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LT1932

Constant-Current DC/DC

LED Driver in ThinSOT

July 2001

The LT

1932 is a fixed frequency step-up DC/DC converter

designed to operate as a constant-current source. Be-

cause it directly regulates output current, the LT1932 is

ideal for driving light emitting diodes (LEDs) whose light

intensity is proportional to the current passing through

them, not the voltage across their terminals.

With an input voltage range of 1V to 10V, the device works

from a variety of input sources. The LT1932 accurately

regulates LED current even when the input voltage is

higher than the LED voltage, greatly simplifying battery-

powered designs. A single external resistor sets LED

current between 5mA and 40mA, which can then be easily

adjusted using either a DC voltage or a pulse width

modulated (PWM) signal. When the LT1932 is placed in

shutdown, the LEDs are disconnected from the output,

ensuring a quiescent current of under 1µA for the entire

circuit. The device’s 1.2MHz switching frequency permits

the use of tiny, low profile chip inductors and capacitors to

minimize footprint and cost in space-conscious portable

applications.

■

Up to 80% Efficiency

■

Inherently Matched LED Current

■

Drives Five White LEDs from 2V

■

Drives Six White LEDs from 2.7V

■

Drives Eight White LEDs from 3V

■

Precise, Adjustable Control of LED Current

■

Disconnects LEDs In Shutdown

■

1.2MHz Fixed Frequency Switching

■

Uses Tiny Ceramic Capacitors

■

Uses Tiny 1mm-Tall Inductors

■

Regulates Current Even When V

> V

OUT

■

Operates with V

as Low as 1V

■

Low Profile (1mm) ThinSOT

Package

■

Cellular Telephones

■

Handheld Computers

■

Digital Cameras

■

Portable MP3 Players

■

Pagers

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.

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

Li-Ion Driver for Four White LEDs Efficiency

Final Electrical Specifications

, LTC and LT are registered trademarks of Linear Technology Corporation.

15mA

1932 TA01

6.8µH

2.7V TO 4.2V

LT1932

SHDN

C1: TAIYO YUDEN JMK212BJ475

C2: TAIYO YUDEN EMK212BJ105

D1:ZETEX ZHCS400

L1: SUMIDA CLQ4D106R8 OR PANASONIC ELJEA6R8

SET

1.50k

PWM

DIMMING

CONTROL

4.7µF

1µF

LED

SET

GND

LED CURRENT (mA)

EFFICIENCY (%)

5101520

1932 TA02

= 4.2V

= 2.7V

ThinSOT is a trademark of Linear Technology Corporation.

Summary of content (16 pages)

