LT1937 Datasheet by Analog Devices Inc.

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L7H”! “I2 LT1937 TECHNOLOGY L7 L7LELUW
1
LT1937
1937f
White LED
Step-Up Converter in
SC70 and ThinSOT
, LTC and LT are registered trademarks of Linear Technology Corporation.
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.
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
Cellular Phones
PDAs, Handheld Computers
Digital Cameras
MP3 Players
GPS Receivers
ThinSOT is a trademark of Linear Technology Corporation.
LED CURRENT (mA)
0
60
EFFICIENCY (%)
65
70
75
80
85
90
V
IN
= 3V
5101520
1937 TA01b
V
IN
= 3.6V
Figure 1. Li-Ion Powered Driver for Three White LEDs
Conversion Efficiency
VIN SW
LT1937
L1
22µH
C1
1µFC2
0.22µF
15mA
1937 F01a
VIN
3V TO 5V
OFF ON SHDN
R1
6.34
C1, C2: X5R OR X7R DIELECTRIC
D1: CENTRAL SEMICONDUCTOR CMDSH-3
L1: MURATA LQH3C-220 OR EQUIVALENT
FB
LED 3
LED 2
LED 1
D1
GND
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
FB V0‘ E : I:° :I i I: j I: j C :i
2
LT1937
1937f
Input Voltage (V
IN
) ................................................. 10V
SW Voltage ............................................................. 36V
FB Voltage .............................................................. 10V
SHDN Voltage ......................................................... 10V
T
JMAX
= 125°C, θ
JA
= 256°C/ W IN FREE AIR
θ
JA
= 120°C ON BOARD OVER GROUND PLANE
ABSOLUTE AXI U RATI GS
WWWU
PACKAGE/ORDER I FOR ATIO
UU
W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
SW 1
GND 2
TOP VIEW
S5 PACKAGE
5-LEAD PLASTIC TSOT-23
FB 3
5 V
IN
4 SHDN
ORDER PART
NUMBER
S5 PART MARKING
LT1937ES5
LTYN
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Operating Voltage 2.5 V
Maximum Operating Voltage 10 V
Feedback Voltage I
SW
= 100mA, Duty Cycle = 66% 86 95 104 mV
FB Pin Bias Current 10 45 100 nA
Supply Current 1.9 2.5 mA
SHDN = 0V 0.1 1.0 µA
Switching Frequency 0.8 1.2 1.6 MHz
Maximum Duty Cycle 85 90 %
Switch Current Limit 320 mA
Switch V
CESAT
I
SW
= 250mA 350 mV
Switch Leakage Current V
SW
= 5V 0.01 5 µA
SHDN Voltage High 1.5 V
SHDN Voltage Low 0.4 V
SHDN Pin Bias Current 65 µA
TA = 25°C, VIN = 3V, VSHDN = 3V, unless otherwise noted.
ELECTRICAL CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
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.
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
(Note 1)
ORDER PART
NUMBER
SC6 PART MARKING
LT1937ESC6
LAAB
SW 1
GND 2
FB 3
6 V
IN
5 GND
4 SHDN
TOP VIEW
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
L7LELUW
3
LT1937
1937f
TYPICAL PERFOR A CE CHARACTERISTICS
UW
Current Limit vs Duty Cycle
Quiescent Current SHDN Pin Bias Current Switching Frequency
Feedback Bias Current
UU
U
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.
