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
LT1937
ABSOLUTE
AXI U
RATI GS
Input Voltage (V
IN
) .................................................
SW Voltage .............................................................
FB Voltage ..............................................................
SHDN Voltage .........................................................
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SW 1
GND 2
FB 3
4 SHDN
5 V
IN
ORDER PART
NUMBER
TOP VIEW
LT1937ES5
S5 PACKAGE
5-LEAD PLASTIC TSOT-23
T
JMAX
= 125°C,
θ
JA
= 256°C/ W IN FREE AIR
θ
JA
= 120°C ON BOARD OVER GROUND PLANE
S5 PART MARKING
LTYN
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Operating Voltage
Maximum Operating Voltage
Feedback Voltage
FB Pin Bias Current
Supply Current
T
A
= 25°C, V
IN
= 3V, V
SHDN
= 3V, unless otherwise noted.
MIN
2.5
10
TYP
MAX
UNITS
V
V
mV
nA
mA
µA
MHz
%
mA
mV
5
0.4
65
µA
V
V
µA
95
45
1.9
0.1
0.8
85
1.2
90
320
350
0.01
1.5
104
100
2.5
1.0
1.6
CONDITIONS
I
SW
= 100mA, Duty Cycle = 66%
SHDN = 0V
Switching Frequency
Maximum Duty Cycle
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
SHDN Voltage High
SHDN Voltage Low
SHDN Pin Bias Current
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
I
SW
= 250mA
V
SW
= 5V
2
U
U
W
W W
U
W
(Note 1)
10V
36V
10V
10V
Extended Commercial
Operating Temperature Range (Note 2)... – 40°C to 85°C
Maximum Junction Temperature .......................... 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
SW 1
GND 2
FB 3
6 V
IN
5 GND
4 SHDN
LT1937ESC6
SC6 PACKAGE
6-LEAD PLASTIC SC70
T
JMAX
= 125°C,
θ
JA
= 256°C/ W IN FREE AIR
θ
JA
= 150°C ON BOARD OVER GROUND PLANE
SC6 PART MARKING
LAAB
86
10
Note 2:
The LT1937E is guaranteed to meet specifications from 0°C to
70°C. Specifications over the –40°C to 85°C operating temperature range
are assured by design, characterization and correlation with statistical
process controls.
1937f
LT1937
TYPICAL PERFOR A CE CHARACTERISTICS
Quiescent Current
2.2
2.0
1.8
1.6
1.4
25°C
100°C
–50°C
SHDN PIN BIAS CURRENT (µA)
300
SHDN = 10V
250
200
150
100
50
SHDN = 3.6V
SHDN = 3V
SWITCHING FREQUENCY (MHz)
I
Q
(mA)
1.2
1.0
0.8
0.6
0.4
0.2
0
0
2
4
V
IN
(V)
1937 G01
6
Feedback Bias Current
60
FEEDBACK BIAS CURRENT (nA)
50
CURRENT LIMIT (mA)
EFFICIENCY (%)
40
30
20
10
0
–50
–25
50
25
0
TEMPERATURE (°C)
PI FU CTIO S
SW (Pin 1):
Switch Pin. Connect inductor/diode here.
Minimize trace area at this pin to reduce EMI.
GND (Pin 2):
Ground Pin. Connect directly to local ground
plane.
FB (Pin 3):
Feedback Pin. Reference voltage is 95mV.
Connect cathode of lowest LED and resistor here. Calcu-
late resistor value according to the formula:
R
FB
= 95mV/I
LED
SHDN (Pin 4):
Shutdown Pin. Connect to 1.5V or higher to
enable device; 0.4V or less to disable device.
GND (Pin 5, SC70 Package):
Ground Pin. Connect to Pin
2 and to local ground plane
V
IN
(Pin 5/Pin 6 SC70 Package):
Input Supply Pin. Must
be locally bypassed.
U W
8
75
1937 G04
SHDN Pin Bias Current
400
350
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Switching Frequency
SHDN = 2.7V
– 25
50
25
TEMPERATURE (°C)
0
75
100
1937 G02
10
0
– 50
0
–50
–25
50
25
0
TEMPERATURE (°C)
75
100
1937 G03
Efficiency vs Temperature
85
V
IN
= 3.6V
3 LEDs
350
Current Limit vs Duty Cycle
300
250
200
150
100
50
0
84
I
LED
= 20mA
83
I
LED
= 15mA
82
I
LED
= 10mA
81
100
80
–50
50
0
TEMPERATURE (°C)
100
1937 G05
0
20
40
60
DUTY CYCLE (%)
80
100
1937 G06
U
U
U
1937f
3
LT1937
BLOCK DIAGRA
V
IN
(PIN 6 FOR
SC70 PACKAGE) 5
V
REF
1.25V
95mV
A1
+
R
C
C
C
Σ
–
SHDN
4
SHUTDOWN
1.2MHz
OSCILLATOR
RAMP
GENERATOR
(PINS 2 AND 5 FOR
SC70 PACKAGE)
Figure 2. LT1937 Block Diagram
OPERATIO
The LT1937 uses a constant frequency, current mode
control scheme to provide excellent line and load regula-
tion. Operation can be best understood by referring to the
block diagram in Figure 2. At the start of each oscillator
cycle, the SR latch is set, which turns on the power switch
Q1. A voltage proportional to the switch current is added
to a stabilizing ramp and the resulting sum is fed into the
positive terminal of the PWM comparator A2. When this
voltage exceeds the level at the negative input of A2, the SR
latch is reset turning off the power switch. The level at the
negative input of A2 is set by the error amplifier A1, and is
simply an amplified version of the difference between the
feedback voltage and the reference voltage of 95mV. In
this manner, the error amplifier sets the correct peak
current level to keep the output in regulation. If the error
amplifier’s output increases, more current is delivered to
the output; if it decreases, less current is delivered.
