LT1937
White LED
Step-Up Converter in
SC70 and ThinSOT
FEATURES
DESCRIPTIO
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The LT 1937 is a step-up DC/DC converter specifically
Inherently Matched LED Current
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designed to drive white LEDs with a constant current. The
High Efficiency: 84% Typical
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device can drive two, three or four LEDs in series from a
Drives Up to Four LEDs from a 3.2V Supply
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Li-Ion cell. Series connection of the LEDs provides iden-
Drives Up to Six LEDs from a 5V Supply
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tical LED currents resulting in uniform brightness and
36V Rugged Bipolar Switch
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eliminating the need for ballast resistors. The LT1937
Fast 1.2MHz Switching Frequency
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switches at 1.2MHz, allowing the use of tiny external
Uses Tiny 1mm Tall Inductors
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components. The output capacitor can be as small as
Requires Only 0.22F Output Capacitor
TM
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0.22F, saving space and cost versus alternative solu-
Low Profile SC70 and ThinSOT Packaging
tions. A low 95mV feedback voltage minimizes power loss
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in the current setting resistor for better efficiency.
APPLICATIO S
The LT1937 is available in low profile SC70 and ThinSOT
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Cellular Phones
packages.
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PDAs, Handheld Computers
, LTC and LT are registered trademarks of Linear Technology Corporation.
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ThinSOT is a trademark of Linear Technology Corporation.
Digital Cameras
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MP3 Players
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GPS Receivers
TYPICAL APPLICATIO
Conversion Efficiency
L1
90
D1
22H
V
IN
3V TO 5V
85
C1 C2
LED 1
1F 0.22F
80
V SW
IN LED 2 15mA
V = 3V
IN
V = 3.6V
IN
LT1937
LED 3 75
OFF ON
SHDN FB
R1 70
GND
6.34
1937 F01a
65
C1, C2: X5R OR X7R DIELECTRIC
D1: CENTRAL SEMICONDUCTOR CMDSH-3
L1: MURATA LQH3C-220 OR EQUIVALENT 60
0 510 15 20
LED CURRENT (mA)
Figure 1. Li-Ion Powered Driver for Three White LEDs
1937 TA01b
1937f
1
EFFICIENCY (%)
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ULT1937
ABSOLUTE AXI U RATI GS (Note 1)
Input Voltage (V ) ................................................. 10V Extended Commercial
IN
SW Voltage ............................................................. 36V Operating Temperature Range (Note 2)... 40C to 85C
FB Voltage .............................................................. 10V Maximum Junction Temperature .......................... 125C
SHDN Voltage ......................................................... 10V Storage Temperature Range ................. 65C to 150C
Lead Temperature (Soldering, 10 sec).................. 300C
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PACKAGE/ORDER I FOR ATIO
ORDER PART ORDER PART
NUMBER NUMBER
TOP VIEW
TOP VIEW
SW 1 5 V
IN
LT1937ES5 SW 1 6 V LT1937ESC6
IN
GND 2
GND 2 5 GND
FB 3 4 SHDN
FB 3 4 SHDN
S5 PACKAGE
S5 PART MARKING SC6 PART MARKING
SC6 PACKAGE
5-LEAD PLASTIC TSOT-23
6-LEAD PLASTIC SC70
T = 125C, = 256C/ W IN FREE AIR
JMAX JA
LTYN T = 125C, = 256C/ W IN FREE AIR LAAB
JMAX JA
= 120C ON BOARD OVER GROUND PLANE
JA
= 150C ON BOARD OVER GROUND PLANE
JA
Consult LTC Marketing for parts specified with wider operating temperature ranges.
T = 25C, V = 3V, V = 3V, unless otherwise noted.
ELECTRICAL CHARACTERISTICS
A IN SHDN
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Operating Voltage 2.5 V
Maximum Operating Voltage 10 V
Feedback Voltage I = 100mA, Duty Cycle = 66% 86 95 104 mV
SW
FB Pin Bias Current 10 45 100 nA
Supply Current 1.9 2.5 mA
SHDN = 0V 0.1 1.0 A
Switching Frequency 0.8 1.2 1.6 MHz
Maximum Duty Cycle 85 90 %
Switch Current Limit 320 mA
Switch V I = 250mA 350 mV
CESAT SW
Switch Leakage Current V = 5V 0.01 5 A
SW
SHDN Voltage High 1.5 V
SHDN Voltage Low 0.4 V
SHDN Pin Bias Current 65 A
Note 1: Absolute Maximum Ratings are those values beyond which the life Note 2: The LT1937E is guaranteed to meet specifications from 0C to
of the device may be impaired. 70C. Specifications over the 40C to 85C operating temperature range
are assured by design, characterization and correlation with statistical
process controls.
1937f
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