V
IN
(V)
0
0
I
Q
(mA)
0.4
0.8
1.2
1.6
2468
1937 G01
10
2.0
0.2
0.6
1.0
1.4
1.8
2.2 –50°C
100°C
25°C
TEMPERATURE (°C)
–50
SHDN PIN BIAS CURRENT (µA)
350
25
1937 G02
200
100
–25 0 50
50
0
400
300
250
150
75 100
SHDN = 10V
SHDN = 3.6V SHDN = 3V
SHDN = 2.7V
TEMPERATURE (°C)
–50
0.8
1.0
1.4
25 75
1937 G03
0.6
0.4
–25 0 50 100
0.2
0
1.2
SWITCHING FREQUENCY (MHz)
TEMPERATURE (°C)
–50
FEEDBACK BIAS CURRENT (nA)
40
50
60
25 75
1937 G04
30
20
–25 0 50 100
10
0
Efficiency vs Temperature
TEMPERATURE (°C)
–50
EFFICIENCY (%)
82
83
1937 G05
81
80 050 100
85
84 I
LED
= 20mA
I
LED
= 15mA
I
LED
= 10mA
V
IN
= 3.6V
3 LEDs
DUTY CYCLE (%)
0
350
300
250
200
150
100
50
060
1937 G06
20 40 80 100
CURRENT LIMIT (mA)
-II— —D "WT #3 E \ «H 1% MAM L7 UHF/‘9
4
LT1937
1937f
+
+
RQ
S
0.2
SW
DRIVER
COMPARATOR
2
SHDN
4
1
V
IN
3
FB
95mV
5
+
Σ
RAMP
GENERATOR
R
C
C
C
1.2MHz
OSCILLATOR
GND
1937 BD1
Q1
A2
A1
SHUTDOWN
V
REF
1.25V
(PIN 6 FOR
SC70 PACKAGE)
(PINS 2 AND 5 FOR
SC70 PACKAGE)
BLOCK DIAGRA
W
OPERATIO
U
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
Figure 2. LT1937 Block Diagram
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 300 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 ILED = 4mA, VIN = 3.6V
L7LELUW
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LT1937
1937f
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
CURRENT
DCR RATING
PART NUMBER () (mA) MANUFACTURER
LQH3C220 0.71 250 Murata
814-237-1431
www.murata.com
ELJPC220KF 4.0 160 Panasonic
714-373-7334
www.panasonic.com
CDRH3D16-220 0.53 350 Sumida
847-956-0666
www.Sumida.com
LB2012B220M 1.7 75 Taiyo Yuden
408-573-4150
www.t-yuden.com
LEM2520-220 5.5 125 Taiyo Yuden
408-573-4150
www.t-yuden.com
APPLICATIO S I FOR ATIO
WUUU
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 PHONE URL
Taiyo Yuden 408-573-4150 www.t-yuden.com
AVX 843-448-9411 www.avxcorp.com
Murata 814-237-1431 www.murata.com
Kemet 408-986-0424 www.kemet.com
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
PART CURRENT DROP CAPACITANCE
NUMBER (mA) (V) (pF) MANUFACTURER
CMDSH-3 100 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
Figure 4. Efficiency Comparison of Different Inductors
LOAD CURRENT (mA)
0
EFFICIENCY (%)
90
85
80
75
70
65
60
55
50 16
1937 F04
4 8 12 20142 6 10 18
MURATA LQH3C-220
PANASONIC ELJPC220KF
SUMIDA CDRH3D16-220
TAIYO YUDEN LB2012B220M
TAIYO YUDEN LEM2520-220
VIN = 3.6V
3LEDs
ofthe L7 UHF/‘9
6
LT1937
1937f
LED Current Control
The LED current is controlled by the feedback resistor (R1
in Figure 1). The feedback reference is 95mV. The LED
current is 95mV/R1. In order to have accurate LED current,
precision resistors are preferred (1% is recommended).
The formula and table for R1 selection are shown below.
R1 = 95mV/I
LED
(1)
Table 4. R1 Resistor Value Selection
I
LED
(mA) R1 ()
5 19.1
10 9.53
12 7.87
15 6.34
20 4.75
Open-Circuit Protection
In the cases of output open circuit, when the LEDs are
disconnected from the circuit or the LEDs fail, the feed-
back voltage will be zero. The LT1937 will then switch at
a high duty cycle resulting in a high output voltage, which
may cause the SW pin voltage to exceed its maximum 36V
rating. A zener diode can be used at the output to limit the
voltage on the SW pin (Figure 5). The zener voltage should
be larger than the maximum forward voltage of the LED
string. The current rating of the zener should be larger than
0.1mA.
