Maxim MAX8790ETP+, MAX8790 Datasheet

General Description

The MAX8790 is a high-efficiency driver for white lightemitting diodes (LEDs). It is designed for large liquidcrystal displays (LCDs) that employ an array of LEDs as the light source. A current-mode step-up controller drives up to six parallel strings of multiple series-connected LEDs. Each string is terminated with ballast that achieves ±1.5% current regulation accuracy, ensuring even brightness for all LEDs. The MAX8790 has a wide input-voltage range from 4.5V to 26V, and provides a fixed 20mA or adjustable 15mA to 25mA full-scale LED current.

The MAX8790 has two dimming control modes to enable a wide variety of applications. In direct DPWM mode, the LED current is directly turned on and off by a PWM signal. In analog dimming mode, an internal phase-locked loop (PLL) circuit translates the PWM signal into an analog signal and linearly controls the LED current down to 12.5%. Below 12.5%, digital dimming is added to allow lower average LED current down to 1%. Both control methods provide 100:1 dimming range.

The MAX8790 has multiple features to protect the controller from fault conditions. Separate feedback loops limit the output voltage if one or more LEDs fail open or short. The controller features cycle-by-cycle current limit to provide consistent operation and soft-start capability. A thermal-shutdown circuit provides another level of protection.

The step-up controller uses an external MOSFET, which provides good efficiency and allows for scalable output power and maximum operating voltage. Low feedback voltage at each LED string (450mV) helps reduce power loss. The MAX8790 features selectable switching frequency (500kHz, 750kHz, or 1MHz), which allows trade-offs between external component size and operating efficiency.

The MAX8790 is available in a thermally enhanced, lead-free, 20-pin, 4mm x 4mm, Thin QFN package.

Applications

Notebook, Subnotebook, and Tablet Computer Displays

Automotive Systems

Handy Terminals

Features

o Drives Six Parallel Strings with Multiple Series-

Connected LEDs per String

o ±1.5% Current Regulation Accuracy Between

Strings

o Low 450mV Feedback Voltage at Full Current

Improves Efficiency

o Step-Up Controller Regulates the Output Just

Above the Highest LED String Voltage

o Full-Scale LED Current Adjustable from 15mA to

25mA, or Preset 20mA

o Wide 100:1 Dimming Range

o Programmable Dimming Control: Direct DPWM or

Analog Dimming

o Built-In PLL for Synchronized Dimming Control

o Open and Short LED Protections

o Output Overvoltage Protection

o Wide Input Voltage Range from 4.5V to 26V

o External MOSFET Allows a Large Number of LEDs

per String

o 500kHz/750kHz/1MHz Switching Frequency

o Small, 20-Pin, 4mm x 4mm Thin QFN Package

MAX8790

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

________________________________________________________________

Maxim Integrated Products

1

Simplified Operating Circuit

Ordering Information

19-0658; Rev 0; 11/06

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

+

Denotes a lead-free package.

EVALUATION KIT

AVAILABLE

PART

TEMP RANGE

PIN-PACKAGE

PKG

CODE

MAX8790ETP+

20 Thin QFN

T2044-3

Pin Configuration appears at end of data sheet.

-40°C to +85°C (4mm x 4mm)

V

D1

IN

MAX8790

GND

L1

EXT

CS

OV

FB1 FB2 FB3 FB4 FB5 FB6

V

IN

C

IN

0.1μF

SHDN

V

CC

FSET ISET

BRT

N.C.

OSC

N.C.

CPLL

CCV

ENA

EP

OUT

N1

Rs

R1

R2

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1. VIN= 12V, V

SHDN

= VIN, CCV = 0.1µF, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

IN, SHDN, to GND .................................................-0.3V to +28V

FB_ to GND ............................................................-0.3V to +28V

V

CC

, BRT, ENA, OSC, OV to GND ...........................-0.3V to +6V

ISET, CCV, CS, FSET, CPLL, EXT to GND .-0.3V to (V

CC

+ 0.3V)

Continuous Power Dissipation (T

A

= +70°C)

20-Pin Thin QFN (derate 16.9mW/°C above +70°C) ...1349mW

Operating Temperature Range ...........................-40°C to +85°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-60°C to +150°C

Lead Temperature (soldering, 10s) .................................+300°C

MAX8790

Six-String White LED Driver with Active Current Balancing for LCD Panel Applications

