MAX8790ETP+T - Maxim Integrated - Datasheet.Directory

General Description

The MAX8790 is a high-efficiency driver for white light-

emitting diodes (LEDs). It is designed for large liquid-

crystal 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 bright-

ness 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 control-

ler 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 frequen-

cy (500kHz, 750kHz, or 1MHz), which allows trade-offs

between external component size and operating efficiency.

The MAX8790 is available in a thermally enhanced,

lead(Pb)-free, 20-pin, 4mm x 4mm, TQFN package.

Applications

●Notebook, Subnotebook, and Tablet Computer Displays

●Handy Terminals

Features

●Drives Six Parallel Strings with Multiple Series-

Connected LEDs per String

●±1.5% Current Regulation Accuracy Between Strings

●Low 450mV Feedback Voltage at Full Current

Improves Efficiency

●Step-Up Controller Regulates the Output Just Above

the Highest LED String Voltage

●Full-Scale LED Current Adjustable from 15mA to

25mA, or Preset 20mA

●Wide 100:1 Dimming Range

●Programmable Dimming Control: Direct DPWM or

Analog Dimming

●Built-In PLL for Synchronized Dimming Control

●Open and Short LED Protections

●Output-Overvoltage Protection

●Wide Input Voltage Range from 4.5V to 26V

●External MOSFET Allows a Large Number of LEDs

per String

●500kHz/750kHz/1MHz Switching Frequency

●Small, 20-Pin, 4mm x 4mm TQFN Package

Pin Configuration appears at end of data sheet.

19-0658; Rev 1; 5/14

+Denotes a lead(Pb)-free/RoHS-compliant package.

PART TEMP RANGE PIN-PACKAGE PKG

CODE

MAX8790ETP+ -40°C to +85°C 20 Thin QFN

(4mm x 4mm) T2044-3

FB1

BRT

CCV

GND

VIN

EXT

FB2

FB3

FB4

FB5

FB6

L1 D1

VCC

ISET

CIN

FSET

CPLL

N.C.

OSC

N.C.

ENA

SHDN

0.1µF

VOUT

MAX8790

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

Simplied Operating Circuit

Ordering Information

EVALUATION KIT AVAILABLE

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

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

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

ISET, CCV, CS, FSET, CPLL, EXT to GND

...-0.3V to (VCC + 0.3V)

Continuous Power Dissipation (TA = +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

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

IN Input Voltage Range VIN = VCC 4.5 5.5 V

VCC = bypassed to GND through 1µF capacitor 5.5 26.0

IN Quiescent Current VSHDN = high VIN = 26V 1 2 mA

VIN = VCC = 5V 1 2

SHDN = GND 10 µA

VCC Output Voltage VSHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA 4.7 5.0 5.3 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 VCC -

0.1 VCC V

EXT Low Level -10mA from EXT to VCC 0 0.1 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 VCC -

0.4 V

OSC Midlevel Threshold 1.5 VCC -

2.0 V

OSC Low-Level Threshold 0.4 V

Operating Frequency

VOSC = VCC 0.9 1.0 1.1 MHz

VOSC = open 675 750 825 kHz

VOSC = GND 450 500 550

Minimum Duty Cycle PWM mode 10 %

Pulse skipping, no load 0

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

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

www.maximintegrated.com Maxim Integrated

│

Absolute Maximum Ratings

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.

Electrical Characteristics

(Circuit of Figure 1. VIN = 12V, VSHDN = 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 VSHDN = 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 = VCC -1 +1 µA

OV Leakage Current -0.1 +0.1 µA

LED CURRENT

Full-Scale FB_ Output Current

ISET = VCC, BRT = 100% 19.40 20.00 20.60

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

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

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

0.4 V

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

ISET = VCC, BRT = 100% -1.5 +1.5 %

ISET = VCC, BRT = 20% -2.0 +2.0 %

Minimum FB_ Regulation Voltage

RISET = 80kΩ to GND, BRT = 100% 300 500 800

mVISET = VCC, BRT = 100% 270 450 720

ISET = VCC, 12.5% 150 275 500

Maximum FB_ Ripple ISET = VCC, COUT = 1µF, OSC = VCC (Note 1) 120 200 mVP-P

FB_ On-Resistance VFB_ = 50mV 13 20 Ω

FB_ Leakage Current SHDN = GND, VFB_ = 26V 1 µA

VSHDN = VIN, BRT = GND, VFB_ = 15V 10 28

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 VCC +

0.20

VCC +

0.6

VCC +

1.45 V

FAULT Shutdown Timer VFB_ > 5.6V (typ) 50 65 80 ms

Thermal-Shutdown Threshold (Note 1) 170 °C

PHASE-LOCKED LOOP

FSET High-Level Threshold PLL disabled VCC -

0.4 V

BRT Frequency Capture Range RFSET = 500kΩ 150 200 250 Hz

RFSET = 250kΩ 300 400 500

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

www.maximintegrated.com Maxim Integrated

│

Electrical Characteristics (continued)

(Circuit of Figure 1. VIN = 12V, VSHDN = 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 VIN = VCC 4.5 5.5 V

VCC bypassed to GND through 1µF cap 5.5 26.0

IN Quiescent Current VSHDN = high VIN = 26V 2 mA

VIN = VCC = 5V 2

SHDN = GND 10 µA

VCC Output Voltage VSHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA 4.7 5.3 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 VCC -

0.1 V

EXT Low Level -10mA from EXT to VCC 0.1 V

EXT On-Resistance EXT high or low 5 Ω

OSC High-Level Threshold VCC -

0.4 V

OSC Midlevel Threshold 1.5 VCC

-2.0 V

OSC Low-Level Threshold 0.4 V

Operating Frequency

VOSC = VCC 0.9 1.1 MHz

VOSC = open 675 825 kHz

VOSC = GND 450 550

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 VSHDN = 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 = VCC -1 +1 µA

OV Leakage Current -0.1 +0.1 µA

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

www.maximintegrated.com Maxim Integrated

│

Electrical Characteristics

(Circuit of Figure 1. VIN = 12V, VSHDN = VIN, CCV = 0.1μF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)

Note 1: Specifications are guaranteed by design, not production tested.