APPLICATIO S I FOR ATIO
WUUU
Dimming Control
There are four different types of dimming control circuits:
1. Using a PWM Signal to SHDN Pin
With the PWM signal applied to the SHDN pin, the LT1937
is turned on or off by the PWM signal. The LEDs operate
at either zero or full current. The average LED current
increases proportionally with the duty cycle of the PWM
signal. A 0% duty cycle will turn off the LT1937 and
corresponds to zero LED current. A 100% duty cycle
corresponds to full current. The typical frequency range of
the PWM signal is 1kHz to 10kHz. The magnitude of the
PWM signal should be higher than the minimum SHDN
voltage high. The switching waveforms of the SHDN pin
PWM control are shown in Figures 6a and 6b.
Figure 5. LED Driver with Open-Circuit Protection
200µs/DIV 1937 F06a
FB
100mV/DIV
SHDN
2V/DIV
(6a) 1kHz
20µs/DIV 1937 F06b
FB
100mV/DIV
SHDN
2V/DIV
(6b) 10kHz
Figure 6. PWM Dimming Control Using the SHDN Pin
PWM
LT1937
SHDN
V
IN
SW
LT1937
L
22µH
C
OUT
0.22µF
C
IN
1µF
1937 F05
V
IN
SHDN
R1
6.34
R2
1k
FB
D
GND
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LT1937
1937f
APPLICATIO S I FOR ATIO
WUUU
2. Using a DC Voltage
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
The dimming control using a DC voltage is shown in
Figure␣ 7. As the DC voltage increases, the voltage drop on
R2 increases and the voltage drop on R1 decreases. Thus,
the LED current decreases. The selection of R2 and R3 will
make the current from the variable DC source much
smaller than the LED current and much larger than the FB
pin bias current. For V
DC
range from 0V to 2V, the selection
of resistors in Figure 7 gives dimming control of LED
current from 0mA to 15mA.
3. Using a Filtered PWM Signal
The filtered PWM signal can be considered as an adjust-
able DC voltage. It can be used to replace the variable DC
voltage source in dimming control. The circuit is shown in
Figure 8.
Figure 7. Dimming Control Using a DC Voltage
Figure 8. Dimming Control Using a Filtered PWM Signal
Figure 9. Dimming Control Using a Logic Signal
V
IN
= 3.6V 50µs/DIV 1937 F09
THREE LEDs
15mA
FB
100mV/DIV
SHDN
2V/DIV
I
IN
100mA/DIV
Figure 10. Start-Up Waveforms Without Soft-Startup Circuit
4. Using a Logic Signal
For applications that need to adjust the LED current in
discrete steps, a logic signal can be used as shown in
Figure 9. R1 sets the minimum LED current (when the
NMOS is off). R
INC
sets how much the LED current
increases when the NMOS is turned on. The selection of
R1 and R
INC
follows formula (1) and Table 4.
Start-up and Inrush Current
To achieve minimum start-up delay, no internal soft-start
circuit is included in LT1937. When first turned on without
an external soft-start circuit, inrush current is about 200mA
as shown in Figure␣ 10. If soft-start is desired, the recom-
mended circuit and the waveforms are shown in Figure 11.
If both soft-start and dimming are used, a 10kHz PWM
signal on SHDN is not recommended. Use a lower fre-
quency or implement dimming through the FB pin as
shown in Figures 7, 8 or 9.
VDC
R2
5k
R1
6.3
1937 F07
R3
90k
LT1937
FB
PWM
R2
5k
R1
6.3
0.1µF
1937 F08
R3
90k
10k
LT1937
FB
LOGIC
SIGNAL 2N7002 R1
1937 F09
R
INC
LT1937
FB
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8
LT1937
1937f
APPLICATIO S I FOR ATIO
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(11a) Recommended Soft-Startup Circuit
V
IN
= 3.6V 50µs/DIV 1937 F11b
THREE LEDs
15mA
FB
100mV/DIV
SHDN
2V/DIV
I
IN
100mA/DIV
Figure 11. Recommended Soft-Startup Circuit and Waveforms
(11b) Soft-Startup Waveforms
Board Layout Consideration
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To maximize efficiency, switch rise and fall times are made
as short as possible. To prevent electromagnetic interfer-
ence (EMI) problems, proper layout of the high frequency
switching path is essential. The voltage signal of the SW
pin has sharp rise and fall edges. Minimize the length and
15
4
R1
2
3
R2
R3
DIMMING
CONTROL
C
O
C
IN
V
IN
GND
SHDN
1937 F12a
DL
Figure 12. Recommended Component Placement
area of all traces connected to the SW pin and always use
a ground plane under the switching regulator to minimize
interplane coupling. In addition, the ground connection for
the feedback resistor R1 should be tied directly to the GND
pin and not shared with any other component, ensuring a
clean, noise-free connection. Recommended component
placement is shown in Figure 12.
