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MAX8790A

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

Maxim Integrated 

MAX8790A

Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

Detailed Description

The MAX8790A is a high-efficiency driver for arrays of

white LEDs. It contains a fixed-frequency, currentmode,

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 MAX8790A 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 MAX8790A supports both analog and digital

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

range. The MAX8790A’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 ana-

log dimming mode, an internal PLL, digital comparator,

and DAC circuit translate the PWM signal into an analog

signal that linearly controls the LED current, down to a

PWM duty factor of 12.5%.

The MAX8790A 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.

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 MAX8790A includes a 5V linear regulator that pro-

vides 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 internal LDO is disabled when SHDN is low.

VIN

7V TO 21V

CIN

0.1µF

L1

4.7µH

SHDN

IN

EXT

VCC

1µF

ENA

ISET

CS

BRT

FSET

MAX8790A

GND

511kΩ

N.C.

OSC

OV

1.2kΩ

CCV

0.1µF

FB1

CPLL

FB2

0.1µF

FB3

FB4

EP

FB5

FB6

Figure 1. Typical Operating Circuit

D1

COUT

N1

RS

56mΩ

R1

1MΩ

R2

37.4kΩ

VOUT

UP TO 35V

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