LINEAR TECHNOLOGY LT3475, LT3475-1 Technical data

FEATURES

LT3475/LT3475-1

Dual Step-Down

1.5A LED Driver

U

DESCRIPTIO

■

True Color PWMTM Delivers Constant Color with

3000:1 Dimming Range

■

Wide Input Range: 4V to 36V Operating, 40V

Maximum

■

Accurate and Adjustable Control of LED Current

from 50mA to 1.5A

■

High Side Current Sense Allows Grounded Cathode

LED Operation

■

Open LED (LT3475) and Short Circuit Protection

■

LT3475-1 Drives LED Strings Up to 25V

■

Accurate and Adjustable 200kHz to 2MHz

Switching Frequency

■

Anti-Phase Switching Reduces Ripple

■

Uses Small Inductors and Ceramic Capacitors

■

Available in the Compact 20-Lead TSSOP Thermally

Enhanced Surface Mount Package

U

APPLICATIO S

■

Automotive and Avionic Lighting

■

Architectural Detail Lighting

■

Display Backlighting

■

Constant-Current Sources

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

All other trademarks are the property of their respective owners. Patents Pending.

The LT®3475/LT3475-1 are dual step-down DC/DC
converters designed to operate as a constant-current
source. An internal sense resistor monitors the output
current allowing accurate current regulation ideal for
driving high current LEDs. The high side current sense al­lows grounded cathode LED operation. High output current
accuracy is maintained over a wide current range, from
50mA to 1.5A, allowing a wide dimming range. Unique
PWM circuitry allows a dimming range of 3000:1, avoid­ing the color shift normally associated with LED current
dimming.

The high switching frequency offers several advantages,
permitting the use of small inductors and ceramic capaci­tors. Small inductors combined with the 20 lead TSSOP
surface mount package save space and cost versus
alternative solutions. The constant switching frequency
combined with low-impedance ceramic capacitors result
in low, predictable output ripple.

With its wide input range of 4V to 36V, the LT3475/LT3475-1
regulate a broad array of power sources. A current mode
PWM architecture provides fast transient response and
cycle-by-cycle current limiting. Frequency foldback and
thermal shutdown provide additional protection.

TYPICAL APPLICATIO

Dual Step-Down 1.5A LED Driver

V

IN

2.2μF

5V TO 36V

10μH 10μH

0.1μF

1.5A LED

CURRENT

0.22μF

*DIMMING

CONTROL

4.7μF

BOOST1 BOOST2

SW1 SW2

OUT1 OUT2
LED1 LED2

PWM1 PWM2
V

C1

REF R

V

ADJ1

V

IN

LT3475

GND

SHDN

U

V

V

ADJ2

Effi ciency

95

VIN = 12V

0.22μF

DIMMING*
CONTROL

C2

T

fSW = 600kHz*SEE APPLICATIONS SECTION FOR DETAILS

24.3k

0.1μF

1.5A LED
CURRENT

2.2μF

3475 TA01

90

85

80

75

70

EFFICIENCY (%)

65

60

55

0

TWO SERIES CONNECTED
WHITE 1.5A LEDS

SINGLE WHITE 1.5A LED

0.5 1
LED CURRENT (A)

3475 TA01b

1.5

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LT3475/LT3475-1

WW

W

U

ABSOLUTE AXI U RATI GS

(Note 1)

VIN Pin .........................................................(-0.3V), 40V

BOOST Pin Voltage ...................................................60V

BOOST Above SW Pin ...............................................30V

OUT, LED, Pins (LT3475) ...........................................15V

OUT, LED Pins (LT3475-1) .........................................25V

PWM Pin ...................................................................15V

Pin ......................................................................6V

V

ADJ

, RT, REF Pins ..........................................................3V

V

C

SHDN Pin ...................................................................V

Maximum Junction Temperature (Note 2)............. 125°C

Operating Temperature Range (Note 3)

LT3475E/LT3475E-1 ............................. –40°C to 85°C

LT3475I/LT3475I-1 ............................. –40°C to 125°C

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

Lead Temperature Range (Soldering, 10 sec) .......300°C

IN

PIN CONFIGURATION

TOP VIEW

1

OUT1

2

LED1

3

BOOST1

4

SW1

5

V

IN

V

IN

SW2

BOOST2

LED2

OUT2

20-LEAD PLASTIC TSSOP

T

= 125°C, θJA = 30°C/W, θJC = 8°C/W

JMAX

EXPOSED PAD (PIN 21) IS GROUND AND MUST

BE ELECTRICALLY CONNECTED TO THE PCB.