(SOT-23 Package)
16
5
4
R1
2
3
R2
R3
DIMMING
CONTROL
C
O
C
IN
V
IN
GND
SHDN
1937 F12b
DL
(SC70 Package)
R2
1k
R1
6.34
5k
D2: MMBT
2.2nF
D1
D2 COUT
1937 F11a
LT1937
FB
L7LELUW
9
LT1937
1937f
TYPICAL APPLICATIO S
U
Li-Ion to Two White LEDs Two LED Efficiency
V
IN
SW
LT1937
L
22µH
C
OUT
1µF
C
IN
1µF
C
IN
: TAIYO YUDEN JMK107BJ105
C
OUT
: AVX 0603ZD105
D: CENTRAL CMDSH2-3
L: MURATA LQH3C220
1937 TA05a
V
IN
3V TO 5V
SHDN
R1
2
5k
90k
FB
D
V
DC
DIMMING
GND
LED CURRENT (mA)
0
EFFICIENCY (%)
78
80
82
40
1937 TA05a
76
74
70 10 20 30
72
86
84 V
IN
= 3.6V
V
IN
= 3V
V
IN
SW
LT1937
L
22µH
C
OUT
0.22µF
C
IN
1µF
C
IN
: TAIYO YUDEN JMK107BJ105
C
OUT
: AVX 0603YD224
D: CENTRAL CMDSH-3
L: MURATA LQH3C220
1937 TA01a
V
IN
3V TO 5V
SHDN
R1
4
5k
90k
FB
D
V
DC
DIMMING
GND
LED CURRENT (mA)
0
60
EFFICIENCY (%)
65
70
75
80
85
90
V
IN
= 3V
5101520
1937 TA01b
V
IN
= 3.6V
Li-Ion to Three White LEDs Three LED Efficiency
L7 UHF/‘9
10
LT1937
1937f
TYPICAL APPLICATIO S
U
Li-Ion to Five White LEDs Five LED Efficiency
V
IN
SW
LT1937
L
22µH
C
OUT
0.22µF
C
IN
1µF
C
IN
: TAIYO YUDEN JMK107BJ105
C
OUT
: TAIYO YUDEN GMK212BJ224
D: CENTRAL CMDSH-3
L: MURATA LQH3C220
1937 TA03a
V
IN
3V TO 5V
SHDN
R1
4
5k
90k
FB
D
V
DC
DIMMING
GND
LOAD CURRENT (mA)
0
65
EFFICIENCY (%)
70
75
80
85
2468
1937 TA03b
10 12
V
IN
= 3.6V
V
IN
= 3V
TYPICAL APPLICATIO S
U
5V to Seven White LEDs Seven LED Efficiency
V
IN
SW
LT1937
L
22µH
C
OUT
0.22µF
C
IN
1µF
C
IN
: TAIYO YUDEN JMK107BJ105
C
OUT
: TAIYO YUDEN GMK212BJ224
D: CENTRAL CMDSH-3
L: MURATA LQH3C220
1937 TA04a
V
IN
5V
SHDN
R1
4
5k
90k
FB
D
V
DC
DIMMING
GND
LOAD CURRENT (mA)
0
65
EFFICIENCY (%)
70
75
80
85
510
1937 TA04b
15
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11
LT1937
1937f
U
PACKAGE DESCRIPTIO
S5 Package
5-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1635)
SC6 Package
6-Lead Plastic SC70
(Reference LTC DWG # 05-08-1638)
1.50 – 1.75
(NOTE 4)
2.80 BSC
0.30 – 0.45 TYP
5 PLCS (NOTE 3)
DATUM ‘A’
0.09 – 0.20
(NOTE 3)
S5 TSOT-23 0302
PIN ONE
2.90 BSC
(NOTE 4)
0.95 BSC
1.90 BSC
0.80 – 0.90
1.00 MAX 0.01 – 0.10
0.20 BSC
0.30 – 0.50 REF
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
3.85 MAX
0.62
MAX 0.95
REF
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
1.4 MIN
2.62 REF