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

PARAMETER CONDITIONS MIN TYP MAX UNITS

LED CURRENT

Full-Scale FB_ Output Current

ISET = VCC, BRT = 100% 19.2 20.8

mARISET = 80kΩ to GND, BRT = 100% 24.0 26.0

RISET = 133kΩ to GND, BRT = 100% 14.4 15.6

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

0.4 V

ISET Voltage 1.12 1.26 V

20% Output Current ISET = VCC, BRT = 20% 3.8 4.2 mA

Current Regulation Between

Strings

ISET = VCC, BRT = 100% -2 +2 %

ISET = VCC, BRT = 20% -3 +3

Minimum FB_ Regulation Voltage

RISET = 80kΩ to GND, BRT = 100% 280 840

mVISET= VCC, BRT = 100% 250 760

ISET = VCC, BRT = 12.5% 140 530

Maximum FB_ Ripple ISET= VCC, COUT = 1µF, OSC = VCC (Note 1) 200 mVP-P

FB_ On-Resistance VFB_ = 50mV 20 Ω

FB_ Leakage Current

SHDN = GND, VFB_ = 26V 1 µA

SHDN = VIN, BRT = GND, VFB_ = 15V 28

BRT Input Frequency 100 500 Hz

FAULT PROTECTION

OV Threshold Voltage 1.16 1.30 V

FB_ Overvoltage Threshold VCC +

0.2

VCC +

1.45 V

FAULT Shutdown Timer VFB_ > 5.6V (typ) 50 80 ms

PHASE-LOCKED LOOP

FSET High-Level Threshold PLL disabled VCC -

0.4 V

BRT Frequency Capture Range RFSET = 500kΩ 150 250 Hz

RFSET = 250kΩ 300 500 Hz

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

www.maximintegrated.com Maxim Integrated

│

Electrical Characteristics (continued)

(Circuit conguration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.)

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

1 10 100 1000

NORMALIZED TO VIN = 20V, AND ILED = 20mA

VIN = 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 (%)

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

0 20 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

-0.01

-0.02

-0.03

-0.04

-0.05

7 12 17

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 5 10 15 20 25 30

SUPPLY CURRENT vs. INPUT VOLTAGE

(DPWM DIMMING)

MAX8790 toc07

SUPPLY CURRENT (mA)

INPUT VOLTAGE (V)

7 12 17

BRT = 100%

BRT = 0%

BOOST CONVERTER EFFICIENCY

vs. INPUT VOLTAGE (BRT = 100%)

MAX8790 toc01

BOOST CONVERTER EFFICIENCY (%)

INPUT VOLTAGE (V)

7 12 17

500kHz

750kHz

1MHz

SHUTDOWN CURRENT vs. INPUT VOLTAGE

MAX8790 toc08

SHUTDOWN CURRENT (A)

INPUT VOLTAGE (V)

7 12 17

SWITCHING WAVEFORMS

(BRT = 100%)

MAX8790 toc09

200ns/div

500mA/div

0mA

VLX

10V/div

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

Maxim Integrated

│

www.maximintegrated.com

Typical Operating Characteristics

(Circuit conguration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.)

SWITCHING WAVEFORMS

(BRT = 15%, ANALOG DIMMING)

MAX8790 toc10

1µs/div

500mA/div

0mA

VLX

10V/div

STARTUP WAVEFORMS

(BRT = 100%, DPWM DIMMING)

4ms/div

SHDN

5V/div

VOUT

20V/div

VCCV

2V/div

1A/div

MAX8790 toc11

LED CURRENT WAVEFORMS

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

2ms/div

BRT

5V/div

VFB1

5V/div

ILED

100mA/div

1A/div

0mA

MAX8790 toc12

LED CURRENT WAVEFORMS

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

1ms/div

BRT

5V/div

VFB1

5V/div

ILED

100mA/div

1A/div

0mA

MAX8790 toc13

LED CURRENT WAVEFORMS

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

1ms/div

0mA

BRT

5V/div

VFB1

2V/div

ILED

50mA/div

500mA/div

0mA

MAX8790 toc15

LED CURRENT WAVEFORMS

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

1ms/div

BRT

5V/div

VFB1

1V/div

ILED

50mA/div

1A/div

0mA

MAX8790 toc14

LED-OPEN FAULT PROTECTION

(BRT = 100%, LED OPEN ON FB3)

20ms/div

VFB3

1V/div

VFB1

10V/div

VOUT

20V/div

1A/div

MAX8790 toc16

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

Maxim Integrated

│

www.maximintegrated.com

Typical Operating Characteristics (continued)

(Circuit conguration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.)

PIN NAME FUNCTION

1 OSC Oscillator Frequency Selection Pin. Connect OSC to VCC to set the step-up converter’s oscillator frequency to

1MHz. Connect OSC to GND to set the frequency to 500kHz. Float OSC to set the frequency to 750kHz.

2 ENA

Analog Dimming Enable. ENA sets the PWM control mode. Set ENA LOW to enable direct DPWM dimming.

Set ENA HIGH to enable analog dimming. In both modes, the duty cycle of the PWM signal at the BRT input

controls the LED current characteristics. See the Dimming Control section for a complete description.

3BRT

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.

4SHDN 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.

5 FB1 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.

6 FB2 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.

7 FB3 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.

8 GND Ground

9 FB4 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.

10 FB5 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.

LED-SHORT FAULT PROTECTION

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

10ms/div

VFB3

1V/div

VFB1

10V/div

VOUT

20V/div

1A/div

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

7 12 17

750kHz

500kHz

1MHz

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

www.maximintegrated.com Maxim Integrated

│

Pin Description

Typical Operating Characteristics (continued)

PIN NAME FUNCTION

11 FB6 LED String 6 Cathode Connection. FB6 is the open-drain output of an internal regulator, which controls current

through FB6. FB6 can sink up to 25mA. If unused, connect FB6 to GND.

12 CS Step-Up Controller Current-Sense Input. Connect the CS input to a ground-referenced sense resistor to

measure the current in the external MOSFET switch.

13 EXT External MOSFET Gate-Drive Output

14 OV Overvoltage Sense. Connect OV to the center tap of a resistive voltage-divider from VOUT to ground. The

detection threshold for voltage limiting at OV is 1.23V (typ).

15 VCC

5V Linear Regulator Output. VCC provides power to the MAX8790 and is also used to bias the gate driver for

the external MOSFET. Bypass VCC to GND with a ceramic capacitor of 1µF or greater. If VIN is less than or

equal to 5.5V, connect VCC to IN to the disable the internal LDO and use the external 5V supply to VCC. When

SHDN is low, the internal linear regulator is disabled.

16 IN Supply Input. VIN biases the internal 5V linear regulator that powers the device. Bypass IN to GND directly at

the pin with a 0.1µF or greater ceramic capacitor.

17 CCV

Step-Up Converter Compensation Pin. Connect a 0.1µF ceramic capacitor and 1.2-free

Ω resistor from CCV to GND. When the MAX8790 shuts down, CCV is discharged to 0V through an internal

20kΩ resistor.

18 ISET

Full-Scale LED Current Adjustment Pin. The resistance from ISET to GND controls the full-scale current in each

LED string:

ILEDmax = 20mA x 100kΩ/RISET

The acceptable resistance range is 80kΩ < RISET < 133kΩ, which corresponds to full-scale LED current of

25mA > ILEDmax > 15mA. Connect ISET to VCC for a default full-scale LED current of 20mA.