6

7

8

9

10

21

FE PACKAGE

PWM1

20

V

19

ADJ1

V

18

C1

REF

17

SHDN

16

GND

15

R

14

T

V

13

C2

V

12

ADJ2

PWM2

11

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LT3475EFE#PBF LT3475EFE#TRPBF LT3475EFE 20-Lead Plastic TSSOP –40°C to 85°C

LT3475IFE#PBF LT3475IFE#TRPBF LT3475IFE 20-Lead Plastic TSSOP –40°C to 125°C

LT3475EFE-1#PBF LT3475EFE-1#TRPBF LT3475FE-1 20-Lead Plastic TSSOP –40°C to 85°C

LT3475IFE-1#PBF LT3475IFE-1#TRPBF LT3475FE-1 20-Lead Plastic TSSOP –40°C to 125°C

Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container.
Consult LTC Marketing for information on non-standard lead based fi nish parts.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/

The

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifi cations are at TA = 25°C. VIN = 12V, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Input Voltage

Input Quiescent Current Not Switching 6 8 mA

Shutdown Current

SHDN = 0.3V, V

● denotes the specifi cations which apply over the full operating

BOOST

= V

OUT

BOOST

= 0V

= 16V, V

= 4V unless otherwise noted (Note 3)

OUT

●

3.7 4 V

0.01 2 μA

2

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LT3475/LT3475-1

ELECTRICAL CHARACTERISTICS

The ● denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at T

= 25°C. VIN = 12V, V

A

PARAMETER CONDITIONS MIN TYP MAX UNITS

LED Pin Current V

ADJ

V

ADJ

Tied to V

Tied to V

REF

REF

• 2/3

• 7/30

LT3475E/LT3475E-1 0°C to 85°C

REF Voltage

Reference Voltage Line Regulation 4V < V

Reference Voltage Load Regulation 0 < I

Pin Bias Current (Note 4)

V

ADJ

Switching Frequency R

Maximum Duty Cycle R

Switching Phase R

Foldback Frequency R

< 40V 0.05 %/V

IN

< 500μA 0.0002 %/μA

REF

= 24.3k

T

= 24.3k

T

R

= 4.32k

T

R

= 100k

T

= 24.3k 150 180 210 Deg

T

= 24.3k, V

T

= 0V 80 kHz

OUT

SHDN Threshold (to Switch) 2.5 2.6 2.74 V

SHDN Pin Current (Note 5) V

SHDN

=

2.6V

PWM Threshold 0.3 0.8 1.2 V

Switching Threshold 0.8 V

V

C

Source Current VC = 1V 50 μA

V

C

Sink Current VC = 1V 50 μA

V

C

LED to V

LED to V

V

V

V

Transresistance 500 V/A

C

Current Gain 1 mA/μA

C

to Switch Current Gain 2.6 A/V

C

Clamp Voltage 1.8 V

C

Pin Current in PWM Mode VC = 1V, V

C

PWM

= 0.3V

OUT Pin Clamp Voltage (LT3475) 13.5 14 14.5 V

OUT Pin Current in PWM Mode V

OUT

= 4V, V

PWM

= 0.3V

Switch Current Limit (Note 6) 2.3 2.7 3.2 A

Switch V

CESAT

BOOST Pin Current I

ISW =1.5A 350 500 mV

=1.5A 25 40 mA

SW

Switch Leakage Current 0.1 10 μA

Minimum Boost Voltage Above SW 1.8 2.5 V

BOOST

= 16V, V

= 4V unless otherwise noted (Note 3)

OUT

0.97

●

0.94

0.336

0.325

●

0.31

●

1.22 1.25 1.27 V

●

●

530 600 640 kHz

●

90 95

1.00

1.03

1.04

0.350

0.364

0.375

0.385

40 400 nA

80
98

7911 μA

●

●

10 400 nA

25 50 μA

A
A
A
A
A

%
%
%

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: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specifi ed maximum operating junction
temperature may impair device reliability.

