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 sig-

nal 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 con-

troller 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 pro-

vide consistent operation and soft-start capability. A ther-

mal-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 ope-

rating 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

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

________________________________________________________________

Maxim Integrated Products

1

FB1

N1

BRT

CCV

GND

IN

EXT

CS

OV

FB2

FB3

FB4

FB5

FB6

L1

D1

ISET

FSET

CPLL

EP

N.C.

OSC

N.C.

ENA

R2

R1

Rs

SHDN

0.1

μF

MAX8790

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+

-40°C to +85°C

20 Thin QFN

(4mm x 4mm)

T2044-3

Pin Configuration appears at end of data sheet.