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LT3591

3591f

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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

TYPICAL APPLICATION

FEATURES

APPLICATIONS

DESCRIPTION

White LED Driver with

Integrated Schottky in

3mm × 2mm DFN

The LT

3591 is a ﬁ xed frequency step-up DC/DC converter

speciﬁ cally designed to drive up to ten 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 high switching frequency allows the use of tiny induc-

tors and capacitors. A single pin performs both shutdown

and accurate LED dimming control. Few external compo-

nents are needed: open-LED protection and the Schottky

diode are all contained inside a low proﬁ le 3mm × 2mm

DFN package.

Li-Ion Driver for Ten White LEDs

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Drives Up to Ten 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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80:1 True Color PWM

Dimming Range

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42V Open LED Protection

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1MHz Switching Frequency

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±5% Reference Accuracy

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Range: 2.5V to 12V

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Requires Only 2.2µF Output Capacitor

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Low Proﬁ le 8-Lead DFN Package

(3mm × 2mm × 0.75mm)

Conversion Efﬁ ciency

CTRL

22µH

3V TO 5V

SENSE

10Ω

3591 TA01a

LT3591

SHUTDOWN AND

DIMMING CONTROL

CAP

LED

GND

2.2µF

1µF

LED CURRENT (mA)

EFFICIENCY (%)

3591 TA01b

= 3.6V

10 LEDs

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

True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are

the property of their respective owners.

Summary of content (16 pages)