Note 3: The LT3475E and LT3475E-1 are guaranteed to meet performance
specifi cations from 0°C to 85°C. Specifi cations over the –40°C to 85°C
operating temperature range are assured by design, characterization and
correlation with statistical process controls. The LT3475I and LT3475I-1
are guaranteed to meet performance specifi cations over the –40°C to
125°C operating temperature range.

Note 4: Current fl ows out of pin.
Note 5: Current fl ows into pin.
Note 6: Current limit is guaranteed by design and/or correlation to static

test. Slope compensation reduces current limit at higher duty cycles.

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LT3475/LT3475-1

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TYPICAL PERFOR A CE CHARACTERISTICS

LED Current vs V

1.50

TA = 25°C

1.25

1.00

0.75

0.50

LED CURRENT (A)

0.25

0

0

0.25 0.5 0.75 1

Switch Current Limit
vs Duty Cycle

3.0

2.5

2.0

1.5

1.0

CURRENT LIMIT (A)

0.5

0

0

20 40 60 80

ADJ

V

(V)

ADJ

TYPICAL

MINIMUM

DUTY CYCLE (%)

3475 G01

TA = 25°C

3475 G04

1.25

100

LED Current vs Temperature Switch On Voltage

1.2

1.0

0.8

0.6

0.4

LED CURRENT (A)

0.2

0

–50

–25 0

V

= V

• 2/3

ADJ

REF

V

= V

ADJ

TEMPERATURE (˚C)

• 7/30

REF

50 100 125

25 75

3475 G02

600

TA = 25°C

500

400

300

200

SWITCH ON VOLTAGE (mV)

100

0

0

Switch Current Limit vs
Temperature

3.5

3.0

2.5

2.0

1.5

CURRENT LIMIT (A)

1.0

0.5

0

–50

–25 0

50 100 125

25 75

TEMPERATURE (°C)

3475 G05

Current Limit vs Output Voltage

3.0

TA = 25°C

2.5

2.0

1.5

1.0

CURRENT LIMIT (A)

0.5

0

0

0.5 1.0
SWITCH CURRENT (A)

0.5 2.5 3.0 3.52.01.0

1.5

1.5

V

(V)

OUT

2.0

3475 G03

4.0

3475 G06

Oscillator Frequency
vs Temperature

700

RT = 24.3kΩ

650

600

550

500

OSCILLATOR FREQUENCY (kHz)

450

400

–50

–25 0

TEMPERATURE (˚C)

4

50 100 125

25 75

3475 G07

Oscillator Frequency Foldback Oscillator Frequency vs R

700

TA = 25°C

= 24.3kΩ

R

T

600

500

400

300

200

OSCILLATOR FREQUENCY (kHz)

100

0

0.5 1.0 1.5 2.5

0

V

OUT

2.0

(V)

3475 G08

TA = 25°C

1000

OSCILLATOR FREQUENCY (kHz)

10

1

RT (kΩ)

T

10 100

3475 G09

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TYPICAL PERFOR A CE CHARACTERISTICS

Boost Pin Current

35

TA = 25°C

30

25

20

15

10

BOOST PIN CURRENT (mA)

5

0

0.5 1.0 2.0

0

SWITCH CURRENT (A)

1.5

3475 G10

Quiescent Current

7

TA = 25°C

6

5

4

3

2

INPUT CURRENT (mA)

1

0

10 20 40

0

30

VIN (V)

3475 G11

LT3475/LT3475-1

Open-Circuit Output Voltage and
Input Current

50

TA = 25°C

45

40

35

30

25

20

15

OUTPUT VOLTAGE (V)

10

5

0

0

INPUT CURRENT

LT3475-1

10

V

LT3475-1

LT3475

OUTPUT VOLTAGE

LT3475

20

(V)

IN

14

12

INPUT CURRENT (mA)

10

8

6

4

2

0

30

40

3475 G12

Minimum Input Voltage, Single

Reference Voltage

1.28

1.27

1.26

(V)

1.25

REF

V

1.24

1.23

1.22
–50

–25 0

50 100 125

25 75

TEMPERATURE (˚C)

3475 G13

1.5A White LED

6

TA = 25°C

5

4

TO RUN

(V)

3

IN

V

2

1

0

0

0.5 1
LED CURRENT (A)

UUU

PI FU CTIO S

OUT1, OUT2 (Pins 1, 10): The OUT pin is the input to the
current sense resistor. Connect this pin to the inductor
and the output capacitor.