Minimum Output Current
The LT1937 can regulate three series LEDs connected at
low output currents, down to approximately 4mA from a
4.2V supply, without pulse skipping, using the same
external components as specified for 15mA operation. As
current is further reduced, the device will begin skipping
pulses. This will result in some low frequency ripple,
although the LED current remains regulated on an average
basis down to zero. The photo in Figure 3 details circuit
operation driving three white LEDs at a 4mA load. Peak
inductor current is less than 50mA and the regulator
operates in discontinuous mode, meaning the inductor
current reaches zero during the discharge phase. After the
inductor current reaches zero, the switch pin exhibits
ringing due to the LC tank circuit formed by the inductor
in combination with switch and diode capacitance. This
ringing is not harmful; far less spectral energy is contained
in the ringing than in the switch transitions. The ringing
can be damped by application of a 300Ω resistor across
the inductor, although this will degrade efficiency.
V
SW
5V/DIV
I
L2
50mA/DIV
V
OUT
100mV/DIV
0.2µs/DIV
1937 F03
Figure 3. Switching Waveforms at I
LED
= 4mA, V
IN
= 3.6V
1937f
4
+
–
W
FB
3
1 SW
COMPARATOR
DRIVER
A2
R
S
Q
Q1
–
+
0.2Ω
2 GND
1937 BD1
U
LT1937
APPLICATIO S I FOR ATIO
Inductor Selection
A 22µH inductor is recommended for most LT1937 appli-
cations. Although small size and high efficiency are major
concerns, the inductor should have low core losses at
1.2MHz and low DCR (copper wire resistance). Some
inductors in this category with small size are listed in
Table 1. The efficiency comparison of different inductors
is shown in Figure 4.
Table 1. Recommended Inductors
DCR
(Ω)
0.71
CURRENT
RATING
(mA)
250
PART NUMBER
LQH3C220
MANUFACTURER
Murata
814-237-1431
www.murata.com
Panasonic
714-373-7334
www.panasonic.com
Sumida
847-956-0666
www.Sumida.com
Taiyo Yuden
408-573-4150
www.t-yuden.com
Taiyo Yuden
408-573-4150
www.t-yuden.com
ELJPC220KF
4.0
160
CDRH3D16-220
0.53
350
LB2012B220M
1.7
75
LEM2520-220
5.5
125
90
V
IN
= 3.6V
85 3LEDs
80
EFFICIENCY (%)
75
70
65
60
55
50
0
2
4
6 8 10 12 14 16 18 20
LOAD CURRENT (mA)
1937 F04
MURATA LQH3C-220
PANASONIC ELJPC220KF
SUMIDA CDRH3D16-220
TAIYO YUDEN LB2012B220M
TAIYO YUDEN LEM2520-220
Figure 4. Efficiency Comparison of Different Inductors
U
Capacitor Selection
The small size of ceramic capacitors makes them ideal for
LT1937 applications. X5R and X7R types are recom-
mended because they retain their capacitance over wider
voltage and temperature ranges than other types such as
Y5V or Z5U. A 1µF input capacitor and a 0.22µF output
capacitor are sufficient for most LT1937 applications.
Table 2. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER
Taiyo Yuden
AVX
Murata
Kemet
PHONE
408-573-4150
843-448-9411
814-237-1431
408-986-0424
URL
www.t-yuden.com
www.avxcorp.com
www.murata.com
www.kemet.com
W
U U
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast reverse recovery, are the ideal choices for LT1937
applications. The forward voltage drop of a Schottky diode
represents the conduction losses in the diode, while the
diode capacitance (C
T
or C
D
) represents the switching
losses. For diode selection, both forward voltage drop and
diode capacitance need to be considered. Schottky diodes
with higher current ratings usually have lower forward
voltage drop and larger diode capacitance, which can
cause significant switching losses at the 1.2MHz switch-
ing frequency of the LT1937. A Schottky diode rated at
100mA to 200mA is sufficient for most LT1937 applica-
tions. Some recommended Schottky diodes are listed in
Table 3.
Table 3. Recommended Schottky Diodes
FORWARD VOLTAGE
DIODE
CURRENT DROP CAPACITANCE
(mA)
(V)
(pF)
MANUFACTURER
100
0.58 at
7.0 at
Central
100mA
10V
631-435-1110
www.centralsemi.com
CMDSH2-3
200
0.49 at
15 at
Central
200mA
10V
631-435-1110
www.centralsemi.com
BAT54
200
0.53 at
10 at
Zetex
100mA
25V
631-543-7100
www.zetex.com
PART
NUMBER
CMDSH-3
1937f
5
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