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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 0.22F Output Capacitor Low Profile SC70 and ThinSOTTM Packaging
APPLICATIO S
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The LT (R)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 identical 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 0.22F, saving space and cost versus alternative solutions. 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 22H C1 1F VIN LT1937 OFF ON SHDN GND FB SW
Conversion Efficiency
D1 C2 0.22F 15mA
EFFICIENCY (%)
90 85 80 VIN = 3V 75 70 65 60 0 5 10 15 LED CURRENT (mA) 20
1937 TA01b
VIN 3V TO 5V
LED 1 LED 2 LED 3 R1 6.34
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
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VIN = 3.6V
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LT1937
ABSOLUTE
AXI U
RATI GS
Input Voltage (VIN) ................................................. SW Voltage ............................................................. FB Voltage .............................................................. SHDN Voltage .........................................................
PACKAGE/ORDER I FOR ATIO
TOP VIEW SW 1 GND 2 FB 3 4 SHDN 5 VIN
ORDER PART NUMBER
TOP VIEW
LT1937ES5
S5 PACKAGE 5-LEAD PLASTIC TSOT-23
TJMAX = 125C, JA = 256C/ W IN FREE AIR JA = 120C 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
TA = 25C, VIN = 3V, VSHDN = 3V, unless otherwise noted.
MIN 2.5 10 TYP MAX UNITS V V mV nA mA A MHz % mA mV 5 0.4 65 A V V A 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
ISW = 100mA, Duty Cycle = 66%
SHDN = 0V Switching Frequency Maximum Duty Cycle Switch Current Limit Switch VCESAT 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. ISW = 250mA VSW = 5V
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(Note 1)
10V 36V 10V 10V
Extended Commercial Operating Temperature Range (Note 2)... - 40C to 85C Maximum Junction Temperature .......................... 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER
SW 1 GND 2 FB 3 6 VIN 5 GND 4 SHDN
LT1937ESC6
SC6 PACKAGE 6-LEAD PLASTIC SC70
TJMAX = 125C, JA = 256C/ W IN FREE AIR JA = 150C ON BOARD OVER GROUND PLANE
SC6 PART MARKING LAAB
86 10
Note 2: The LT1937E is guaranteed to meet specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls.
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LT1937 TYPICAL PERFOR A CE CHARACTERISTICS
Quiescent Current
2.2 2.0 1.8 1.6 1.4 25C 100C -50C
SHDN PIN BIAS CURRENT (A)
300 SHDN = 10V 250 200 150 100 50 SHDN = 3.6V SHDN = 3V
SWITCHING FREQUENCY (MHz)
IQ (mA)
1.2 1.0 0.8 0.6 0.4 0.2 0 0 2 4 VIN (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. Calculate resistor value according to the formula: RFB = 95mV/ILED 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 VIN (Pin 5/Pin 6 SC70 Package): Input Supply Pin. Must be locally bypassed.
UW
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 VIN = 3.6V 3 LEDs
350
Current Limit vs Duty Cycle
300 250 200 150 100 50 0
84
ILED = 20mA
83
ILED = 15mA
82
ILED = 10mA
81
100
80 -50
50 0 TEMPERATURE (C)
100
1937 G05
0
20
40 60 DUTY CYCLE (%)
80
100
1937 G06
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LT1937
BLOCK DIAGRA
VIN (PIN 6 FOR SC70 PACKAGE) 5
VREF 1.25V
95mV
A1
+
RC CC
-
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 regulation. 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 300 resistor across the inductor, although this will degrade efficiency.