LED1, LED2 (Pins 2, 9): The LED pin is the output of
the current sense resistor. Connect the anode of the LED
here.

(Pins 5, 6): The VIN pins supply current to the internal

V

IN

circuitry and to the internal power switches and must be
locally bypassed.

SW1, SW2 (Pins 4, 7): The SW pin is the output of the
internal power switch. Connect this pin to the inductor,
switching diode and boost capacitor.

Minimum Input Voltage, Two Series
Connected 1.5A White LEDs

10

TA = 25°C

TO START

LED VOLTAGE

1.5

3475 G14

(V)

IN

V

9

8

7

6

5

TO RUN

0

TO START

LED VOLTAGE

0.5 1
LED CURRENT (A)

3475 G15

BOOST1, BOOST2 (Pins 3, 8): The BOOST pin is used to
provide a drive voltage, higher than the input voltage, to
the internal bipolar NPN power switch.

GND (Pins 15, Exposed Pad Pin 21): Ground. Tie the GND
pin and the exposed pad directly to the ground plane. The
exposed pad metal of the package provides both electrical
contact to ground and good thermal contact to the printed
circuit board. The exposed pad must be soldered to the
circuit board for proper operation. Use a large ground plane
and thermal vias to optimize thermal performance.

1.5

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LT3475/LT3475-1

UUU

PI FU CTIO S

RT (Pin 14): The RT pin is used to set the internal
oscillator frequency. Tie a 24.3k resistor from R

to GND

T

for a 600kHz switching frequency.

SHDN

(Pin 16): The

SHDN

pin is used to shut down the

switching regulator and the internal bias circuits. The

2.6V switching threshold can function as an accurate
undervoltage lockout. Pull below 0.3V to shut down the
LT3475/LT3475-1. Pull above 2.6V to enable the LT3475/
LT3475-1. Tie to V

IN

SHDN

function is unused.

if the

REF (Pin 17): The REF pin is the buffered output of the
internal reference. Either tie the REF pin to the V

ADJ

pin
for a 1.5A output current, or use a resistor divider to
generate a lower voltage at the V

pin. Leave this pin

ADJ

unconnected if unused.

BLOCK DIAGRAM

VC1, VC2 (Pins 18, 13): The VC pin is the output of the
internal error amp. The voltage on this pin controls the
peak switch current. Use this pin to compensate the
control loop.

, V

V

ADJ1

the internal voltage-to-current amplifi er. Connect the V

(Pins 19, 12): The V

ADJ2

pin is the input to

ADJ

ADJ

pin to the REF pin for a 1.5A output current. For lower
output currents, program the V
formula: I

= 1.5A • V

LED

ADJ

/1.25V.

pin using the following

ADJ

PWM1, PWM2 (Pins 20, 11): The PWM pin controls the
connection of the V
the PWM pin is low, the V

pin to the internal circuitry. When

C

pin is disconnected from the

C

internal circuitry and draws minimal current. If the PWM
feature is unused, leave this pin unconnected.

V

IN

C

IN

V

SHDN

IN

INT REG

AND

D1 D2

BOOST1

C1 C2

Q1 Q2

SW1 SW2

L1 L2

D3 D4

C

OUT1

D

LED1

C

C1

DRIVER

OUT1

0.067Ω 100Ω 0.067Ω100Ω

LED1 LED2

gm1 gm2

PWM 1

V

C1

Q3 Q4

1.25k

C1 C2

QR

QS

∑

UVLO

SLOPE COMP SLOPE COMP

MOSC 1 MOSC 2

SLAVE

OSC

FREQUENCY

FOLDBACK

–
+

2V 2V

V

ADJ1

R

T

V

R

IN

T

MASTER

OSC

∑

SLAVE

OSC

FREQUENCY
FOLDBACK

–
+

1.25V

REF

V

ADJ2

EXPOSED

PAD

QR

QS

DRIVER

1.25k

GND

BOOST2

OUT2

PWM2

V

C2

D

LED 2

C

C

OUT2

C2

3475 BD

6

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