2 _______________________________________________________________________________________

PARAMETER CONDITIONS MIN TYP MAX UNITS

IN Input Voltage Range

IN Quiescent Current

VCC Output voltage V

VCC Short-Circuit Current 15 56 130 mA

VCC UVLO Threshold Rising edge, hysteresis = 20mV 4.00 4.25 4.45 V

STEP-UP CONVERTER

EXT High Level 10mA from EXT to GND

EXT Low Level -10mA from EXT to V EXT On-Resistance EXT high or low 2 5 Ω EXT Sink/Source Current EXT forced to 2V 1 A

OSC High-Level Threshold

OSC Midlevel Threshold 1.5

OSC Low-Level Threshold 0.4 V

Operating Frequency

Minimum Duty Cycle

Maximum Duty Cycle 94 95 % CS Trip Voltage Duty cycle = 75% 85 100 115 mV

CONTROL INPUT

SHDN Logic-Input High Level 2.1 V SHDN Logic-Input Low Level 0.8 V

BRT, ENA Logic-Input High Level 2.1 V BRT, ENA Logic-Input Low Level 0.8 V

V

= V

IN

CC

V

= bypassed to GND through 1µF cap 5.5 26.0

CC

V

= high

SHDN

SHDN = GND

= 5V, 6V < VIN < 26V, 0 < I

SHDN

CC

V

= V

V V

OSC

CC

= open 675 750 825 = GND 450 500 550

PWM mode 10 Pulse skipping, no load 0

4.5 5.5

VIN = 26V 1 2

= VCC = 5V 1 2

V

IN

10 µA

< 10mA 4.7 5.0 5.3 V

VCC

V

-

CC

0.1

V

CC

V

0 0.1 V

V

-

CC

0.4

V

VCC -

2.0

0.9 1.0 1.1 MHz

V

mA

V

kHz

%

MAX8790

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1. VIN= 12V, V

SHDN

= VIN, CCV = 0.1µF, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)

PARAMETER CONDITIONS MIN TYP MAX UNITS

INPUT LEAKAGE

SHDN Leakage Current SHDN = 26V +35 µA

CS Leakage Current VCS = GND +40 +50 µA OSC Leakage Current -3 +3 µA BRT, ENA Leakage Current -1 +1 µA FSET, ISET Leakage Current FSET = ISET = V OV Leakage Current -0.1 +0.1 µA

LED CURRENT

Full-Scale FB_ Output Current

ISET High-Level Threshold Default setting for 20mA full-scale LED current

ISET Voltage 1.12 1.19 1.26 V 20% Output Current ISET = VCC, BRT = 20% 3.84 4.00 4.16 mA

Current Regulation Between Strings

Minimum FB_ Regulation Voltage

Maximum FB_ Ripple ISET = V FB_ On-Resistance V

FB_ Leakage Current

BRT Input Frequency 100 500 Hz Minimum BRT Duty Cycle PLL active 12.5 %

FAULT PROTECTION

OV Threshold Voltage 1.16 1.23 1.30 V

FB_ Overvoltage Threshold

FAULT Shutdown Timer V Thermal-Shutdown Threshold (Note 1) 170 °C

PHASE-LOCKED LOOP

FSET High-Level Threshold PLL disabled

BRT Frequency Capture Range

CC

ISET = V R

ISET

R

ISET

ISET = V ISET = V R

ISET

ISET = V ISET = V

FB_

SHDN = GND, V SHDN = V

FB_

R

FSET

R

FSET

, BRT = 100% 19.40 20.00 20.60

CC

= 80kΩ to GND, BRT = 100% 24.25 25.00 25.75

= 133kΩ to GND, BRT = 100% 14.40 15.00 15.60

, BRT = 100% -1.5 +1.5 %

CC

, BRT = 20% -2.0 +2.0 %

CC

= 80kΩ to GND, BRT = 100% 300 500 800

, BRT = 100% 270 450 720

CC

, 12.5% 150 275 500

CC

CC , COUT

= 50mV 13 20 Ω

IN

> 5.6V (typ) 50 65 80 ms

= 500kΩ 150 200 250 = 250kΩ 300 400 500

= 1µF, OSC = VCC (Note 1) 120 200 mV

= 26V 1

FB_

, BRT = GND, V

FB_

= 15V

-1 +1 µA

V

-

CC

0.4

V

10 28

VCC +

0.20

V

CC

0.4

V

+

CC

0.6

-

V

1.45

CC

+

mA

mV

P-P

µA

V

Hz

MAX8790

Six-String White LED Driver with Active Current Balancing for LCD Panel Applications