VSW 5V/DIV
IL2 50mA/DIV VOUT 100mV/DIV 0.2s/DIV
1937 F03
Figure 3. Switching Waveforms at ILED = 4mA, VIN = 3.6V
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+
-
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FB 3 1 SW COMPARATOR DRIVER A2 R S Q Q1
-
+
0.2
2 GND
1937 BD1
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LT1937
APPLICATIO S I FOR ATIO
Inductor Selection
A 22H inductor is recommended for most LT1937 applications. 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 () 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 VIN = 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
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Capacitor Selection The small size of ceramic capacitors makes them ideal for LT1937 applications. X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U. A 1F input capacitor and a 0.22F 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
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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 (CT or CD) 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 switching frequency of the LT1937. A Schottky diode rated at 100mA to 200mA is sufficient for most LT1937 applications. 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
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LT1937
APPLICATIO S I FOR ATIO
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/ILED
Table 4. R1 Resistor Value Selection
ILED (mA) 5 10 12 15 20 R1 () 19.1 9.53 7.87 6.34 4.75
Open-Circuit Protection
LT1937
In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail, the feedback 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.
L 22H VIN CIN 1F VIN LT1937 SHDN GND FB R1 6.34
1937 F05
D COUT 0.22F SW R2 1k
Figure 5. LED Driver with Open-Circuit Protection
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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. (1)
SHDN PWM FB 100mV/DIV SHDN 2V/DIV 200s/DIV
1937 F06a
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(6a) 1kHz
FB 100mV/DIV
SHDN 2V/DIV
20s/DIV
1937 F06b
(6b) 10kHz Figure 6. PWM Dimming Control Using the SHDN Pin
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LT1937
APPLICATIO S I FOR ATIO
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 VDC 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 adjustable DC voltage. It can be used to replace the variable DC voltage source in dimming control. The circuit is shown in Figure 8.
LT1937 FB R3 90k VDC R1 6.3
1937 F07
R2 5k
Figure 7. Dimming Control Using a DC Voltage
LT1937 FB R3 90k 0.1F R2 5k R1 6.3
1937 F08
10k PWM
Figure 8. Dimming Control Using a Filtered PWM Signal
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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). RINC sets how much the LED current increases when the NMOS is turned on. The selection of R1 and RINC 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 recommended 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 frequency or implement dimming through the FB pin as shown in Figures 7, 8 or 9.
LT1937 FB RINC LOGIC SIGNAL 2N7002 R1
1937 F09
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Figure 9. Dimming Control Using a Logic Signal
IIN 100mA/DIV
FB 100mV/DIV SHDN 2V/DIV VIN = 3.6V THREE LEDs 15mA 50s/DIV
1937 F09
Figure 10. Start-Up Waveforms Without Soft-Startup Circuit
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LT1937
APPLICATIO S I FOR ATIO
D1 2.2nF
LT1937 FB
D2
R2 1k 5k R1 6.34
COUT SHDN 2V/DIV VIN = 3.6V THREE LEDs 15mA 50s/DIV
1937 F11b
D2: MMBT
1937 F11a
(11a) Recommended Soft-Startup Circuit
Figure 11. Recommended Soft-Startup Circuit and 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 interference (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
L D CO 1 2 3 R2 R1
1937 F12a
5
CIN GND
4 SHDN R2
DIMMING CONTROL
R3
(SOT-23 Package) Figure 12. Recommended Component Placement
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IIN 100mA/DIV FB 100mV/DIV
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(11b) Soft-Startup Waveforms
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.