19 FSET

PLL Free-Running Frequency Control Pin. The resistance from FSET to GND controls the PLL oscillator’s free-

running frequency, fPLL:

fPLL = 1 / (10 x RFSET x 800pF)

The capture range is 0.6 x fPLL to fPLL. The acceptable resistance range for FSET is 250kΩ < RFSET < 754kΩ,

which corresponds to a frequency range of 500Hz > fPLL > 166Hz. The resulting capture frequency range is

100Hz to 500Hz.

20 CPLL Phase-Locked Loop-Compensation Capacitor Pin. The capacitance at CPLL compensates the PLL loop

response. Connect a 0.1µF ceramic capacitor from CPLL to GND.

EP EP Exposed Backside Pad. Solder to the circuit board ground plane with sufcient copper connection to ensure

low thermal resistance. See the PCB Layout Guidelines section.

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

www.maximintegrated.com Maxim Integrated

│

Pin Description (continued)

Detailed Description

The MAX8790 is a high-efficiency driver for arrays of

white LEDs. It contains a fixed-frequency, current-mode,

PWM step-up controller, 5V linear regulator, dimming

control circuit, and six regulated current sources (see

Figure 2). When enabled, the step-up controller boosts

the output voltage to provide sufficient headroom for

the current sources to regulate their respective string

currents. The MAX8790 features selectable switching

frequency (500kHz, 750kHz, or 1MHz), which allows

trade-offs between external component size and operat-

ing efficiency. The control architecture automatically skips

pulses at light loads to improve efficiency and prevents

overcharging the output capacitor.

A PWM logic input signal, BRT, controls the LED bright-

ness. The MAX8790 supports both analog and digital

control of the LED current, and achieves 100:1 dimming

range. The MAX8790’s dimming control circuit consists of

a PLL, a digital comparator, and a DAC. In direct DPWM

mode, the step-up controller and current source are

directly turned on and off by the PWM signal. In analog

dimming mode, an internal PLL, digital comparator, and

DAC circuit translate the PWM signal into an analog sig-

nal that linearly controls the LED current, down to a PWM

duty factor of 12.5%.

The MAX8790 has multiple features to protect the control-

ler from fault conditions. Separate feedback loops limit

the output voltage if one or more LEDs fail open or short.

During operation, if one of the feedback string voltages

exceeds the VCC to 0.6V (typ) protection threshold, the

controller shuts down and latches off after an internal

timer expires. The controller features cycle-by-cycle cur-

rent limit to provide consistent operation and soft-start

capability. A thermal-shutdown circuit provides another

level of protection.

The MAX8790 includes a 5V linear regulator that provides

the internal bias and gate drive for the step-up controller.

When an external 5V is available, the internal LDO can

be overdriven to decrease power dissipation. Otherwise,

connect the IN pin to an input greater than 5.5V. The inter-

nal LDO is disabled when SHDN is low.

Figure 1. Typical Operating Circuit

FB1

BRT

CCV

GND

VIN

7V TO 21V

EXT

CPLL FB2

FB3

FB4

FB5

FB6

4.7µH D1

VCC

ISET

FSET

CIN

ENA

OSC

N.C.

0.1µF

1µF

511kΩ

1.2kΩ

37.4kΩ

1MΩ

56mΩ

SHDN

0.1µF

COUT

VOUT

UP TO 35V

MAX8790

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

www.maximintegrated.com Maxim Integrated

│

Figure 2. Control Circuit Block Diagram

5V LINEAR

REGULATOR

OSCILLATOR SLOPE

COMPENSATION

CLOCK

FB OVERVOLTAGE

COMPARATOR

ERROR

COMPARATOR

OUTPUT OVERVOLTAGE

COMPARATOR

EXT

ERROR

AMPLIFIER

CURRENT SOURCE

FB1

GND

FB2

65ms TIMER

SHUTDOWN

LATCH

CONTROL AND

DRIVER LOGIC

CURRENT SENSE

HVC FB6

FB5

FB4

FB3

FB2

LVC

TRI-LEVEL

COMPARATOR

VCC

OSC

REF ADJ

CLK

REF

VCC - 0.4V

VCC + 0.6V

1.25V

ISET

PLL

5 LSBs

5 MSBs

CPLL

BRT

ENA

OSC

256 x fBRT

DIGITAL

COMPARATOR

8-BIT

LATCH

8-BIT DAC

SAT

DIGITAL CONTROL

CURRENT SOURCE

8-BIT

COUNTER

FSET

CCV

VCC

SHDN

FB3

CURRENT SOURCE

FB4

CURRENT SOURCE

FB5

CURRENT SOURCE

FB6

CURRENT SOURCE

10Ω

MAX8790 Six-String White LED Driver with Active

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Fixed-Frequency Step-Up Controller

The MAX8790’s fixed-frequency, current-mode, step-up

controller automatically chooses the lowest active FB_

voltage to regulate the feedback voltage. Specifically, the

difference between the lowest FB_ voltage and the current

source-control signal plus an offset (VSAT) is integrated at

the CCV output. The resulting error signal is compared to

the external switch current plus slope compensation to

terminate the switch on-time. As the load changes, the

error amplifier sources or sinks current to the CCV output

to adjust the required peak inductor current. The slope-

compensation signal is added to the current-sense signal

to improve stability at high duty cycles.

At light loads, the MAX8790 automatically skips pulses

to improve efficiency and prevent overcharging the out-

put capacitor. In SKIP mode, the inductor current ramps

up for a minimum on-time of approximately 150ns, then

discharges the stored energy to the output. The switch

remains off until another pulse is needed to boost the

output voltage.

Internal 5V Linear Regulator VCC and UVLO

The MAX8790 includes an internal low-dropout linear

regulator (VCC). When VIN is higher than 5.5V and

SHDN is high, this linear regulator generates a 5V supply

to power an internal PWM controller, control logic, and

MOSFET driver. This linear regulator can deliver at least

10mA of total additional load current. If VIN is less than or

equal to 5.5V, VCC and IN can be connected together and

powered from an external 5V supply. There is an internal

diode from VCC to IN, so VIN must be greater than VCC

(see Figure 2).

The MAX8790 includes UVLO protection. The controller is

disabled until VCC exceeds the UVLO threshold of 4.25V

(typ). Hysteresis on UVLO is approximately 20mV.

The VCC pin should be bypassed to GND with a 1μF or

greater ceramic capacitor.

Figure 3. Low-Input-Voltage Application Circuit

FB1

BRT

CCV

GND

VIN

2.8V TO 5.5V

EXTERNAL

5V SUPPLY

EXT

CPLL FB2

FB3

FB4

FB5

FB6

0.9µH D1

VCC

ISET

FSET

CIN

ENA

OSC

N.C.

