LINEAR TECHNOLOGY LT3598 Technical data

LT3598

F

6-String 30mA LED Driver

with 1.5% Current Matching

FEATURES

n

True Color PWM™ Dimming Delivers Up to 3000:1

Dimming Ratio

n

Drives Six Strings of LEDs at Up to 30mA

n

1.5% Accurate LED Current Matching

n

Wide Input Voltage Range: 3.2V to 30V

n

Output Voltage Up to 44V

n

Regulates Current Even When VIN > V

n

Disconnects LEDs in Shutdown

n

Programmable Open LED Protection (Regulated)

n

OPENLED Alert Pin

n

Programmable LED Current Derating

n

Adjustable Frequency: 200kHz to 2.5MHz

n

Synchronizable to an External Clock

n

Parallel Channels for Higher Current per LED String

n

Thermally Enhanced 4mm × 4mm QFN Package

OUT

APPLICATIONS

n

Notebook Computer Display

n

Medium Size Displays

n

Automotive LCD Display

DESCRIPTION

The LT®3598 is a fi xed frequency step-up DC/DC converter
designed to drive up to six strings of LEDs at an output
voltage up to 44V. LED dimming can be achieved with
analog dimming on the CTRL pin, and with pulse width
modulation dimming on the PWM pin. The LT3598 accu­rately regulates LED current even when the input voltage
is higher than the LED output voltage. The switching fre­quency is programmable from 200kHz to 2.5MHz through
an external resistor.

Additional features include programmable overvoltage
protection, switching frequency synchronization to an
external clock, LED current derating based on junction
temperature and/or LED temperature, LED string disable
control, OPENLED alert pin and output voltage limiting
when all LED strings are disconnected. The LT3598 is
available in a thermally enhanced 24-pin (4mm × 4mm)
QFN package.

LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
True Color PWM is a trademarkof Linear Technology Corporation. All other trademarks are the
property of their respective owners. Protected by U.S. Patents, including

7199560, 7321203.

TYPICAL APPLICATION

90% Effi cient LED Driver for 60 White LEDs

PV

IN

8V TO 40V

2.2μF

2.2μF

51.1k

60.4k

V

IN

5V

SHDN

PWM

SYNC

10k

100k

100k

100k

10μH

V

IN

OPENLED

SHDN

PWM
SYNC
RT

V

REF

CTRL

T

SETISET

47pF

14.7k

SW

LT3598

SS GND

10nF

V

V

OUT

O_SW

LED1
LED2
LED3
LED4
LED5
LED6

FB

V

C

2.61k

15nF

1.00M

30.9k

3598 TA01a

20mA

4.7μ

1.5
ALL SIX LED STRINGS

1.0

0.5

0

MATCHING (%)

–0.5

–1.0

–1.5

–50

LED Current Matching

050

–25

25 75 100 125

TEMPERATURE (°C)

3598 TA01b

3598f

1

LT3598

(Note 1)

SHDN ................................................................VIN + 3V

, OPENLED ...........................................................30V

V

IN

SW Voltage ...............................................................45V

, V

V

OUT

LED1 to LED6 ............................................................44V

PWM, SYNC, CTRL, RT, SS, V

, FB Voltage..........................................................6V

V

REF

, T

I

SET

Operating Junction Temperature Range

(Note 2) .................................................. –40°C to 125°C

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

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

Voltage .................................................44V

O_SW

...................................6V

C

.....................................................................6V

SET

PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS

TOP VIEW

O_SWVOUT

V

SW

VINSHDN

GND

24 23 22 21 20 19

LED1

1

LED2

2

LED3

3

LED4

4

LED5

5

LED6

6

24-LEAD (4mm × 4mm) PLASTIC QFN

T

EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB

JMAX

25

7 8 9

OPENLED

10 11 12

C

V

SET

I

CTRL

UF PACKAGE

= 125°C, θJA = 37°C/W

18

V

REF

SS

17

RT

16

PWM

15

SYNC

14

13

NC

FB

SET

T

ORDER INFORMATION

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

LT3598EUF#PBF LT3598EUF#TRPBF 3598
LT3598IUF#PBF LT3598IUF#TRPBF 3598
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:

For more information on tape and reel specifi cations, go to:

ELECTRICAL CHARACTERISTICS

The l denotes the specifi cations which apply over the full operating

http://www.linear.com/leadfree/

http://www.linear.com/tapeandreel/

temperature range, otherwise specifi cations are at TA = 25°C. VIN = 5V, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum Operating Voltage 3 3.2 V
Maximum Operating Voltage 30 V
Reference Voltage

Reference Voltage Line Regulation 3 < VIN < 30V, VC = 0.3V 0.01 0.03 %/V
Maximum V
FB Pin Bias Current V
FB Error Amp Transconductance ΔI = 5μA 300 μmhos
FB Error Amp Voltage Gain 600 V/V
FB Pin Voltage 1.22 1.24 1.26 V
Current Loop Amp Transconductance 21 μmhos

Pin Current Out of Pin 200 μA

REF

= 1.230V (Note 3) 100 250 nA

FB

24-Lead (4mm × 4mm) Plastic QFN
24-Lead (4mm × 4mm) Plastic QFN

= VIN unless otherwise noted. (Note 2)

SHDN

1.216

l

1.210

–40°C to 125°C
–40°C to 125°C

1.230 1.260

1.260

3598f

V
V

2

LT3598

ELECTRICAL CHARACTERISTICS

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

= 25°C. VIN = 5V, V

A

PARAMETER CONDITIONS MIN TYP MAX UNITS

Current Loop Amp Voltage Gain 80 V/V

Sink Current 10 μA

V

C

Quiescent Current V
Quiescent Current in Shutdown V

Voltage V

I

SET

LED Current R

= 5V, PWM = 0V, Not Switching 3.5 5 mA

SHDN

= 0V 0 1 μA

SHDN

= 1.5V, V

CTRL

= 14.7kΩ 19.5 20 20.7 mA

ISET

TSET

= 1.5V, R

LED String Current Matching 20mA LED Current
LED Open Detection Threshold 0.2 0.25 V
OPENLED Sink Current 2mA
Minimum LED Regulation Voltage 0.8 V
LED1-6 Leakage Current V

CTRL Pin Bias Current V

= 1V, V

LED1-6

V

= 42V, V

LED1-6

= 0.8V (Note 4) 50 125 nA

CTRL

= 5V, PWM = 0V

OUT

= 44V, PWM = 0V

OUT

Switching Frequency RT = 309kΩ

RT = 51.1kΩ
RT = 14.7kΩ

T

Voltage 602 mV

SET

Maximum Switch Duty Cycle RT = 309kΩ

RT = 51.1kΩ

RT = 14.7kΩ
Switch Current Limit (Note 5) 1.5 2 2.5 A
Switch V

CESAT

Switch Leakage Current V
SHDN Pin Current V

ISW = 0.5A 0.12 V

= 40V 0.2 5 μA

SW

V

SHDN
SHDN

= 0V, V
= 5V, V

IN
IN

= 0V
= 5V

SHDN Voltage High 1.6 V
SHDN Voltage Low 0.4 V

Soft-Start Charging Current V

= 0.1V 5 10 15 μA

SS

PWM Input High Voltage 1V
PWM Input Low Voltage 0.4 V
PWM Pin Bias Current PWM = 3.3V 0.1 1 μA
SYNC Input High Voltage 1.5 V
SYNC Input Low Voltage 0.4 V
SYNC Pin Bias Current SYNC = 0V

SYNC = 3.3V
V

Switch Resistance 1000 Ω

O_SW

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 LT3598E is guaranteed to meet performance specifi cations
from 0°C to 125°C junction temperature. Specifi cations over the –40°C
to 125°C operating junction temperature range are assured by design,

= VIN unless otherwise noted. (Note 2)

SHDN

= 14.7kΩ 0.985 1.000 1.015 V

ISET

l

171

0.9

2.25

90

l

87
80

±0.5 ±1.5 %

0.1

0.2

190

1

2.5

1
2

209

1.1

2.75

μA
μA

kHz
MHz
MHz

95
90
86

0.1
30

25

0.1

60

50

1

μA
μA

μA

1

μA

%
%
%

characterization and correlation with statistical process controls. The
LT3598I is guaranteed over the full –40°C to 125°C operating junction
temperature range.