PAGE 1
Final Electrical Specifications LT1932 Constant-Current DC/DC LED Driver in ThinSOT July 2001 U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LT®1932 is a fixed frequency step-up DC/DC converter designed to operate as a constant-current source. Because it directly regulates output current, the LT1932 is ideal for driving light emitting diodes (LEDs) whose light intensity is proportional to the current passing through them, not the voltage across their terminals.
PAGE 2
LT1932 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) VIN Voltage ............................................................. 10V SHDN Voltage ......................................................... 10V SW Voltage ............................................................. 36V LED Voltage ............................................................. 36V RSET Voltage ............................................................. 1V Junction Temperature ...................
PAGE 3
LT1932 U W TYPICAL PERFOR A CE CHARACTERISTICS Switch Saturation Voltage (VCESAT) Switch Current Limit TJ = 25°C 250 200 TJ = –50°C 150 100 SWITCHING FREQUENCY (MHz) 300 1.8 VIN = 1.2V 600 TJ = 125°C PEAK CURRENT (mA) SWITCH SATURATION VOLTAGE (mV) 350 VIN = 10V 500 400 300 200 100 50 0 Switching Frequency 2.
PAGE 4
LT1932 U U U PI FU CTIO S SW (Pin 1): Switch Pin. This is the collector of the internal NPN power switch. Minimize the metal trace area connected to this pin to minimize EMI. RSET (Pin 4): A resistor between this pin and ground programs the LED current (that flows into the LED pin). This pin is also used to provide LED dimming. GND (Pin 2): Ground Pin. Tie this pin directly to local ground plane. SHDN (Pin 5): Shutdown Pin. Tie this pin higher than 0.85V to turn on the LT1932; tie below 0.
PAGE 5
LT1932 U W U U APPLICATIO S I FOR ATIO Inductor Selection Several inductors that work well with the LT1932 are listed in Table 1. Many different sizes and shapes are available. Consult each manufacturer for more detailed information and for their entire selection of related parts. As core losses at 1.2MHz are much lower for ferrite cores that for the cheaper powdered-iron ones, ferrite core inductors should be used to obtain the best efficiency. Choose an inductor that can handle at least 0.
PAGE 6
LT1932 U W U U APPLICATIO S I FOR ATIO Capacitor Selection Low ESR (equivalent series resistance) capacitors should be used at the output to minimize the output ripple voltage. Because they have an extremely low ESR and are available in very small packages, multilayer ceramic capacitors are an excellent choice. X5R and X7R type capacitors are preferred because they retain their capacitance over wider voltage and temperature ranges than other types such as Y5V or Z5U. A 1µF or 2.
PAGE 7
LT1932 U W U U APPLICATIO S I FOR ATIO Open-Circuit Protection For applications where the string of LEDs can be disconnected or could potentially become an open circuit, a zener diode can be added across the LEDs to protect the LT1932 (see Figure 4). If the device is turned on without the LEDs present, no current feedback signal is provided to the LED pin. The LT1932 will then switch at its maximum duty cycle, generating an output voltage 10 to 15 times greater than the input voltage.
PAGE 8
LT1932 U W U U APPLICATIO S I FOR ATIO Dimming Using a Filtered PWM Signal Dimming Using a DC Voltage While the direct PWM method provides the widest dimming range and the purest white light output, it causes the LT1932 to enter into Burst ModeTM operation. This operation may be undesirable for some systems, as it may reflect some noise to the input source at the PWM frequency. The solution is to filter the control signal by adding a 10k resistor and a 0.
PAGE 9
LT1932 U W U U APPLICATIO S I FOR ATIO Board Layout Considerations 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 radiation and high frequency resonance problems, proper layout of the high frequency switching path is essential.
PAGE 10
LT1932 U TYPICAL APPLICATIO S Efficiency Single Cell Driver for One White LED L1 4.7µH VIN 1V TO 1.5V 80 D1 75 1 VIN SW VIN = 1.5V LT1932 C1 4.7µF 2.5V PWM DIMMING CONTROL 5 LED SHDN 24.9k RSET GND 4 2 3 C2 4.7µF EFFICIENCY (%) 6 15mA 70 VIN = 1.1V 65 60 55 RSET 1.50k 50 0 C1, C2: TAIYO YUDEN JMK212BJ475 D1: ZETEX ZHCS400 L1: MURATA LQH3C4R7M24 (408) 573-4150 (631) 543-7100 (814) 237-1431 2.5 1932 TA03a L1 4.7µH 80 2.5V PWM DIMMING CONTROL SHDN 24.
PAGE 11
LT1932 U TYPICAL APPLICATIO S Efficiency 2-Cell Driver for Two White LEDs L1 4.7µH VIN 1.8V TO 3V 85 D1 80 1 VIN SW VIN = 3V LT1932 C1 4.7µF 2.5V DC DIMMING CONTROL 5 LED SHDN 60.4k RSET GND 4 2 3 C2 2.2µF EFFICIENCY (%) 6 15mA 75 VIN = 1.8V 70 65 60 RSET 1.50k 55 0 C1: TAIYO YUDEN JMK212BJ475 C2: TAIYO YUDEN LMK212BJ225 D1: ZETEX ZHCS400 L1: MURATA LQH3C4R7M24 (408) 573-4150 (408) 573-4150 (631) 543-7100 (814) 237-1431 5 1932 TA15a L1 4.
PAGE 12
LT1932 U TYPICAL APPLICATIO S 2-Cell Driver for Four White LEDs L1 4.7µH VIN 1.8V TO 3V Efficiency 85 D1 80 C1 4.7µF 6 1 VIN SW LT1932 PWM DIMMING CONTROL 5 SHDN LED RSET GND 4 2 3 C2 1µF EFFICIENCY (%) VIN = 3V 75 VIN = 1.8V 70 65 15mA 60 RSET 1.50k 55 0 C1: TAIYO YUDEN JMK212BJ475 C2: TAIYO YUDEN EMK212BJ105 D1: ZETEX ZHCS400 L1: MURATA LQH3C4R7M24 (408) 573-4150 (408) 573-4150 (631) 543-7100 (814) 237-1431 5 1932 TA07a L1 4.
PAGE 13
LT1932 U TYPICAL APPLICATIO S Li-Ion Driver for Two White LEDs L1 6.8µH 85 D1 6 1 VIN SW LT1932 C1 4.7µF 3.3V PWM DIMMING CONTROL 5 RSET GND 4 2 31.6k 3 LED SHDN C2 2.2µF 15mA EFFICIENCY (%) VIN 2.7V TO 4.2V Efficiency 80 VIN = 4.2V 75 VIN = 2.7V 70 65 60 RSET 1.50k 55 0 C1: TAIYO YUDEN JMK212BJ475 C2: TAIYO YUDEN LMK212BJ225 D1: ZETEX ZHCS400 L1: PANASONIC ELJEA6R8 (408) 573-4150 (408) 573-4150 (631) 543-7100 (714) 373-7334 5 1932 TA08a L1 6.
PAGE 14
LT1932 U TYPICAL APPLICATIO S Li-Ion Driver for Four White LEDs L1 6.8µH C1 4.7µF 85 D1 6 1 VIN SW LT1932 PWM DIMMING CONTROL 5 LED SHDN RSET GND 4 2 3 C2 1µF EFFICIENCY (%) VIN 2.7V TO 4.2V Efficiency 15mA 80 VIN = 4.2V 75 VIN = 2.7V 70 65 60 RSET 1.50k 55 0 C1: TAIYO YUDEN JMK212BJ475 C2: TAIYO YUDEN EMK212BJ105 D1: ZETEX ZHCS400 L1: PANASONIC ELJEA6R8 (408) 573-4150 (408) 573-4150 (631) 543-7100 (714) 373-7334 5 1932 TA10a C1 4.
PAGE 15
LT1932 U TYPICAL APPLICATIO S Li-Ion Driver for Eight White LEDs L1 4.7µH VIN 3V TO 4.2V Efficiency 85 D1 80 1 VIN SW VIN = 4.2V EFFICIENCY (%) 6 LT1932 C1 4.7µF 3.3V DC DIMMING CONTROL 5 LED SHDN RSET GND 4 2 80.6k 3 C2 1µF 75 70 VIN = 3V 65 60 RSET 1.
PAGE 16
LT1932 U TYPICAL APPLICATIO Li-Ion Driver for Ten White LEDs L1 10µH VIN 2.7V TO 4.2V Efficiency 80 D1 VIN = 4.2V 75 VIN = 2.7V 1 VIN SW LT1932 C1 4.7µF 5 LED SHDN RSET GND 4 2 C2 4.