0.1µF

1µF

511kΩ

1.2kΩ

59kΩ

1MΩ

30mΩ

SHDN

COUT

VOUT

UP TO 22V

MAX8790

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

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Startup

At startup, the MAX8790 checks each FB_ pin to deter-

mine if the respective current string is enabled. Each FB_

pin is internally pulled up with a 10μA current source. If

an FB_ pin is connected to GND, the corresponding string

current source is disabled. This feedback scan takes

approximately 264μs, after which the step-up converter

begins switching.

Shutdown

When the SHDN pin is less than 0.8V, the MAX8790

shuts down the internal LDO, the reference, current

sources, and all control circuitry. The resulting supply cur-

rent is less than 10μA. While the n-channel MOSFET is

turned off, the step-up regulator’s output is connected to

IN through the external inductor and rectifier diode.

Frequency Selection

A tri-level OSC input sets the internal oscillator frequency

for the step-up converter, as shown in Table 1. High-

frequency (1MHz) operation optimizes the regulator for

the smallest component size, at the expense of effi-

ciency due to increased switching losses. Low-frequency

(500kHz) operation offers the best overall efficiency, but

requires larger components and PCB area.

Overvoltage Protection

To protect the step-up converter when the load is open,

or the output voltage becomes excessive for any reason,

the MAX8790 features a dedicated overvoltage feedback

input (OV). The OV pin is connected to the center tap of a

resistive voltage-divider from the highvoltage output (see

Figure 1). When the MAX8790 is powered up, if none of

the LED strings on FB1–FB6 are connected to the step-

up converter output, the step-up converter regulates the

output voltage to VOUT = 1.23V(1 + R1 / R2). When VOV

exceeds 1.23V, a comparator turns off N1. The step-up

converter switch is reenabled after the output voltage

drops below the protection threshold.

LED Current Sources

Maintaining uniform LED brightness and dimming capa-

bility are critical for LCD backlight applications. The

MAX8790 is equipped with a bank of six matched current

sources. These specialized current sources are accurate

to within ±1.5% and can be switched on and off within

10μs, enabling PWM frequencies of up to 2kHz. All LED

full-scale currents are identical and are set through the

ISET pin (15mA < ILED < 25mA).

The minimum voltage drop across each current source is

approximately 450mV at 20mA. The low voltage drop helps

reduce dissipation while maintaining sufficient compliance

to control the LED current within the required tolerances.

The LED current sources can be disabled by grounding

the respective FB_ pin at startup. When the IC is powered

up, the controller scans settings for all FB_ pins. If an

FB_ pin is not grounded, an internal circuit pulls this pin

high, and the controller enables the corresponding cur-

rent source to regulate the string current. If the FB_ pin is

grounded, the controller disables the corresponding cur-

rent regulator. The current regulator cannot be disabled by

grounding any of the FB_ pins after the IC is powered up.

All FB_ pins in use are measured and the highest signal

(HVC) and the lowest signal (LVC) are extracted for two

feedback loops. HVC is used to identify excessive dis-

sipation across the current-source inputs. When HVC is

greater than VCC + 0.6V (typ) for greater than 65ms (see

the Current-Source Fault Protection section), a fault latch

is set and the MAX8790 is shut down. The LDO output is

not affected by the fault latch. LVC is fed into the step-up

converter’s error amplifier to regulate the step-up con-

verter’s output voltage.

Current-Source Fault Protection

The LED current sources are protected against string

open, short, and gross mismatch faults, using overvoltage

detection circuitry on each FB_ pin. If any of these three

fault conditions persists for a preset duration, the MAX8790

is latched off. The duration of the fault time depends on the

dimming mode and the duty cycle of the BRT input (DBRT).

In the DPWM mode, the timeout interval is:

tTIMEOUT_DPWM = 65ms/DBRT

In analog dimming mode, the fault time is fixed at 65ms

for DBRT greater than 12.5%. When DBRT is less than

12.5%, the timeout interval is:

tTIMEOUT_ANALOG = 8.125ms/DBRT

The fault latch can be cleared by cycling the power or tog-

gling the shutdown pin SHDN.

Open-Current Source Protection

The MAX8790 step-up converter output voltage is regu-

lated according to the minimum value of the enable FB_

voltages. If an individual LED string is open, the respec-

tive FB_ is pulled down to near ground. In this situation,

the step-up converter output voltage increases but is

Table 1. Frequency Selection

OSC SWITCHING FREQUENCY (kHz)

GND 500

Open 750

VCC 1000

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

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clamped to a level set with the OV feedback input. When

this elevated output voltage is applied to the undamaged

strings, excessive voltage drop develops across the FB_

pins. If the resulting HVC signal exceeds VCC + 0.6V for

greater than 65ms, the fault latch is triggered to protect

the circuit.

LED-Short and String Mismatch Protection

Normally, white LEDs have variations in forward-voltage

drop of 3.1V to 3.6V. The MAX8790 can tolerate slight

mismatches between LED strings. When the sum of the

LED forward voltages creates a mismatch in the strings

so the HVC signal exceeds VCC + 0.6V for greater than

65ms, the fault latch is triggered in much the same way as

the circuit responds to open string faults. Similar protec-

tion is activated when an LED is shorted.

The larger the number of series-connected LEDs (N), the

smaller the tolerable mismatch between LEDs:

CC SAT

Error V 0.6V V<+−

∑

VSAT ≈ 450mV and VCC = 5V

Error 5.150V<

∑

5.150V

Average Error Per LED

For N = 8, the average error per LED = 644mV.

For N = 10, the average error per LED = 510mV.

The larger the total mismatch, the larger the voltage drop

required across each current source to correct for the

error, and therefore the larger the dissipation within the

MAX8790.

Dimming Control

The MAX8790 features both analog and digital dimming

control. Analog dimming can provide potentially higher

converter efficiency because of low voltage drop across

each WLED when the current is low. Digital dimming

(DPWM) provides less WLED color distortion since the

WLED current is held at full scale when the WLED is on.

The MAX8790’s dimming control circuit consists of a PLL,

a digital comparator, and a DAC. The controller provides

100:1 dimming range through either analog or digital

control methods. Both methods translate the duty cycle

of the BRT input into a control signal for the LED current

sources. In analog dimming mode, the current-source

outputs are DC and the BRT duty cycle (12.5% < DBRT <

100%) modulates the amplitude of the currents. For DBRT

< 12.5%, the LED current is digitally modulated to reduce

the average LED current down to 1% of full scale. The

PLL detects the BRT frequency and phase, and adjusts

the current-source amplitude and duty cycle synchro-

nously (see Figure 4).

