LINEAR TECHNOLOGY LT3491 Technical data

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 × 0.75mm)

U

APPLICATIO S

■

Cellular Phones

■

PDAs, Handheld Computers

■

Digital Cameras

■

MP3 Players

■

GPS Receivers

LT3491

White LED Driver with

Integrated Schottky in SC70

and 2mm × 2mm DFN

U

DESCRIPTIO

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 exter­nal 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.

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

All other trademarks are the property of their respective owners.

TYPICAL APPLICATIO

Li-Ion Driver for Four White LEDs

SHUTDOWN AND

DIMMING CONTROL

V

3V TO 5V

IN

L1

10µH

C1
1µF

C1: TAIYO YUDEN LMK212BJ105MD
C2: TAIYO YUDEN GMK316BJ105ML
L1: MURATA LQH32CN100

V

SW

CTRL

IN

CAP

LT3491

LED

GND

U

R

SENSE

10Ω

3491 TA01a

C2
1µF

80

VIN = 3.6V
4 LEDs

75

70

65

60

55

EFFICIENCY (%)

50

45

40

0

Efficiency

5

10

LED CURRENT (mA)

15

20

3491 TA01b

3491fa

1

LT3491

WWWU

ABSOLUTE AXI U RATI GS

(Note 1)

Input Voltage (VIN) ................................................. 12V

SW Voltage ............................................................. 32V

CAP Voltage ............................................................ 32V

CTRL Voltage .......................................................... 12V

UU

W

PACKAGE/ORDER I FOR ATIO

TOP VIEW

6

1

V

IN

GND

2

SW

3

DC PACKAGE

6-LEAD (2mm × 2mm) PLASTIC DFN

T

= 125°C, θJA = 102°C/W, θJC = 20°C/ W

EXPOSED PAD (PIN 7) SHOULD BE CONNECTED TO PCB GROUND

JMAX

ORDER PART NUMBER DC PART MARKING

LT3491EDC

Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF

Lead Free Part Marking: http://www.linear.com/leadfree/

Consult LTC Marketing for parts specified with wider operating temperature ranges.

CTRL

5

7

LED

CAP

4

LCHJ

LED Voltage ............................................................ 32V

Operating Temperature Range (Note 2) .. – 40°C to 85°C

Maximum Junction Temperature ......................... 125°C

Storage Temperature Range ................ –65°C to 150°C

Lead Temperature (Soldering, 10sec, SC-70) ......... 300°C

TOP VIEW

SW 1
GND 2
GND 3
GND 4

SC8 PACKAGE

8-LEAD PLASTIC SC70

T

= 125°C, θJA = 270°C/ W

JMAX

8 CAP
7 LED
6 CTRL
5 V

IN

ORDER PART NUMBER DC PART MARKING

LT3491ESC8

LBXQ

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.

= 3V, V

V

IN

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Operating Voltage 2.5 V
LED Current Sense Voltage (V
CAP, LED Pin Bias Current V
V

, V

CAP

LED

Supply Current V

Switching Frequency 1.8 2.3 2.8 MHz
Maximum Duty Cycle
Switch Current Limit
Switch V
Switch Leakage Current VSW = 16V 0.1 5 µA
V

for Full LED Current V

CTRL

V

to Shut Down IC 50 mV

CTRL

V

to Turn On IC

CTRL

CTRL Pin Bias Current 100 nA
CAP Pin Overvoltage Protection
Schottky Forward Drop I
Schottky Leakage Current VR = 20V 4 µA

= 3V, unless otherwise specified.

CTRL

– V

CAP

)V

LED

= 30V

CAP

= 16V, V

CAP

●

190 200 210 mV

= 16V 20 40 µA

LED

Common Mode Minimum Voltage 2.5 V

= 16V, V

CAP

= 15V, CTRL = 3V 2.6 4 mA

LED

CTRL = 0V 8 10 µA

●

88 92 %

●

260 350 mA

CESAT

ISW = 200mA 200 mV

= 30V

CAP

SCHOTTKY

= 100mA 0.8 V

●

1.5 V

●

100 mV

●

26 27 28 V

3491fa

2

ELECTRICAL CHARACTERISTICS

LT3491

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.