4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 1. VIN= 12V, V

SHDN

= VIN, CCV = 0.1µF, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)

PARAMETER CONDITIONS MIN TYP MAX UNITS

IN Input Voltage Range

IN Quiescent Current

VCC Output Voltage V

VCC Short-Circuit Current 12 130 mA

VCC UVLO Threshold Rising edge, hysteresis = 20mV 4.00 4.45 V

STEP-UP CONVERTER

EXT High Level 10mA from EXT to GND

EXT Low Level -10mA from EXT to V EXT On-Resistance EXT high or low 5 Ω

OSC High-Level Threshold

OSC Midlevel Threshold 1.5

OSC Low-Level Threshold 0.4 V

Maximum Duty Cycle 94 % CS Trip Voltage Duty cycle = 75% 85 115 mV

CONTROL INPUT

SHDN Logic-Input High Level 2.1 V SHDN Logic-Input Low Level 0.8 V

BRT, ENA Logic-Input High Level 2.1 V BRT, ENA Logic-Input Low Level 0.8 V

INPUT LEAKAGE

SHDN Leakage Current SHDN = 26V +35 µA

CS Leakage Current VCS = GND +50 µA OSC Leakage Current -3 +3 µA BRT, ENA Leakage Current -1 +1 µA FSET, ISET Leakage Current FSET = ISET = V OV Leakage Current -0.1 +0.1 µA

V

= V

IN

CC

V

bypassed to GND through 1µF cap 5.5 26.0

CC

V

= high

SHDN

SHDN = GND

= 5V, 6V < VIN < 26V, 0 < I

SHDN

V

= V

V V

OSC

CC

= open 675 825Operating Frequency = GND 450 550

VIN = 26V 2

= VCC = 5V 2

V

IN

< 10mA 4.7 5.3 V

VCC

CC

4.5 5.5

V

CC

0.1

V

CC

0.4

0.9 1.1 MHz

-1 +1 µA

10 µA

-

V

0.1 V

-

V

VCC -

2.0

V

mA

V

kHz

MAX8790

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

_______________________________________________________________________________________ 5

Note 1: Specifications are guaranteed by design, not production tested. Note 2: Specifications to -40°C are guaranteed by design, not production tested.

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 1. VIN= 12V, V

SHDN

= VIN, CCV = 0.1µF, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)

PARAMETER CONDITIONS MIN TYP MAX UNITS

LED CURRENT

ISET High-Level Threshold Default setting for 20mA full-scale LED current

ISET Voltage 1.12 1.26 V 20% Output Current ISET = VCC, BRT = 20% 3.8 4.2 mA

Current Regulation Between Strings

Maximum FB_ Ripple ISET= VCC, C FB_ On-Resistance V

FB_ Leakage Current

BRT Input Frequency 100 500 Hz

FAULT PROTECTION

OV Threshold Voltage 1.16 1.30 V

FB_ Overvoltage Threshold

FAULT Shutdown Timer V

ISET = V R

ISET

R

ISET

ISET = V ISET = V R

ISET

ISET= V ISET = V

FB_

SHDN = GND, V SHDN = V

FB_

, BRT = 100% 19.2 20.8

CC

= 80kΩ to GND, BRT = 100% 24.0 26.0Full-Scale FB_ Output Current = 133kΩ to GND, BRT = 100% 14.4 15.6

V

CC

0.4

, BRT = 100% -2 +2

CC

, BRT = 20% -3 +3

CC

= 80kΩ to GND, BRT = 100% 280 840

, BRT = 100% 250 760Minimum FB_ Regulation Voltage

CC

, BRT = 12.5% 140 530

CC

= 1µF, OSC = VCC (Note 1) 200 mV

OUT

= 50mV 20 Ω

= 26V 1

FB_

, BRT = GND, V

IN

> 5.6V (typ) 50 80 ms

= 15V 28

FB_

V

CC

0.2

-

V

+

VCC +

1.45

PHASE-LOCKED LOOP

V

-

FSET High-Level Threshold PLL disabled

R

= 500kΩ 150 250 Hz

BRT Frequency Capture Range

FSET

R

= 250kΩ 300 500 Hz

FSET

CC

0.4

V

mA

%

mV

P-P

µA

V

MAX8790

Six-String White LED Driver with Active Current Balancing for LCD Panel Applications

6 _______________________________________________________________________________________

Typical Operating Characteristics

(Circuit configuration 1, VIN= 12V, V

SHDN

= VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA= +25°C, unless otherwise noted.)

