LM1937
LM1937
WHITE LED STEP-UP CONVERTER
FEATURES
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
DESCRIPTION
The LM1937 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 identical LED currents resulting in uniform
brightness and eliminating the need for ballast resistors.
The output capacitor can be as small as 0.22μF, saving
space versus alternative solutions. A low 95mV
feedback voltage minimizes power loss for better
efficiency.
APPLICATIONS
Cellular Phones
PDAs, Handheld Computers
Digital Cameras
MP3 Players
GPS Receivers
TYPICAL APPLICATION
V
IN
3V TO 5V
Conversion Efficiency
90
D1
C2
0.22μF
15mA
C1
1 F
EFFICIENCY (%)
L1
22 H
85
80
V
IN
=3V
LED 1
V
IN
LM1937
OFF ON
SHDN
GND
SW
LED 2
LED 3
FB
R1
6.34Ω
75
70
65
V
IN
=3.6V
1937 F01a
60
C1, C2: X5R OR X7R DIELECTRIC
D1: CENTRAL SEMICONDUCTOR CMDSH-3
L1: MURATA LQH3C-220 OR EQUIVALENT
0
10
5
15
LED CURRENT (mA)
20
Figure 1. Li-Ion Powered Driver for Three White LEDs
PIN FUNCTIONS
TOP VIEW
ORDER PART NUMBER
LM1937CM5
3
2
1
4
PIN
1
2
3
4
5
5
PIN NAME
SW
GND
FB
SHDN
V
IN
PIN DESCRIPTION
Switch Pin.
(Minimize trace area at this pin to reduce EMI.)
Ground Pin. Connect directly to local ground plane.
Feedback Pin. Reference voltage is 95 mV.
(Calculate resistor value according to the formula: R
FB
=95 mV / I
LED
.)
Shutdown Pin. (Connect to 1.5 V or higher to enable device; 0.4 V or less to disable device.)
Input Supply Pin. (Must be locally bypassed.)
SOT-23-5L
1
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