Figure 4. LED Current Control Using Analog Dimming Mode

BRT

ILEDMAX

ILED

ANALOG DIMMING MODE

D = 50% D = 30% D = 12.5% D = 6.25%

D = tON

tON

tBRT

MAX8790 Six-String White LED Driver with Active

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In digital dimming mode, the step-up controller and cur-

rent source are directly turned on and off by the PWM sig-

nal. The current pulse magnitude, or full-scale current, is

set by ISET and is independent of PWM duty factor. The

current-source outputs are PWM signals synchronized to

the BRT input signal (see Figure 5).

The full-scale current in both methods is specified by

resistance from the ISET pin to ground:

LEDmax ISET

20mA 100k

×Ω

The acceptable resistance range is 80kΩ < RISET <

133kΩ, which corresponds to full-scale LED current of

25mA > ILEDmax > 15mA. Connect ISET to VCC for a

default full-scale LED current of 20mA. When ENA is high,

the analog dimming is enabled, when ENA is low, digital

dimming is enabled.

When the current-source output is pulse-width modulated,

current-source turn-on is synchronized with the BRT signal.

Synchronization and low jitter in the PWM signals help

reduce flicker noise in the display. The current through

each FB_ pin is controlled only during the step-up con-

verter’s on-time. During the converter’s off-time, the cur-

rent sources are turned off. The output voltage does not

discharge and stays high. Each FB_ pin can withstand 28V,

which is the pin’s maximum rated voltage.

Table 2 summarizes the characteristics of both analog

and digital dimming methods.

A PLL translates the duty cycle of the BRT input into a

reference for the MAX8790’s current sources. A resistor

from the FSET pin to ground controls the PLL’s freerunning

frequency:

PLL FSET

f10 R 800pF

=××

The PLL’s loop filter bandwidth is set with a capacitor from

the CPLL pin to ground. This filter integrates the phase

difference between the BRT input signal and the PLL oscil-

lator. The filter bandwidth determines the PLL’s dynamic

response to frequency changes in the BRT signal. For

Figure 5. LED Current Control Using DPWM Dimming Mode

Table 2. Dimming Mode

MODE ENA PLL FREQUENCY CPLL DESCRIPTION

Analog + DPWM > 2.1V 250kΩ < RFSET < 754kΩ 0.1µF

Analog dimming from 100% to 12.5% brightness. From

12.5% to 1% brightness, DPWM dimming is employed.

BRT frequency range is 100Hz to 500Hz.

Direct DPWM < 0.8V VFSET > VCC - 0.4V, disables PLL OPEN

Direct dimming by BRT signal. BRT frequency can be

100Hz to 2kHz; 50µs minimum BRT on-time limits the

minimum brightness.

D = 30% D = 12.5% D = 6.25%

BRT

ILEDMAX

ILED

D = 50%

D = tON

tON

tBRT

DPWM DIMMING MODE

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

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│

most applications, a 0.1μF capacitor is adequate for oscil-

lator frequencies in the 166Hz < fPLL < 500Hz range. The

PLL frequency capture window is 0.6 x fPLL to fPLL.

The PLL is disabled in DPWM mode; consequently, the

BRT frequency is not limited by fPLL. The maximum BRT

frequency is determined by the minimum BRT ontime of

50μs and the minimum acceptable dimming factor. If a 1%

dimming factor is needed, the maximum BRT frequency is

200Hz. If a 10% dimming factor is acceptable, the maxi-

mum BRT frequency is 2kHz.

In analog dimming mode, load-current transients can

occur when the BRT duty cycle abruptly changes on the

fly. Large regulation transients induce a flash on the LED

load that is observable with the naked eye and should

therefore be avoided. Such annoying flashes can be

eliminated by dynamically changing the ENA pin setting.

When a capacitor is connected to the CPLL pin and the

ENA pin is grounded, the PLL continues to run but does

not affect the dimming. When fast PLL lockup transitions

are required, the ENA pin can be momentarily pulled to

ground; after the PLL is locked up, ENA can be pulled

high to reenable PLL in dimming control.

Table 3. Component List

CIRCUIT FIGURE 1 FIGURE 1 FIGURE 1 FIGURE 3

Switching

Frequency 1MHz 750kHz 500kHz 750kHz

White LED

3.2V (typ), 3.5V (max) at

20mA

Nichia NSSW008C

3.2V (typ), 3.5V (max) at

20mA

Nichia NSSW008C

3.2V (typ), 3.5V (max) at

20mA

Nichia NSSW008C

3.2V (typ), 3.5V (max) at

20mA

Nichia NSSW008C

Number of

White LEDs

6 series x 6 parallel,

20mA (max)

8 series x 6 parallel,

20mA (max)

10 series x 6 parallel, 25mA

(max)

6 series x 6 parallel,

20mA (max)

Input Voltage 4.5V to 5.5V, VCC = IN 7V to 21V 7V to 21V 2.8V to 5.5V, VCC = 5V

Inductor L1 2.2µH, 2.5A power inductor

Sumida CDRH5D16-2R2

4.7µH, 2.05A power

inductor

Sumida CDRH5D16-4R7

4.7µH, 3.6A power inductor

Sumida CDRH8D28-4R7

0.9µH, 4.7A power inductor

Sumida CDRH5D16-0R9

Input

Capacitors

10µF ±10%, 10V X5R

ceramic capacitor (1206)

Murata GRM31MR61A106K

10µF ±10%, 25V X5R

ceramic capacitor (1206)

Murata

GRM31CR61E106KA

10µF ±10%, 25V X5R

ceramic capacitor (1206)

Murata

GRM31CR61E106KA

10µF ±10%, 10V X5R

ceramic capacitor (1206)

Murata GRM31MR61A106K

COUT Output

Capacitor

2.2µF ±10%, 50V X7R

ceramic capacitor (1x)

Murata GRM31CR71H225K

2.2µF ±10%, 50V X7R

ceramic capacitor (1206)

(1x)

Murata GRM31CR71H225K

4.7µF ±10%, 50V X7R

ceramic capacitor (1210)

(1x)

Murata GRM32ER71H475K

2.2µF ±10%, 50V X7R

ceramic capacitor (1x)

Murata GRM31CR71H225K

MOSFET N1

30V, 3A n-channel MOSFET

(6-pin SC70)

Vishay Si1402DH

60V, 2.8A n-channel

MOSFET (6-pin TSOP)

Fairchild Semiconductor

FDC5612

Sanyo Semiconductor

CPH6424

60V, 6A n-channel

MOSFET (PowerPAK 1212-

Vishay Si7308DN

30V, 4.9A n-channel

MOSFET (6-pin TSOP)

Vishay Si3456BDV

Diode

Rectier D1

2A, 30V Schottky diode

Nihon EC21QS03L

2A, 40V Schottky diode

Toshiba CMS11

Nihon EC21QS04

3A, 60V Schottky diode

Nihon EC31QS06

3A, 30V Schottky diode

Nihon EC31QS03L

Sense

Resistor

50mΩ ±1%, 1/2W

IRC LRC-LRF-1206LF-01-

R050-F

56mΩ ±1%, 1/2W

IRC LRC-LRF-1206LF-01-

R056-F

40mΩ ±1%, 1/2W

IRC LRC-LRF-1206LF-01-

R040-F

30mΩ ±1%, 1/2W

IRC LRC-LRF-1206LF-01-

R030-F

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

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│

Thermal Shutdown

The MAX8790 includes a thermal-protection circuit. When

the local IC temperature exceeds +170°C (typ), the con-

troller and current sources shut down and do not restart

until the die temperature drops by 15°C.

