UTC L1937
WHITE LED STEP-UP
CONVERTER
LINEAR INTEGRATED CIRCUIT
2
1
DESCRIPTION
The UTC L1937 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 UTC
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 solutions. A low 95mV feedback voltage
minimizes power loss in the current setting resistor for
better efficiency.
3
5
4
SOT-25
1
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 0.22μ Output Capacitor
F
SOT-363
*Pb-free plating product number: L1937L
APPLICATIONS
* Marking code: LTCG
* Cellular Phones
* PDAs, Handheld Computers
* Digital Cameras
* MP3 Players
* GPS Receivers
PIN FUNCTION
PIN NO. PIN NO.
PIN NAME
(SOT-25) (SOT-363).
1
2
3
4
5
6
5
Switch
Ground
Feedback
Shutdown
Input Supply
Ground
SYMBOL
SW
GND
FB
FUNCTION
Connect inductor/diode here. Minimize trace area at this pin to
reduce EMI.
Connect directly to local ground plane.
Reference voltage is 95mV. Connect cathode of lowest LED and
resistor here. Calculate resistor value according to the formula:
R
FB
= 95mV/I
LED
Connect to 1.5V or higher to enable device;0.4V or less to
disable device.
Input Supply Pin. Must be locally bypassed.
Connect to Pin 2 and to local ground plane
SHDN
V
IN
GND
UTC
UNISONIC TECHNOLOGIES CO., LTD.
1
QW-R103-021,A
UTC L1937
BLOCK DIAGRAM
V
IN
5
LINEAR INTEGRATED CIRCUIT
FB
3
COMPARATOR
V
REF
1.25V
95mV
SW
1
( PIN 6 FOR SOT-363
PACKAGE )
-
A1
+
Rc
Cc
-
A2
+
Σ
DRIVER
R
S
Q
Q1
+
-
0.2Ω
SHDN
4
SHUTDOWN
RAMP
GENERATOR
2
1.2MHz
OSCILIATOR
GND
( PINS 2 AND 5 FOR
SOT-363 PACKAGE )
ABSOLUTE MAXIMUM RATINGS
PARAMETER
Input Voltage
SW Voltage
FB Voltage
SHDN Voltage
Operating Temperature
Storage Temperature
Maximum Junction Temperature
Lead Temperature (Soldering, 10 sec)
SYMBOL
V
IN
V
SW
V
FB
V
SHDN
Topr
Tstg
T
j
T
Lead
RATINGS
10
36
10
10
-40 ~ +85
-65 ~ +150
125
300
UNIT
V
V
V
V
℃
℃
℃
℃
ELECTRICAL CHARACTERISTICS
(T
a
=25℃, V
IN
=3V, V
SHDN
=3V, unless otherwise noted.)
PARAMETER
Operating Voltage
Feedback Voltage
FB Pin Bias Current
Supply Current
Switching Frequency
Maximum Duty Cycle
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
SHDN Voltage High
SHDN Voltage Low
SHDN Pin Bias Current
SYMBOL
V
IN
V
FB
I
FB
Icc
f
osc
DC
I
SW
TEST CONDITIONS
I
SW
=100mA,Duty Cycle=66%
MIN
86
10
TYP MAX UNIT
95
45
1.9
0.1
1.2
90
320
350
0.01
10
104
100
2.5
1.0
1.6
V
mV
nA
mA
µA
MHz
%
mA
mV
µA
V
V
µA
SHDN=0V
0.8
85
V
CESAT(SW)
I
SW
=250mA
I
SW(OFF)
V
SW
=5V
V
IH
V
IL
I
SHDN
5
0.4
1.5
65
UTC
UNISONIC TECHNOLOGIES CO., LTD.
2
QW-R103-021,A
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