1.22 REF
1.15 – 1.35
(NOTE 4)
1.80 – 2.40
0.15 – 0.30
6 PLCS (NOTE 3)
0.10 – 0.18
(NOTE 3)
SC6 SC70 0302
1.80 – 2.20
(NOTE 4)
0.65 BSC
PIN 1
0.80 – 1.00
1.00 MAX 0.00 – 0.10
0.10 – 0.40
0.10 – 0.30
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. DETAILS OF THE PIN 1 INDENTIFIER ARE OPTIONAL,
BUT MUST BE LOCATED WITHIN THE INDEX AREA
7. EIAJ PACKAGE REFERENCE IS EIAJ SC-70
3.26 MAX
0.47
MAX 0.65
REF
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.96 MIN
2.1 REF
1.16 REF
INDEX AREA
(NOTE 6)
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
L7 UHF/‘9
12
LT1937
1937f
PART NUMBER DESCRIPTION COMMENTS
LT1615 Micropower Step-Up Converter in ThinSOT Up to 36V Output; 20µA I
Q
, V
IN
: 1V to 15V,
Can Drive Up to Six LEDs, ThinSOT Package
LT1618 Constant Current/Voltage Step-Up DC/DC 1.4MHz, Drives Up to 20 LEDs, MS10 Package
LT1932 White LED Step-Up Converter in ThinSOT 1.2MHz, V
IN
= 1V to 10V, Drives Up to Eight LEDs from 3V Input,
ThinSOT Package
LT1944/LT1944-1 Dual Micropower Step-Up Converter V
IN
= 1.2V to 15V, Two Independent DC/DCs, Up to 36V
OUT
, 20µA I
Q
,
MS10 Package
LTC®3200/LTC3200-5 Low Noise White LED Charge Pump Converter 2MHz, 100mA, No Inductor Required, MS8/ThinSOT Packages
For up to 6 LEDs
LTC3201 Ultralow Noise White LED Charge Pump Converter 1.8MHz, 100mA, No Inductor Required, DAC Brightness Adj,
For up to 6 LEDs MS8 Package
LTC3202 Low Noise White LED Fractional Charge Pump Converter 1.5MHz, 125mA, No Inductor Required, Digital Brightness Adjust,
For up to 6 LEDs MS8 Package
LT/TP 0702 2K • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 2002
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
FAX: (408) 434-0507
www.linear.com
Li-Ion to Four White LEDs
Four LED Efficiency
V
IN
SW
LT1937
L
22µH
C
OUT
0.22µF
C
IN
1µF
C
IN
: TAIYO YUDEN JMK107BJ105
C
OUT
: AVX 0603YD224
D: CENTRAL CMDSH-3
L: MURATA LQH3C220
1937 TA02a
V
IN
3V TO 5V
SHDN
R1
4
5k
90k
FB
D
V
DC
DIMMING
GND
LOAD CURRENT (mA)
0
60
EFFICIENCY (%)
70
75
80
85
510
1937 TA02b
15 20
V
IN
= 3V V
IN
= 3.6V
U
TYPICAL APPLICATIO
RELATED PARTS
0.2µs/DIV
1937 TA02c
V
SW
10V/DIV
I
SW
100mA/DIV
V
OUT
100mV/DIV
V
IN
= 3.6V
FOUR LEDs
15mA
Switching Waveforms

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