PAGE 1
LT3591 White LED Driver with Integrated Schottky in 3mm × 2mm DFN DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ Drives Up to Ten White LEDs from a 3V Supply High Side Sense Allows “One Wire Current Source” Internal Schottky Diode One Pin Dimming and Shutdown 80:1 True Color PWMTM Dimming Range 42V Open LED Protection 1MHz Switching Frequency ±5% Reference Accuracy VIN Range: 2.5V to 12V Requires Only 2.2µF Output Capacitor Low Proﬁle 8-Lead DFN Package (3mm × 2mm × 0.
PAGE 2
LT3591 ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) TOP VIEW Input Voltage (VIN) ................................................... 12V CTRL Voltage ........................................................... 12V SW Voltage .............................................................. 45V CAP Voltage ............................................................. 45V LED Voltage .............................................................
PAGE 3
LT3591 TYPICAL PERFORMANCE CHARACTERISTICS Switch Saturation Voltage (VCESAT) Schottky Forward Voltage Drop 25°C 400 300 125°C 200 – 50°C 100 500 SHUTDOWN CURRENT (µA) SCHOTTKY FORWARD CURRENT (mA) 125°C 400 300 25°C 200 – 50°C 100 0 0 100 200 300 400 500 600 700 800 SWITCH CURRENT (mA) 200 400 600 800 1000 SCHOTTKY FORWARD DROP (mV) Sense Voltage (VCAP – VLED) vs VCTRL – 50°C 25°C 6 125°C 3 1200 0 3 6 9 12 VIN (V) 3591 G03 Open-Circuit Output Clamp Voltage Input Current in Output
PAGE 4
LT3591 TYPICAL PERFORMANCE CHARACTERISTICS Quiescent Current (VCTRL = 3V) Schottky Leakage Current vs Temperature Current Limit vs Temperature 6 15 25°C SCHOTTKY LEAKAGE CURRENT (µA) 1000 5 QUIESCENT CURRENT (mA) TA = 25°C, unless otherwise speciﬁed.
PAGE 5
LT3591 PIN FUNCTIONS VIN (Pin 1): Input Supply Pin. Must be locally bypassed. CTRL (Pin 8): Dimming and Shutdown Pin. Connect this pin below 50mV to disable the driver. As the pin voltage is ramped from 0V to 1.5V, the LED current ramps from 0 to ILED ( = 200mV/RSENSE). The CTRL pin must not be left ﬂoating. GND (Pin 2): Ground Pin. Should be tied directly to local ground plane. SW (Pin 4): Switch Pin. Minimize trace area at this pin to minimize EMI. Connect the inductor at this pin.
PAGE 6
LT3591 OPERATION The LT3591 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. the LED current. The LT3591 enters into shutdown when CTRL is pulled lower than 50mV. 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.
PAGE 7
LT3591 APPLICATIONS INFORMATION INDUCTOR SELECTION CAPACITOR SELECTION A 22µH inductor is recommended for most LT3591 applications. Although small size and high efﬁciency are major concerns, the inductor should have low core losses at 1MHz 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. The small size of ceramic capacitors make them ideal for LT3591 applications.
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LT3591 APPLICATIONS INFORMATION SCHOTTKY DIODE The LT3591 has a built-in Schottky diode. The internal schottky saves board space in space constrained applications. In less space sensitive applications, an external schottky diode connected between the SW node and the CAP node increases efﬁciency one to two percent. It is important to use a properly rated schottky diode that can handle the peak switch current of the LT3591.
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LT3591 APPLICATIONS INFORMATION ILED (mA) RSENSE (Ω) 5 40 10 20 15 13.3 20 10 DIMMING CONTROL There are three different types of dimming control circuits. The LED current can be set by modulating the CTRL pin with a DC voltage, a ﬁltered PWM signal or directly with a PWM signal. Using a DC Voltage For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current. The CTRL pin voltage can be modulated to set the dimming of the LED string.
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LT3591 APPLICATIONS INFORMATION PWM 5V/DIV The calculations show that for a 100Hz signal the dimming range is 83 to 1. In addition, the minimum PWM duty cycle of 1.2% ensures that the LED current has enough time to settle to its ﬁnal value. Figure 8 shows the dimming range achievable for different frequencies with a settling time of 120µs. 10000 PWM DIMMING RANGE and ground as shown in Figure 6. A Si2308 MOSFET can be used since its source is connected to ground.
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LT3591 APPLICATIONS INFORMATION LOW INPUT VOLTAGE APPLICATIONS BOARD LAYOUT CONSIDERATIONS The LT3591 can be used in low input voltage applications. The input supply voltage to the LT3591 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 LT3591.
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LT3591 TYPICAL APPLICATIONS Li-Ion Driver for Ten White LEDs Efﬁciency D1 L1 22µH 85 *OPTIONAL VIN = 3.6V 10 LEDs 80 SW VIN NO SCHOTTKY CAP RSENSE 10Ω LT3591 SHUTDOWN AND DIMMING CONTROL CTRL LED GND C2 2.2µF C1 1µF EFFICIENCY (%) VIN 3V TO 5V 75 70 EXTERNAL SCHOTTKY 65 60 55 5 3591 TA02a 0 C1:TAIYO YUDEN EMK107BJ105MA C2: MURATA GRM31CR71H225KA88 3491 TA02b Efﬁciency SHUTDOWN AND DIMMING CONTROL 80 CTRL L1 10µH RSENSE 3.92Ω LT3591 SW LED GND C2 4.
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LT3591 TYPICAL APPLICATIONS 24V to Four White LEDs at 100mA Efﬁciency 95 C2 4.
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LT3591 TYPICAL APPLICATIONS Li-Ion Driver for Seven White LEDs Conversion Efﬁciency SHUTDOWN AND DIMMING CONTROL 85 VIN = 3.6V 7 LEDs 80 CTRL VIN L1 22µH C1 1µF 75 CAP RSENSE 10Ω LT3591 SW LED C2 2.2µF GND EFFICIENCY (%) VIN 3V TO 5V 70 65 60 55 50 0 5 10 15 LED CURRENT (mA) 20 3591 TA07b 3591 TA07a C1: TAIYO YUDEN EMK107BJ105MA C2: MURATA GRM31CR71H225KA88 L1: TAIYO YUDEN NR4018T220M Li-Ion Driver for Eight White LEDs Conversion Efﬁciency SHUTDOWN AND DIMMING CONTROL 85 VIN = 3.
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LT3591 PACKAGE DESCRIPTION DDB Package 8-Lead Plastic DFN (3mm × 2mm) (Reference LTC DWG # 05-08-1702 Rev B) 0.61 ±0.05 (2 SIDES) 0.70 ±0.05 2.55 ±0.05 1.15 ±0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.20 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (2 SIDES) R = 0.115 TYP 5 R = 0.05 TYP 0.40 ± 0.10 8 2.00 ±0.10 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 0.56 ± 0.05 (2 SIDES) 0.200 REF 0.75 ±0.05 0 – 0.05 4 0.25 ± 0.05 1 PIN 1 R = 0.20 OR 0.
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LT3591 TYPICAL APPLICATION Li-Ion Driver for Nine White LEDs Conversion Efﬁciency 85 SHUTDOWN AND DIMMING CONTROL VIN = 3.6V 9 LEDs 80 VIN 3V TO 5V VIN L1 22µH CAP SW C1 1µF RSENSE 10Ω LT3591 C2 2.2µF LED GND EFFICIENCY (%) CTRL 75 70 65 60 55 0 5 20 15 10 LED CURRENT (mA) 3591 TA09b 3591 TA09a C1: TAIYO YUDEN EMK107BJ105MA C2: MURATA GRM31CR71H225KA88 L1: TAIYO YUDEN NR4018T220M RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1618 Constant-Current, Constant Voltage 1.