Design Procedure

All MAX8790 designs should be prototyped and tested

prior to production. Table 3 provides a list of power com-

ponents for the typical applications circuit. Table 4 lists

component suppliers. External component value choice is

primarily dictated by the output voltage and the maximum

load current, as well as maximum and minimum input

voltages. Begin by selecting an inductor value. Once L is

known, choose the diode and capacitors.

Inductor Selection

The inductance, peak current rating, series resistance,

and physical size should all be considered when selecting

an inductor. These factors affect the converter’s operat-

ing mode, efficiency, maximum output load capability,

transient response time, output voltage ripple, and cost.

The maximum output current, input voltage, output volt-

age, and switching frequency determine the inductor

value. Very high inductance minimizes the current ripple,

and therefore reduces the peak current, which decreases

core losses in the inductor and I2R losses in the entire

power path. However, large inductor values also require

more energy storage and more turns of wire, which

increases physical size and I2R copper losses in the

inductor. Low inductor values decrease the physical size,

but increase the current ripple and peak current. Finding

the best inductor involves the compromises among circuit

efficiency, inductor size, and cost.

When choosing an inductor, the first step is to determine

the operating mode: continuous conduction mode (CCM)

or discontinuous conduction mode (DCM). The MAX8790

has a fixed internal slope compensation, which requires a

minimum inductor value. When CCM mode is chosen, the

ripple current and the peak current of the inductor can be

minimized. If a small-size inductor is required, DCM mode

can be chosen. In DCM mode, the inductor value and size

can be minimized but the inductor ripple current and peak

current are higher than those in CCM. The controller can

be stable, independent of the internal slope compensation

mode, but there is a maximum inductor value requirement

to ensure the DCM operating mode.

The equations used here include a constant LIR, which is

the ratio of the inductor peak-to-peak ripple current to the

average DC inductor current at the full-load current. The

controller operates in DCM mode when LIR is higher than

2.0, and it switches to CCM mode when LIR is lower than

2.0. The best trade-off between inductor size and con-

verter efficiency for step-up regulators generally has an

LIR between 0.3 and 0.5. However, depending on the AC

characteristics of the inductor core material and ratio of

inductor resistance to other power-path resistances, the

best LIR can shift up or down. If the inductor resistance

is relatively high, more ripple can be accepted to reduce

the number of required turns and increase the wire diam-

eter. If the inductor resistance is relatively low, increasing

inductance to lower the peak current can reduce losses

throughout the power path. If extremely thin high-resis-

tance inductors are used, as is common for LCD panel

applications, LIR higher than 2.0 can be chosen for DCM

operating mode.

Once a physical inductor is chosen, higher and lower

values of the inductor should be evaluated for efficiency

improvements in typical operating regions. The detail

design procedure can be described as follows:

Calculate the approximate inductor value using the

typical input voltage (VIN), the maximum output cur-

rent (IOUT(MAX)), the expected efficiency (ηTYP) taken

from an appropriate curve in the Typical Operating

Characteristics, and an estimate of LIR based on the

above discussion:

IN_MIN OUT IN_MIN TYP

OUT OUT(MAX) OSC

V VV

LV I f LIR



−

 η





= 



×



Table 4. Component Suppliers

SUPPLIER PHONE WEBSITE

Murata 770-436-1300 www.murata.com

Nichia 248-352-6575 www.nichia.com

Sumida 847-545-6700 www.sumida.com

Toshiba 949-455-2000 www.toshiba.com/taec

Vishay 203-268-6261 www.vishay.com

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

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│

The MAX8790 has a minimum inductor value limitation

for stable operation in CCM mode at low input voltage

because of the internal fixed slope compensation. The

minimum inductor value for stability is calculated by the

following equation:

( )

OUT(MAX) DIODE IN(MIN) S

CCM(MIN) OSC(MIN)

V V 2V R

5 1m V f

+ −× ×

=×

where 51mV is a scale factor based on slope compensa-

tion, and RS is the current-sense resistor. To determine

the minimum inductor value, the RS can be temporarily

calculated using the following equation:

S _TMP IN(DC, MAX)

100mV

R1.2 I

=×

where 100mV is the current-limit sense voltage.

The minimum inductor value should be recalculated after

the RS is determined (see the Sense-Resistor Selection

section).

Choose an available inductor value from an appropriate

inductor family. Calculate the maximum DC input current

at the minimum input voltage VIN(MIN), using conserva-

tion of energy and the expected efficiency at that operat-

ing point (ηMIN) taken from an appropriate curve in the

Typical Operating Characteristics:

MIN

OUT(MAX) OUT

IN(DC, MAX) IN(MIN)

=×η

Calculate the ripple current at that operating point and the

peak current required for the inductor:

( )

IN(MIN) OUT(MAX) IN(MIN)

RIPPLE OUT(MAX) OSC

VV V

ILV f

×−

=××

RIPPLE

PEAK IN(DC, MAX)

II 2

= +

When DCM operating mode is chosen to minimize the

inductor value, the calculations are different from that in

the above CCM mode. The maximum inductor value for

DCM mode is calculated by the following equation:

IN(MIN)

DCM(MAX) OUT(MAX) DIODE

IN(MIN)

OSC(MAX) OUT(MAX) OUT(MAX)

2f V I





=−×





×η

×× ×

The peak inductor current in DCM mode is calculated

using the following equation:

( )

OUT(max) OUT(MAX) OUT(MAX) DIODE IN(MIN)

PEAK

OSC(MIN) OUT(MAX) DIODE

I 2V V V V

ILf V V

×× × + −

=× ×η× +

The inductor’s saturation current rating should exceed

IPEAK and the inductor’s DC current rating should exceed

IIN(DC,MAX). For good efficiency, choose an inductor with

less than 0.1Ω series resistance.