BOOST CONVERTER EFFICIENCY

vs. INPUT VOLTAGE (BRT = 100%)

MAX8790 toc01

BOOST CONVERTER EFFICIENCY (%)

INPUT VOLTAGE (V)

94

93

92

91

90

89

88

87

86

71217

500kHz

750kHz

1MHz

NORMALIZED POWER vs. TOTAL LED CURRENT

(ANALOG AND DPWM DIMMING)

MAX8790 toc02

NORMALIZED POWER

TOTAL LED CURRENT (mA)

1.2

1.0

0.8

0.6

0.4

0.2

0

1 10 100 1000

NORMALIZED TO VIN = 20V, AND I

LED

= 20mA

V

IN

= 7V

TOTAL LED

POWER, ANALOG

TOTAL INPUT

POWER, DPWM

TOTAL INPUT

POWER, ANALOG

TOTAL LED

POWER, DPWM

LED CURRENT vs. BRT DUTY CYCLE

(BRT AT 200Hz)

MAX8790 toc03

LED CURRENT (mA)

BRT DUTY CYCLE (%)

25

20

15

10

5

0

1 10 100

IDENTICAL FOR DPWM DIMMING AND ANALOG DIMMING

LED CURRENT

vs. AMBIENT TEMPERATURE (BRT = 100%)

MAX8790 toc04

LED CURRENT (mA)

AMBIENT TEMPERATURE (°)

21.0

20.8

20.6

20.4

20.2

20.0

19.8

19.6

19.4

19.2

19.0

020 6040 80

LED CURRENT REGULATION

vs. INPUT VOLTAGE

MAX8790 toc05

LED CURRENT REGULATION (%)

INPUT VOLTAGE (V)

0.05

0.04

0.03

0.02

0.01

0

-0.01

-0.02

-0.03

-0.04

-0.05

71217

ANALOG DIMMING BRT = 10%

DPWM DIMMING BRT = 100%

DPWM DIMMING BRT = 10%

FB_ VOLTAGE vs. LED CURRENT

(ANALOG DIMMING)

MAX8790 toc06

FB_ REGULATION VOLTAGE (V)

LED STRING CURRENT (mA)

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0 5 10 15 20 25 30

SUPPLY CURRENT vs. INPUT VOLTAGE

(DPWM DIMMING)

MAX8790 toc07

SUPPLY CURRENT (mA)

INPUT VOLTAGE (V)

7

6

5

4

3

2

1

0

71217

BRT = 100%

BRT = 0%

SHUTDOWN CURRENT vs. INPUT VOLTAGE

MAX8790 toc08

SHUTDOWN CURRENT (μA)

INPUT VOLTAGE (V)

7

6

5

4

3

2

1

0

71217

SWITCHING WAVEFORMS

(BRT = 100%)

MAX8790 toc09

200ns/div

I

L

500mA/div 0mA

V

LX

10V/div

0V

MAX8790

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

_______________________________________________________________________________________

7

Typical Operating Characteristics (continued)

(Circuit configuration 1, VIN= 12V, V

SHDN

= VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA= +25°C, unless otherwise noted.)

SWITCHING WAVEFORMS

(BRT = 15%, ANALOG DIMMING)

1μs/div

MAX8790 toc10

LED CURRENT WAVEFORMS

(BRT = 1% AT 200Hz, DPWM DIMMING)

LED CURRENT WAVEFORMS

(BRT = 50% AT 200Hz, DPWM DIMMING)

2ms/div

MAX8790 toc14

MAX8790 toc12

0V

V

LX

10V/div

0V

I

L

500mA/div 0mA

STARTUP WAVEFORMS

(BRT = 100%, DPWM DIMMING)