VIN CO
L D 1 2 3 6 5 4 CIN
VIN
GND
SHDN R1
1937 F12b
DIMMING CONTROL
R3
(SC70 Package)
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LT1937
TYPICAL APPLICATIO S
Li-Ion to Two White LEDs
L 22H CIN 1F D COUT 1F 86 84 82 VIN = 3.6V
VIN 3V TO 5V
EFFICIENCY (%)
VDC DIMMING VIN LT1937 SHDN GND FB R1 2 SW 90k 5k
CIN: TAIYO YUDEN JMK107BJ105 COUT: AVX 0603ZD105 D: CENTRAL CMDSH2-3 L: MURATA LQH3C220
Li-Ion to Three White LEDs
L 22H CIN 1F D COUT 0.22F
EFFICIENCY (%)
90 85 80
VIN 3V TO 5V
VDC DIMMING VIN LT1937 SHDN GND FB R1 4 SW 90k 5k
CIN: TAIYO YUDEN JMK107BJ105 COUT: AVX 0603YD224 D: CENTRAL CMDSH-3 L: MURATA LQH3C220
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Two LED Efficiency
VIN = 3V 80 78 76 74 72
1937 TA05a
70
0
10
20 LED CURRENT (mA)
30
40
1937 TA05a
Three LED Efficiency
VIN = 3V 75 70 65 60 0 5
VIN = 3.6V
1937 TA01a
10 15 LED CURRENT (mA)
20
1937 TA01b
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LT1937
TYPICAL APPLICATIO S
Li-Ion to Five White LEDs
L 22H CIN 1F
VIN 3V TO 5V
EFFICIENCY (%)
VDC DIMMING VIN LT1937 SHDN GND FB R1 4 SW 90k 5k
CIN: TAIYO YUDEN JMK107BJ105 COUT: TAIYO YUDEN GMK212BJ224 D: CENTRAL CMDSH-3 L: MURATA LQH3C220
5V to Seven White LEDs
L 22H CIN 1F
D COUT 0.22F
VIN 5V
EFFICIENCY (%)
VDC DIMMING VIN LT1937 SHDN GND FB R1 4 SW 90k 5k
CIN: TAIYO YUDEN JMK107BJ105 COUT: TAIYO YUDEN GMK212BJ224 D: CENTRAL CMDSH-3 L: MURATA LQH3C220
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Five LED Efficiency
85
COUT 0.22F
80 VIN = 3V 75 VIN = 3.6V 70
1937 TA03a
65 0 2 4 6 8 LOAD CURRENT (mA) 10 12
1937 TA03b
Seven LED Efficiency
85
80
75
70
1937 TA04a
65 0 5 10 LOAD CURRENT (mA) 15
1937 TA04b
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LT1937
PACKAGE DESCRIPTIO
0.62 MAX
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.20 BSC 1.00 MAX DATUM `A'
0.30 - 0.50 REF 0.09 - 0.20 (NOTE 3) 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
0.47 MAX
0.65 REF
3.26 MAX 2.1 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 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 0.10 - 0.18 (NOTE 3) 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
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
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S5 Package 5-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1635)
0.95 REF 2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 2.80 BSC 1.50 - 1.75 (NOTE 4) PIN ONE 0.30 - 0.45 TYP 5 PLCS (NOTE 3) 0.95 BSC 0.80 - 0.90 0.01 - 0.10 1.90 BSC
S5 TSOT-23 0302
SC6 Package 6-Lead Plastic SC70
(Reference LTC DWG # 05-08-1638)
1.80 - 2.20 (NOTE 4)
1.16 REF
0.96 MIN
1.80 - 2.40 1.15 - 1.35 (NOTE 4)
INDEX AREA (NOTE 6)
PIN 1
0.65 BSC
0.15 - 0.30 6 PLCS (NOTE 3)
0.80 - 1.00 0.00 - 0.10
1.00 MAX
SC6 SC70 0302
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LT1937
TYPICAL APPLICATIO
VIN 3V TO 5V
Four LED Efficiency
85
80
EFFICIENCY (%)
VIN = 3V 75
VIN = 3.6V
70
60 0 15 5 10 LOAD CURRENT (mA) 20
1937 TA02b
RELATED PARTS
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1937f LT/TP 0702 2K * PRINTED IN USA
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
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Li-Ion to Four White LEDs
L 22H CIN 1F D COUT 0.22F VDC DIMMING VIN LT1937 SHDN GND FB R1 4 SW 90k 5k CIN: TAIYO YUDEN JMK107BJ105 COUT: AVX 0603YD224 D: CENTRAL CMDSH-3 L: MURATA LQH3C220
1937 TA02a
Switching Waveforms
VSW 10V/DIV ISW 100mA/DIV VOUT 100mV/DIV
1937 TA02c
VIN = 3.6V FOUR LEDs 15mA
0.2s/DIV
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2002

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