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LT3491

3491fa

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Cellular Phones

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PDAs, Handheld Computers

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Digital Cameras

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MP3 Players

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GPS Receivers

Li-Ion Driver for Four White LEDs

White LED Driver with

Integrated Schottky in SC70

and 2mm × 2mm DFN

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Drives Up to Six White LEDs from a 3V Supply

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High Side Sense Allows “One Wire Current Source”

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Internal Schottky Diode

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One Pin Dimming and Shutdown

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27V Open LED Protection

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2.3MHz Switching Frequency

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±5% Reference Accuracy

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Range: 2.5V to 12V

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Requires Only 1µF Output Capacitor

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Wide 300:1 True Color PWM

Dimming Range

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8-Lead SC70 Package

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Low Profile 6-Lead DFN Package

(2mm × 2mm × 0.75mm)

Efficiency

FEATURES

DESCRIPTIO

APPLICATIO S

TYPICAL APPLICATIO

CTRL

10µH

3V TO 5V

SENSE

10Ω

3491 TA01a

LT3491

SHUTDOWN AND

DIMMING CONTROL

CAP

LED

GND

1µF

C1: TAIYO YUDEN LMK212BJ105MD

C2: TAIYO YUDEN GMK316BJ105ML

L1: MURATA LQH32CN100

The LT

3491 is a fixed frequency step-up DC/DC converter

specifically designed to drive up to six white 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 device

features a unique high side LED current sense that enables

the part to function as a “one wire current source;” one

side of the LED string can be returned to ground anywhere,

allowing a simpler one wire LED connection. Traditional

LED drivers use a grounded resistor to sense LED current,

requiring a 2-wire connection to the LED string.

The 2.3MHz switching frequency allows the use of tiny

inductors and capacitors. A single pin performs both

shutdown and accurate LED dimming control. Few exter-

nal components are needed: open-LED protection and the

Schottky diode are all contained inside the tiny SC70 and

2mm × 2mm DFN packages. With such a high level of

integration, the LT3491 provides a high efficiency LED

driver solution in the smallest of spaces.

LED CURRENT (mA)

EFFICIENCY (%)

3491 TA01b

= 3.6V

4 LEDs

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

All other trademarks are the property of their respective owners.

Summary of content (16 pages)