Note 3: Current fl ows out of FB pin.
Note 4: Current fl ows out of CTRL pin.
Note 5: Current limit guaranteed by design and/or correlation to static test.

Current limit is independent of duty cycle and is guaranteed by design.

3598f

3

LT3598

TYPICAL PERFORMANCE CHARACTERISTICS

SHDN Pin Turn-On Threshold SHDN Pin Current Quiescent Current

1.5

1.4

1.3

SHDN THRESHOLD (V)

1.2
–50

050

–25

25 75 100 125

TEMPERATURE (°C)

3598 G01

Reference Voltage Oscillator Frequency Current Limit

1.240

1.235

50
45

–50°C

40

35
30
25

20
15

SHDN PIN CURRENT (μA)

10

5

0

0

5

3.0

2.5

25°C

20

15

10

V

SHDN

2.5MHz

25

(V)

125°C

30

3598 G02

6

5

4

3

QUIESCENT CURRENT (mA)

2

35

–50

2.8

2.4

050

–25

25 75 100 125

TEMPERATURE (°C)

3598 G03

1.230

1.225

REFERENCE VOLTAGE (V)

1.220

1.215

0.40

0.35

0.30

0.25

(V)

0.20

CESAT

V

0.15

0.10

0.05

VIN = 40V

VIN = 5V

050

–50

–25

V

CESAT

0

0.25 0.750.50 1.00 1.25 1.5O

0

25 75 100 125

TEMPERATURE (°C)

125°C

–50°C

SWITCH CURRENT (A)

VIN = 30V

3598 G04

25°C

3598 G07

2.0

1.5

1.0

OSCILLATOR FREQUENCY (MHz)

0.5

0

–50

050

–25

TEMPERATURE (°C)

1MHz

200kHz

25 75 100 125

3598 G05

2.0

1.6

CURRENT (A)

1.2

0.8

0.4
–50

–25

Soft-Start Pin Current Feedback Pin Voltage

15.0

12.5

(μA)

10.0

SS

I

7.5

5.0
–50

050

–25

25 75 100 125

TEMPERATURE (°C)

3598 G08

1.24

1.23

1.22

FEEDBACK VOLTAGE (V)

1.21

1.20
–50

VC = 1.5V

VC = 1V

–25

050

25 75 100 125

TEMPERATURE (°C)

050

25 75 100 125

TEMPERATURE (°C)

3598 G06

3598 G09

4

3598f

TYPICAL PERFORMANCE CHARACTERISTICS

LED Current vs PWM Duty Cycle LED Current vs Temperature LED Current vs CTRL Voltage

100

20.3

25

LT3598

10

1

0.1

LED CURRENT (mA)

0.01

0.001

0.01
PWM DUTY CYCLE (%)

1100.1 100

3598 G10

20.2

20.1

20.0

19.9

LED CURRENT (mA)

19.8

19.7
–25

–50

LED Current vs Temperature OPENLED Sink Current

1.5

1.0

0.5

0.0

MATCHING (%)

–0.5

–1.0

–1.5

–50

050

–25

25 75 100 125

TEMPERATURE (°C)

3598 G13

3.0

2.5

2.0

1.5

1.0

OPENLED CURRENT (mA)

0.5

0

–50

–25

050

25 75 100 125

TEMPERATURE (°C)

050

25 75 100 125

TEMPERATURE (°C)

3598 G11

3598 G14

20

15

10

LED CURRENT (mA)

5

0

0

LED Current Waveforms
(0.1% PWM)

PWM

5V/DIV

SW

20V/DIV

I

L

1A/DIV

I

LED1

50mA/DIV

0.2 0.4
CTRL VOLTAGE (V)

0.6

20μs/DIV

0.8

1.0

1.2

3598 G12

3598 G15

PWM

5V/DIV

SW

20V/DIV

1A/DIV

I

LED1

50mA/DIV

LED Current Waveforms
(0.1% PWM)