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Switch Saturation Voltage (V

350

300

250

200

150

100

50

SWITCH SATURATION VOLTAGE (mV)

0

0

50 100

200 400

150 300

SWITCH CURRENT (mA)

250

CESAT

350

3491 G01

)

Schottky Forward Voltage Drop

400

350

300

250

200

150

100

SCHOTTKY FORWARD CURRET (mA)

50

0

200 400 800

0

SCHOTTKY FORWARD DROP (mV)

600

1000

= 25°C unless otherwise specified)

(T

A

Shutdown Current (V

15

12

9

6

SHUTDOWN CURRENT (µA)

3

0

3491 G02

1200

0

3

VIN (V)

= 0V)

CTRL

6

9

12

3491 G03

Sense Voltage (V
vs V

CTRL

240

200

160

120

80

SENSE VOLTAGE (mV)

40

0

500

0

1000

V

CTRL

V

SW

10V/DIV

V

CAP

50mV/DIV

100mA/DIV

– V

LED

1500

)

2000

3491 G04

CAP

(mV)

Switching Waveform

I

L

V

= 3.6V

IN

FRONT PAGE
APPLICATION CIRCUIT

200ns/DIV

Open-Circuit Output Clamp
Voltage

30

29

28

27

OUTPUT CLAMP VOLTAGE (V)

26

25

0

3

VIN (V)

3491 G07

6

9

V

CAP

5V/DIV

V

CTRL

5V/DIV

200mA/DIV

INPUT CURRENT (mA)

12

3491 G05

Transient Response

I

L

V

= 3.6V

IN

FRONT PAGE
APPLICATION CIRCUIT

1ms/DIV

Input Current in Output Open
Circuit

6

5

4

3

2

1

0

0

36 912

VIN (V)

3491 G08

3491 G06

3491fa

3

LT3491

TEMPERATURE (°C)

–50 –25

1.95

SWITCH FREQUENCY (MHz)

2.05

2.20

0

50

75

3419 G14

2.00

2.15

2.10

25

100

125

TEMPERATURE (°C)

–50 –25

192

SENSE VOLTAGE (mV)

200

212

0

50

75

3491 G17

196

208

204

25

100

125

UW

TYPICAL PERFOR A CE CHARACTERISTICS

(TA = 25°C unless otherwise specified)

Quiescent Current (V

3.0

2.5

2.0

1.5

1.0

QUIESCENT CURRENT (mA)

0.5

0

0

36 912

VIN (V)

CTRL

= 3V)

Open-Circuit Output Clamp Voltage
vs Temperature

30

29

28

27

26

OUTPUT CLAMP VOLTAGE (V)

3491 G09

Switching Current Limt
vs Duty Cycle

450

400

350

300

250

200

150

CURRENT LIMIT (mA)

100

50

0

40

30

25°C

50

60 90

DUTY CYCLE (%)

70

Input Current in Output Open Circuit
vs Temperature

6

VIN = 3V

5

4

3

2

INPUT CURRENT (mA)

1

80

3491 G10

Schottky Leakage Current
vs Temperature

15

12

9

6

3

SCHOTTKY LEAKAGE CURRENT (µA)

0

–50

02550

–25

TEMPERATURE (°C)

Switching Frequency
vs Temperature

VR = 10V

= 16V

V

R

V

= 20V

R

75 100

3491 G11

25

–50 –25

Sense Voltage (V
vs V

240

200

160

120

80

SENSE VOLTAGE (mV)

40

0

0

4

0

TEMPERATURE (°C)

CAP

– V

75

LED

50

25

CTRL

500 1000 1500 2000

V

(mV)

CTRL

100

)

3491 G12

–50°C
25°C
85°C

3491 G15

125

0

–50

–25 0

Sense Voltage (V
vs V

CAP

212

208

204

200

SENSE VOLTAGE (mV)

196

192

5

10

50 100 125

25 75

TEMPERATURE (°C)

– V

CAP

15

V

(V)

CAP

LED

20

)

3491 G13

3491 G16

Sense Voltage (V

CAP

– V

LED

)

vs Temperature

25

3491fa

LT3491

U

PI FU CTIO S

UU

(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.

V

(Pin 5/Pin 1): Input Supply Pin. Must be locally

IN

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 1.5V, the LED current ramps
from 0 to I

( = 200mV/R

LED

). The CTRL pin must not

SENSE

be left floating.

W

BLOCK DIAGRA

LED (Pin 7/Pin 5): Connection Point for the Anode of the
First LED and the Sense Resistor. The LED current can be
programmed by :

LED

=

200
R

SENSE

I

mV

CAP (Pin 8/Pin 4): Output of the Driver. This pin is
connected to the cathode of internal Schottky. Connect the
output capacitor to this pin and the sense resistor from
this pin to the LED pin.

EXPOSED PAD (NA/Pin 7): The Exposed Pad should be
soldered to the PCB ground to achieve the rated thermal
performance.

5

V

IN

–

A2

+

Σ

RAMP

GENERATOR

V

SHDN

START-UP

CONTROL

PIN NUMBERS CORRESPOND TO THE 8-PIN SC70 PACKAGE

REF

1.25V

OSCILLATOR

R

PWM

COMP

C

C

C

1

SW

CAP

DRIVER

Q

R

S

A3

Q1

OVERVOLTAGE

+

PROTECTION

R

8

–

–
+

A1

+

GND

CTRL

PINS 2, 3, 4

6

A = 6.25

+

LED

–

7

3491 F01

Figure 1. Block Diagram

3491fa

5