Considering the typical operating circuit, the maximum

load current (IOUT(MAX)) is 120mA with a 28.72V output

and a minimal input voltage of 7V. Choosing a DCM

operating mode and estimating efficiency of 90% at this

operating point:

DCM(MAX)

28.72V 0.4V

(7V) 0.9 5.8µH

2 0.825MHz 28.72V 120mA



=−×





×=

× ××

An inductance less than LDCM(MAX) is required, so a

4.7μH inductor is chosen. The peak inductor current at

minimum input voltage is calculated as follows:

( )

PEAK

120mA 2 28.72V 28.72V 0.4V 7V

I 1.35A

4.7 H 0.675MHz 0.9 28.72V 0.4V

×× × + −

= =

µ× × × +

Sense-Resistor Selection

The detected signal is fed into the step-up converter con-

trol compensation loop through the CS pin.

The MAX8790’s current-mode step-up converter senses

the switch current from CS to GND with an external resis-

tor, RS. The current-limit sense voltage is a fixed 100mV.

The required resistance is calculated based upon the

peak inductor current at the end of the switch on-time:

( )

CS_EC MAX

SPEAK

V 25.6mV 0.75 D

+ ×−

where 25.6mV is a scale factor from slope compensa-

tion, VCS_EC is the current-sense voltage listed in the

Electrical Characteristics table (85mV), and the DMAX is

the maximum duty cycle at minimum input voltage and

maximum output voltage. In DCM operating mode, it is

calculated by the following equation:

LIM OSC

MAX IN(MIN)

LI f

××

For the typical operating circuit as Figure 1:

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

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│

MAX

4.7µH 1.35A 0.75MHz

D 0.68

××

= =

( )

85mV 25.6mV 0.75 0.68

R 64m

1.35A

+ ×−

<=Ω

Again, RS is calculated as a maximum, so a 56mΩ cur-

rent-sense resistor is chosen.

Output Capacitor Selection

The total output voltage ripple has two components: the

capacitive ripple caused by the charging and discharging

on the output capacitor, and the ohmic ripple due to the

capacitor’s equivalent series resistance (ESR):

RIPPLE RIPPLE(C) RIPPLE(ESR)

OUT(MAX) OUT(MAX) IN(MIN)

RIPPLE(C) OUT OUT(MAX) OSC

VV V

IV V

VC Vf

= +



−



≈





and:

RIPPLE(ESR) PEAK ESR(COUT)

V IR≈

where IPEAK is the peak inductor current (see the Inductor

Selection section).

The output voltage-ripple voltage should be low enough

for the FB_ current-source regulation. The ripple voltage

should be less than 200mVP-P. For ceramic capaci-

tors, the output-voltage ripple is typically dominated by

VRIPPLE(C). The voltage rating and temperature charac-

teristics of the output capacitor must also be considered.

External MOSFET Selection

The MAX8790’s step-up converter uses an external

MOSFET to enable applications with scalable output volt-

age and output power. The boost switching architecture is

simple and ensures that the controller is never exposed

to high voltage. Only the external MOSFET, diode, and

inductor are exposed to the output voltage plus one

Schottky diode forward voltage:

BV F_LED F_SCHOTTKY FB_

V NV V V=×+ +

The MOSFET’s breakdown ratings should be higher than

VBV with sufficient margin to ensure long-term reliability. A

conservative rule of thumb, a minimum 30% margin would

be recommended for MOSFET breakdown voltage. The

external MOSFET should have a current rating of no less

than the IPEAK derived from the Inductor Selection sec-

tion. To improve efficiency, choose a MOSFET with low

RDS(ON). The MAX8790’s gate-drive linear regulator can

provide 10mA. Select the external MOSFET with a total

gate charge so the average current to drive the MOSFET

at maximum switching frequency is less than 10mA:

g(MAX) OSC

Q f 10mA×<

For example, the Si3458DV is specified with 16nC of

max total gate charge at Vg = 10V. For 5V of gate drive,

the required gate charge is 8nC, which equates to 8mA

at 1MHz.

The MOSFET conduction loss or resistive loss is caused

by the MOSFET’s on-resistance (RDS(ON)). This power

loss can be estimated as:

DS(ON) OSC PEAK

RES(MAX) IN(MIN)

R Lf I

PD 3V

×× ×

=×

For the above Si3458DV, the estimated conduction loss is:

RES(MAX)

0.1 4.7µH 750kHz 1.35A

PD 0.04W

3 7V

Ω× × ×

= =

The approximate maximum switching loss can be calcu-

lated as:

turn off PEAK OUT OSC

SW(MAX)

t I Vf

PD 2

−×××

For the above Si3458DV, the approximate switching loss is:

SW(MAX)

10ns 1.35A 28.72V 750kHz

PD 0.145W

×× ×

= =

Rectier Diode Selection

The MAX8790’s high switching frequency demands a

high-speed rectifier. Schottky diodes are recommended

for most applications because of their fast recovery time

and low forward voltage. The diode should be rated to

handle the output voltage and the peak switch current.

Make sure that the diode’s peak current rating is at least

IPEAK calculated in the Inductor Selection section and

that its breakdown voltage exceeds the output voltage.

Setting the Overvoltage Protection Limit

The OV protection circuit should ensure the circuit safe

operation; therefore, the controller should limit the out-

put voltage within the ratings of all MOSFET, diode, and

output capacitor components, while providing sufficient

output voltage for LED current regulation. The OV pin is

connected to the center tap of a resistive voltagedivider

(R1 and R2 in Figure 1) from the high-voltage output.

When the controller detects the OV pin voltage reaching

the threshold VOV_TH, typically 1.23V, OV protection is

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activated. Hence, the step-up converter output overvolt-

age protection point is:

OUT(OVP) OV_TH

V V (1 )

= ×+

In Figure 1, the output OVP voltage is set to:

OUT(OVP)

V 1.23V (1 ) 34.1V

37.4k

Ω

= ×+ =

Ω

Input Capacitor Selection

The input capacitor (CIN) filters the current peaks drawn

from the input supply and reduces noise injection into

the IC. A 10μF ceramic capacitor is used in the typical

operating circuit (Figure 1) because of the high source

impedance seen in typical lab setups. Actual applications

usually have much lower source impe-dance since the

step-up regulator often runs directly from the output of

another regulated supply. In some applications, CIN can

be reduced below the values used in the typical operating

circuit. Ensure a low noise supply at IN by using adequate

CIN. Alternatively, greater voltage variation can be toler-

ated on CIN if IN is decoupled from CIN using an RC

lowpass filter.