4ms/div

MAX8790 toc13

BRT 5V/div

0V

V

FB1

5V/div

0V

MAX8790 toc11

SHDN 5V/div

0V

V

OUT

20V/div

0V

V

CCV

2V/div

0V

I

L

1A/div

0A

LED CURRENT WAVEFORMS

(BRT = 50% AT 200Hz, ANALOG DIMMING)

BRT 5V/div

V

FB1

1V/div

0V

0mA

0A

BRT 5V/div

V

FB1

5V/div

I

LED

100mA/div

I

L

1A/div

1ms/div

LED CURRENT WAVEFORMS

(BRT = 1% AT 200Hz, ANALOG DIMMING)

1ms/div

MAX8790 toc15

0mA

0A

0V

0mA

I

LED

100mA/div

I

L

1A/div

BRT 5V/div

V

FB1

2V/div

I

LED

50mA/div

I

L

500mA/div

1ms/div

LED-OPEN FAULT PROTECTION

(BRT = 100%, LED OPEN ON FB3)

20ms/div

MAX8790 toc16

0mA

0A

0V

0A

I

LED

50mA/div

I

L

1A/div

V

FB3

1V/div

V

FB1

10V/div

V

OUT

20V/div

I

L

1A/div

MAX8790

Six-String White LED Driver with Active Current Balancing for LCD Panel Applications

8 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(Circuit configuration 1, VIN= 12V, V

SHDN

= VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA= +25°C, unless otherwise noted.)

LED-SHORT FAULT PROTECTION

(BRT = 100%, 2 LEDs SHORT ON FB3)

10ms/div

0A

V

FB3

1V/div

V

FB1

10V/div

V

OUT

20V/div

I

L

1A/div

0V

MAX8790 toc17

LED CURRENT BALANCING

vs. INPUT VOLTAGE (BRT = 100%)

MAX8790 toc18

LED CURRENT BALANCING ACCURACY (%)

INPUT VOLTAGE (V)

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0

71217

750kHz

500kHz

1MHz

Pin Description

PIN NAME FUNCTION

1 OSC

2 ENA

3 BRT

4 SHDN

5 FB1

6 FB2

7 FB3

8 GND Ground

9 FB4

10 FB5

O sci l l ator Fr eq uency S el ecti on P i n. C onnect OS C to V 1M H z. C onnect O S C to G N D to set the fr eq uency to 500kH z. Fl oat OS C to set the fr eq uency to 750kH z.

Anal og D i m m i ng E nab l e. E N A sets the P WM contr ol m od e. S et E N A LOW to enab l e d i r ect D P WM d i m m i ng . S et E N A H IGH to enab l e anal og d i m m i ng . In b oth m od es, the d uty cycl e of the P WM si g nal at the BRT i np ut contr ol s the LE D cur r ent char acter i sti cs. S ee the D i m m i ng C ontr ol secti on for a com p l ete d escr i p ti on.

Brightness Control Input. The duty cycle of this digital input signal controls the LED current characteristics. The allowable frequency range is 100Hz to 500Hz in analog dimming mode. The duty cycle can be 100% to 1%. The BRT frequency can go above 500Hz in direct DPWM mode as long as the BRT pulse width is greater than 50µs minimum. See the Dimming Control section for a complete description.

Shutdown Control Input. The MAX8790 shuts down when SHDN is less than 0.8V. Pulling SHDN above

2.1V enables the MAX8790. SHDN can be connected to the input voltage if desired.

LED String 1 Cathode Connection. FB1 is the open-drain output of an internal regulator, which controls current through FB1. FB1 can sink up to 25mA. If unused, connect FB1 to GND.

LED String 2 Cathode Connection. FB2 is the open-drain output of an internal regulator, which controls current through FB2. FB2 can sink up to 25mA. If unused, connect FB2 to GND.

LED String 3 Cathode Connection. FB3 is the open-drain output of an internal regulator, which controls current through FB3. FB3 can sink up to 25mA. If unused, connect FB3 to GND.

LED String 4 Cathode Connection. FB4 is the open-drain output of an internal regulator, which controls current through FB4. FB4 can sink up to 25mA. If unused, connect FB4 to GND.

LED String 5 Cathode Connection. FB5 is the open-drain output of an internal regulator, which controls current through FB5. FB5 can sink up to 25mA. If unused, connect FB5 to GND.

to set the step - up conver ter ’ s osci l l ator fr eq uency to

C C