I

L

2μs/DIV

3598 G16

PWM

5V/DIV

SW

20V/DIV

1A/DIV

I

LED1

50mA/DIV

LED Current Waveforms
(90% PWM)

I

L

100μs/DIV

3598 G17

PWM

5V/DIV

SW

20V/DIV

1A/DIV

I

LED1

50mA/DIV

LED Current Waveforms
(90% PWM)

I

L

5μs/DIV

3598 G18

3598f

5

LT3598

PIN FUNCTIONS

LED1-6 (Pins 1, 2, 3, 4, 5, 6): LED String Output. Connect
the bottom cathode of each LED string to these pins. Tie
pins to V

if the string is not used.

OUT

OPENLED (Pin 7): Open LED Flag When Any LED String
Opens. The output is open-collector. Tie a resistor to other
supply for open LED fl ag function.

(Pin 8): Programs LED Current for Each String. Con-

I

SET

nect a 14.7k resistor between I

and GND to program

SET

each LED string current to 20mA. A 47pF capacitor on the

pin reduces current ripple in each LED string.

I

SET

CTRL (Pin 9): LED Current Control. If the CTRL pin is not
used, tie this pin to V

(Pin 10): Error Amplifi er Output Pin. Tie the external

V

C

through a 10k to 20k resistor.

REF

compensation network to this pin.
FB (Pin 11): Feedback Pin for Overvoltage Protection.

Reference voltage is 1.230V. Connect the resistive divider
tap here. Minimize trace area at FB. Set V
to V

= 1.230(1 + R2/R1) when overvoltage protection

OUT

according

OUT

occurs (see Figure 2).

T

(Pin 12): An external resistor divider from V

SET

REF

pro­grams a decrease in LED current versus internal junction
temperature (setting temperature breakpoint and slope).
If the T

pin is not used, tie this pin to V

SET

REF

.

NC (Pin 13): No Connection.
SYNC (Pin 14): Frequency Synchronization Pin. This

input allows for synchronizing the operating frequency
to an external clock. The RT resistor should be chosen to
program a switching frequency 20% slower than SYNC
pulse frequency. This pin should be grounded if this
feature is not used.

PWM (Pin 15): Input Pin for PWM Dimming Control. Above
1V allows converter switching and below 1V disables
switching with VC pin level maintained. A PWM signal

driving the PWM pin provides accurate dimming control.
The PWM signal can be driven from 0V to 5V. If unused,
the pin should be connected to V

REF

.

RT (Pin 16): A resistor to ground programs switching
frequency between 200kHz and 2.5MHz. For SYNC func­tion, choose the resistor to program a frequency 20%
slower than the SYNC pulse frequency. Do not leave this
pin open.

SS (Pin 17): Soft-Start Pin. Place a soft-start capacitor
here. Upon start-up, a 10μA current charges the capacitor.
Use a larger capacitor for slower start-up. Leave open if
not used.

V

(Pin 18): Bandgap Voltage Reference. Internally set

REF

to 1.230V. This pin can supply up to 100μA. Can be used
to program the CTRL pin voltage using resistor dividers
to ground.

GND (Pin 19): Ground. Tie directly to local ground
plane.

SHDN (Pin 20): Shutdown Pin. Tie to 1.6V or more to en-
able the device. Tie below 0.4V or less to disable device.
Do not fl oat this pin.

V

(Pin 21): Input Supply Pin. Must be locally bypassed

IN

with a capacitor to ground.
SW (Pin 22): Switch Pin. This is the collector of the in-

ternal NPN power switch. Minimize the metal trace area
connected to this pin to minimize EMI.

V

(Pin 23): Output Pin. This pin provides power to

OUT

all LEDs.

V

(Pin 24): Drain of an Internal PMOS. The internal

O_SW

PMOS disconnects the feedback resistors from the V

OUT

pin during shutdown and the PWM transitioned to low.
Exposed Pad (Pin 25): Ground. The Exposed Pad must

be soldered to the PCB.

6

3598f