Select CIN’s RMS ripple current rating to ensure that its

thermal rise is less than approximately 10°C:

RMS

I23

=×

LED Selection and Bias

The series/parallel configuration of the LED load and the

full-scale bias current have a significant effect on regula-

tor performance. LED characteristics vary significantly

from manufacturer to manufacturer. Consult the respec-

tive LED data sheets to determine the range of output

voltages for a given brightness and LED current. In

general, brightness increases as a function of bias cur-

rent. This suggests that the number of LEDs could be

decreased if higher bias current is chosen; however, high

current increases LED temperature and reduces operat-

ing life. Improvements in LED technology are resulting in

devices with lower forward voltage while increasing the

bias current and light output.

LED manufacturers specify LED color at a given LED

current. With lower LED current, the color of the emitted

light tends to shift toward the blue range of the spectrum.

A blue bias is often acceptable for business applications

but not for high-image-quality applications such as DVD

players. Direct DPWM dimming is a viable solution for

reducing power dissipation while maintaining LED color

integrity. Careful attention should be paid to switching

noise to avoid other display quality problems.

Using fewer LEDs in a string improves step-up converter

efficiency, and lowers breakdown voltage requirements of

the external MOSFET and diode. The minimum number

of LEDs in series should always be greater than the maxi-

mum input voltage. If the diode voltage drop is lower than

the maximum input voltage, the voltage drop across the

current-sense inputs (FB_) increases and causes excess

heating in the IC. Between 8 and 12 LEDs in series is

ideal for input voltages up to 20V.

Applications Information

LED VFB_ Variation

The MAX8790 has accurate (±1.5%) matching for each

current source. However, the forward voltage of each

white LED can vary up to ±5% from part to part. The accu-

mulated voltage difference in each string equates to addi-

tional power loss within the IC. For the best efficiency, the

voltage difference between strings should be minimized.

The difference between lowest voltage string and highest

voltage string should be less than 4.5V. Otherwise, the

internal LED short-circuit protection shuts the part off.

Choosing the Appropriate Dimming Mode

Analog dimming mode allows lower peak LED current

and results in higher converter efficiency and lower noise

compared to direct DPWM mode. Unfortunately, the LED

color spectrum can shift as a function of DC current so

DPWM mode is often used to achieve more consistent

display characteristics. (See the LED manufacturer’s data

sheet to determine the extent of the color shift.) When the

MAX8790 is configured with an FSET resistor and CPLL

capacitor, the ENA signal can toggle between modes on

the fly. Care should be exercised when switching between

modes to prevent the current from becoming unstable

during the PLL lock-in time. To avoid such problems, force

the controller into DPWM mode between transitions.

LCD Panel Capacitance

Some LCD panels include a capacitor in parallel with LED

string to improve ESD immunity. Because of the 10μA pul-

lup current source in each FB_ input for string detection,

the MAX8790 can start up with less than 470pF capaci-

tance on each FB_ pin. If the string capacitance CLED

is greater than 470pF, a bank of pullup resistors to VIN

should be added to prevent startup problems (see Figure

6). A delay of 3 x 1MΩ x CLED should be added after VIN

was settled before enabling the MAX8790 to ensure the

FB_ voltage exceeds the 3V internal threshold. A similar

delay should be added after the part is shut down to

ensure proper restart.

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PCB Layout Guidelines

Careful PCB layout is important for proper operation. Use

the following guidelines for good PCB layout:

1) Minimize the area of the high current-switching loop

of the rectifier diode, external MOSFET, sense resis-

tor, and output capacitor to avoid excessive switching

noise. Use wide and short traces for the gate-drive

loop from the EXT pin, to the MOSFET gate, and

through the current-sense resistor, then returning to

the IC GND pin.

2) Connect high-current input and output components

with short and wide connections. The high-current

input loop goes from the positive terminal of the input

capacitor to the inductor, to the external MOSFET,

then to the current-sense resistor, and to the input

capacitor’s negative terminal. The high-current output

loop is from the positive terminal of the input capacitor

to the inductor, to the rectifier diode, to the positive ter-

minal of the output capacitors, reconnecting between

the output capacitor and input capacitor ground termi-

nals. Avoid using vias in the high-current paths. If vias

are unavoidable, use multiple vias in parallel to reduce

resistance and inductance.

3) Create a ground island (PGND) consisting of the input

and output capacitor ground and negative terminal of

the current-sense resistor. Connect all these together

with short, wide traces or a small ground plane.

Maximizing the width of the power ground traces

improves efficiency and reduces output-voltage ripple

and noise spikes. Create an analog ground island

(AGND) consisting of the overvoltage detection-divider

ground connection, the ISET and FSET resistor con-

nections, CCV and CPLL capacitor connections, and

the device’s exposed backside pad. Connect the

AGND and PGND islands by connecting the GND pins

directly to the exposed backside pad. Make no other

connections between these separate ground planes.

4) Place the overvoltage detection-divider resistors as

close to the OV pin as possible. The divider’s center

trace should be kept short. Placing the resistors far

away causes the sensing trace to become antennas

that can pick up switching noise. Avoid running the

sensing traces near LX.

5) Place the IN pin bypass capacitor as close to the

device as possible. The ground connection of the IN

bypass capacitor should be connected directly to GND

pins with a wide trace.

6) Minimize the size of the LX node while keeping it wide

and short. Keep the LX node away from the feedback

node and ground. If possible, avoid running the LX

node from one side of the PCB to the other. Use DC

traces as shields, if necessary.

7) Refer to the MAX8790 evaluation kit for an example of

proper board layout.

Figure 6. Startup Circuit with Large Capacitors on LED Strings

FB1

CLED

1MΩ

SHDN

VIN

TO VIN

EXT

FB2

FB3

FB4

FB5

FB6

L1 D1

COUT

VOUT

MAX8790

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Current Balancing for LCD Panel Applications

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PACKAGE

TYPE

PACKAGE

CODE

OUTLINE

NO.

LAND

PATTERN NO.

20 TQFN T2044+3 21-0139 90-0037

ENA

FB1

OSC

FB6

VCC

1 2

ISET

4 5

15 14 12 11

FSET

CPLL

FB4

GND

FB3

FB2

BRT EXT

CCV

16 10 FB5

4mm x 4mm THIN QFN

TOP VIEW

SHDN

MAX8790ETP+

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

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Chip Information

TRANSISTOR COUNT: 12,042

PROCESS: BiCMOS

Pin Conguration

Package Information

For the latest package outline information and land patterns

(footprints), go to www.maximintegrated.com/packages. Note

that a “+”, “#”, or “-” in the package code indicates RoHS status

only. Package drawings may show a different suffix character, but

the drawing pertains to the package regardless of RoHS status.

REVISION

NUMBER

REVISION

DATE DESCRIPTION PAGES

CHANGED

0 5/14 No /V OPNs; removed automotive reference from Applications section 1

Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses

are implied. Maxim Integrated reserves the right to change the circuitry and specications without notice at any time. The parametric values (min and max limits)

shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.

Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

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Revision History

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