PDAs, Handheld Computers}
s _{Digital Cameras}
s _{MP3 Players}
s _{GPS Receivers}
LT1937
White LED
Step-Up Converter in
SC70 and ThinSOT
DESCRIPTIO
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.
TYPICAL APPLICATIO
V_IN
3V TO 5V
OFF ON
L1
22µH
D1
C1
C2
1µF
LED 1
0.22µF
V_IN
SW _{LED 2}
15mA
LT1937
LED 3
SHDN
FB
GND
R1
6.34Ω
C1, C2: X5R OR X7R DIELECTRIC
D1: CENTRAL SEMICONDUCTOR CMDSH-3
L1: MURATA LQH3C-220 OR EQUIVALENT
1937 F01a
Figure 1. Li-Ion Powered Driver for Three White LEDs
Conversion Efficiency
90
85
80
V_IN = 3V _{VIN = 3.6V}
75
70
65
60
0
5
10
15
20
LED CURRENT (mA)
1937 TA01b
1937f
1