PAGE 1
LT3491 White LED Driver with Integrated Schottky in SC70 and 2mm × 2mm DFN DESCRIPTIO U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Drives Up to Six White LEDs from a 3V Supply High Side Sense Allows “One Wire Current Source” Internal Schottky Diode One Pin Dimming and Shutdown 27V Open LED Protection 2.3MHz Switching Frequency ±5% Reference Accuracy VIN Range: 2.5V to 12V Requires Only 1µF Output Capacitor Wide 300:1 True Color PWMTM Dimming Range 8-Lead SC70 Package Low Profile 6-Lead DFN Package (2mm × 2mm × 0.
PAGE 2
LT3491 W W W AXI U U ABSOLUTE RATI GS (Note 1) Input Voltage (VIN) ................................................. SW Voltage ............................................................. CAP Voltage ............................................................ CTRL Voltage .......................................................... 12V 32V 32V 12V LED Voltage ............................................................ 32V Operating Temperature Range (Note 2) ..
PAGE 3
LT3491 ELECTRICAL CHARACTERISTICS Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3491E is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls.
PAGE 4
LT3491 U W TYPICAL PERFOR A CE CHARACTERISTICS (TA = 25°C unless otherwise specified) Switching Current Limt vs Duty Cycle Quiescent Current (VCTRL = 3V) 3.0 15 450 400 SCHOTTKY LEAKAGE CURRENT (µA) 25°C 2.5 350 CURRENT LIMIT (mA) QUIESCENT CURRENT (mA) Schottky Leakage Current vs Temperature 2.0 1.5 1.0 300 250 200 150 100 0.
PAGE 5
LT3491 U U U PI FU CTIO S (SC70/DFN) SW (Pin 1/Pin 3): Switch Pin. Minimize trace area at this pin to minimize EMI. Connect the inductor at this pin. GND (Pins 2, 3, 4/Pin 2): Ground Pins. All three pins should be tied directly to local ground plane. VIN (Pin 5/Pin 1): Input Supply Pin. Must be locally bypassed. CTRL (Pin 6/Pin 6): Dimming and Shutdown Pin. Connect this pin below 50mV to disable the driver. As the pin voltage is ramped from 0V to 1.
PAGE 6
LT3491 U OPERATIO The LT3491 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 1. At power up, the capacitor at the CAP pin is charged up to VIN (input supply voltage) through the inductor and the internal Schottky diode. If CTRL is pulled higher than 100mV, the bandgap reference, the start-up bias and the oscillator are turned on.
PAGE 7
LT3491 U W U U APPLICATIO S I FOR ATIO INDUCTOR SELECTION A 10µH inductor is recommended for most LT3491 applications. Although small size and high efficiency are major concerns, the inductor should have low core losses at 2.3MHz and low DCR (copper wire resistance). Some small inductors in this category are listed in Table 1. The efficiency comparison of different inductors is shown in Figure 3. Table 2 shows a list of several ceramic capacitor manufacturers.
PAGE 8
LT3491 U W U U APPLICATIO S I FOR ATIO For low DCR inductors, which is usually the case for this application, the peak inrush current can be simplified as follows: IPK = α= ILED (mA) VIN – 0.6 ⎛ α π⎞ • exp ⎜ – • ⎟ ⎝ ω 2⎠ L•ω r 2 •L ω= Table 4. RSENSE Value Selection for 200mV Sense 5 40 10 20 15 13.3 20 10 DIMMING CONTROL There are three different types of dimming control circuits.
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LT3491 U W U U APPLICATIO S I FOR ATIO Using a Filtered PWM Signal A filtered PWM signal can be used to control the brightness of the LED string. The PWM signal is filtered (Figure 5) by a RC network and fed to the CTRL pin. The corner frequency of R1, C1 should be much lower than the frequency of the PWM signal. R1 needs to be much smaller than the internal impedance of the CTRL pin which is 10MΩ (typ). PWM 10kHz TYP LT3491 R1 100k C1 0.
PAGE 10
LT3491 U W U U APPLICATIO S I FOR ATIO The time it takes for the LED current to reach its programmed value sets the achievable dimming range for a given PWM frequency. For example, the settling time of the LED current in Figure 7 is approximately 30µs for a 3V input voltage. The achievable dimming range for this application and 100Hz PWM frequency can be determined using the following method. Example: down to 100mV.
PAGE 11
LT3491 U W U U APPLICATIO S I FOR ATIO LOW INPUT VOLTAGE APPLICATIONS BOARD LAYOUT CONSIDERATIONS The LT3491 can be used in low input voltage applications. The input supply voltage to the LT3491 must be 2.5V or higher. However, the inductor can be run off a lower battery voltage. This technique allows the LEDs to be powered off two alkaline cells. Most portable devices have a 3.3V logic supply voltage which can be used to power the LT3491.
PAGE 12
LT3491 U TYPICAL APPLICATIO S Li-Ion Driver for One White LED Efficiency 60 C2 1µF 50 EFFICIENCY (%) RSENSE 10Ω LED VIN 3V TO 5V CAP VIN L1 10µH SW 45 40 35 30 25 LT3491 C1 1µF VIN = 3.6V 55 SHUTDOWN AND DIMMING CONTROL CTRL GND 20 15 10 3491 TA07a 5 0 C1: TAIYO YUDEN LMK212BJ105MD C2: TAIYO YUDEN GMK316BJ105ML L1: MURATA LQH32CN100 3491 TA07b Li-Ion Driver for Two White LEDs Efficiency 70 C2 1µF EFFICIENCY (%) 60 LED CAP VIN L1 10µH LT3491 SW C1 1µF VIN = 3.
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LT3491 U TYPICAL APPLICATIO S 12V to One White LED at 200mA Efficiency C2 4.7µF 75 RSENSE 1Ω C3 1µF EFFICIENCY (%) PVIN 12V 80 D1 CAP VIN 3V LED L1 15µH VIN SHUTDOWN AND DIMMING CONTROL C1 1µF LT3491 70 65 60 SW 55 3491 TA02a 50 CTRL GND 0 C1, C3: TAIYO YUDEN LMK212BJ105MD C2: TAIYO YUDEN LMK316BJ475ML D1: LUXEON EMITTER LXHL-BWO2 L1: MURATA LQH32CN150 3491 TA02b 12V to Two White LEDs at 200mA Efficiency C2 4.
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LT3491 U TYPICAL APPLICATIO S Li-Ion Driver for Three White LEDs SHUTDOWN AND DIMMING CONTROL 80 CTRL 70 VIN L1 10µH C1 1µF CAP RSENSE 10Ω LT3491 SW VIN = 3.
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LT3491 U PACKAGE DESCRIPTIO SC8 Package 8-Lead Plastic SC70 (Reference LTC DWG # 05-08-1639 Rev Ø) 0.30 MAX 0.50 REF PIN 8 1.80 – 2.20 (NOTE 4) 1.00 REF INDEX AREA (NOTE 6) 1.80 – 2.40 1.15 – 1.35 (NOTE 4) 2.8 BSC 1.8 REF PIN 1 RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.10 – 0.40 0.15 – 0.27 8 PLCS (NOTE 3) 0.50 BSC 0.80 – 1.00 0.00 – 0.10 REF 1.00 MAX GAUGE PLANE 0.15 BSC 0.26 – 0.46 SC8 SC70 0905 REV Ø 0.10 – 0.18 (NOTE 3) NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2.
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LT3491 U TYPICAL APPLICATIO Li-Ion Driver for Six White LEDs Efficiency 80 SHUTDOWN AND DIMMING CONTROL VIN = 3.6V 6 LEDs 75 CTRL VIN L1 10µH RSENSE 10Ω LT3491 SW C1 1µF 70 CAP EFFICIENCY (%) VIN 3V TO 5V LED GND C2 1µF 65 60 55 50 45 40 0 5 10 15 20 LED CURRENT (mA) 3491 TA06b 3491 TA06a C1: TAIYO YUDEN LMK212BJ105MD C2: TAIYO YUDEN GMK316BJ105ML L1: MURATA LQH32CN100 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1618 Constant Current, Constant Voltage 1.