Datasheet

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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

Packaging

■

Cellular Phones

■

PDAs, Handheld Computers

■

Digital Cameras

■

MP3 Players

■

GPS Receivers

ThinSOT is a trademark of Linear Technology Corporation.

LED CURRENT (mA)

EFFICIENCY (%)

= 3V

5101520

1937 TA01b

= 3.6V

Figure 1. Li-Ion Powered Driver for Three White LEDs

Conversion Efficiency

LT1937

22µH

1µF

0.22µF

15mA

1937 F01a

3V TO 5V

OFF ON

SHDN

6.34Ω

C1, C2: X5R OR X7R DIELECTRIC

D1: CENTRAL SEMICONDUCTOR CMDSH-3

L1: MURATA LQH3C-220 OR EQUIVALENT

LED 3

LED 2

LED 1

GND

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Summary of content (12 pages)

PAGE 1
LT1937 White LED Step-Up Converter in SC70 and ThinSOT U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO 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 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.
PAGE 2
LT1937 W W W AXI U U ABSOLUTE RATI GS Input Voltage (VIN) ................................................. SW Voltage ............................................................. FB Voltage .............................................................. SHDN Voltage ......................................................... (Note 1) 10V 36V 10V 10V Extended Commercial Operating Temperature Range (Note 2)... – 40°C to 85°C Maximum Junction Temperature ..........................
PAGE 3
LT1937 U W TYPICAL PERFOR A CE CHARACTERISTICS Quiescent Current 100°C 25°C 1.6 SHDN PIN BIAS CURRENT (µA) 1.8 IQ (mA) 1.4 1.2 1.0 0.8 0.6 0.4 400 1.4 350 1.2 300 SHDN = 10V 250 200 150 SHDN = 3.6V 100 50 0.2 0 0 2 4 6 8 SHDN = 3V – 25 VIN (V) 0 50 25 TEMPERATURE (°C) 1937 G01 Feedback Bias Current 0.4 0.2 75 0 –50 100 40 30 20 75 100 50 25 0 TEMPERATURE (°C) 75 VIN = 3.
PAGE 4
LT1937 W BLOCK DIAGRA VIN (PIN 6 FOR SC70 PACKAGE) 5 FB 3 1 SW – VREF 1.25V 95mV – A1 + COMPARATOR DRIVER RC + A2 R S Q1 Q CC + Σ 0.2Ω – RAMP GENERATOR SHDN 4 (PINS 2 AND 5 FOR SC70 PACKAGE) SHUTDOWN 2 GND 1937 BD1 1.2MHz OSCILLATOR Figure 2. LT1937 Block Diagram U 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.
PAGE 5
LT1937 U W U U APPLICATIO S I FOR ATIO Inductor Selection Capacitor Selection A 22µH 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.
PAGE 6
LT1937 U W U U APPLICATIO S I FOR ATIO LED Current Control Dimming 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. There are four different types of dimming control circuits: R1 = 95mV/ILED (1) Table 4. R1 Resistor Value Selection ILED (mA) R1 (Ω) 5 19.
PAGE 7
LT1937 U W U U APPLICATIO S I FOR ATIO 2. Using a DC Voltage 4. Using a Logic Signal 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.
PAGE 8
LT1937 U W U U APPLICATIO S I FOR ATIO D1 IIN 100mA/DIV 2.2nF LT1937 FB FB 100mV/DIV R2 1k D2 COUT SHDN 2V/DIV R1 6.34Ω 5k VIN = 3.6V THREE LEDs 15mA D2: MMBT 1937 F11a (11a) Recommended Soft-Startup Circuit 50µs/DIV 1937 F11b (11b) Soft-Startup Waveforms 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.
PAGE 9
LT1937 U TYPICAL APPLICATIO S Li-Ion to Two White LEDs L 22µH 86 D VIN = 3.
PAGE 10
LT1937 U TYPICAL APPLICATIO S Li-Ion to Five White LEDs 85 L 22µH D CIN 1µF COUT 0.22µF VDC DIMMING VIN SW 90k LT1937 SHDN 80 EFFICIENCY (%) VIN 3V TO 5V Five LED Efficiency 5k FB VIN = 3V 75 VIN = 3.6V 70 R1 4Ω GND CIN: TAIYO YUDEN JMK107BJ105 COUT: TAIYO YUDEN GMK212BJ224 D: CENTRAL CMDSH-3 L: MURATA LQH3C220 65 1937 TA03a 0 12 Seven LED Efficiency D COUT 0.
PAGE 11
LT1937 U PACKAGE DESCRIPTIO S5 Package 5-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1635) 0.62 MAX 0.95 REF 2.90 BSC (NOTE 4) 1.22 REF 2.80 BSC 1.4 MIN 3.85 MAX 2.62 REF 1.50 – 1.75 (NOTE 4) PIN ONE RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 – 0.45 TYP 5 PLCS (NOTE 3) 0.95 BSC 0.80 – 0.90 0.20 BSC 0.01 – 0.10 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.
PAGE 12
LT1937 U TYPICAL APPLICATIO Li-Ion to Four White LEDs L 22µH VIN 3V TO 5V D CIN 1µF COUT 0.22µF VDC DIMMING VIN SW 90k LT1937 SHDN 5k FB R1 4Ω GND CIN: TAIYO YUDEN JMK107BJ105 COUT: AVX 0603YD224 D: CENTRAL CMDSH-3 L: MURATA LQH3C220 1937 TA02a Four LED Efficiency 85 Switching Waveforms VSW 10V/DIV EFFICIENCY (%) 80 VIN = 3V VIN = 3.6V ISW 100mA/DIV 75 VOUT 100mV/DIV 70 60 0 15 5 10 LOAD CURRENT (mA) 20 VIN = 3.6V FOUR LEDs 15mA 1937 TA02c 0.