MAX8790A(2007) Ver la hoja de datos (PDF) - Maxim Integrated

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MAX8790A
(Rev.:2007)

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

Maxim Integrated 

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, current-

mode, PWM step-up controller, 5V linear regulator, dim-

ming control circuit, and six regulated current sources

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

boosts the output voltage to provide sufficient head-

room 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

operating efficiency. The control architecture automati-

cally 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 con-

sists 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

controller 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) protec-

tion threshold, the controller shuts down and latches off

after an internal timer expires. The controller features

cycle-by-cycle current limit to provide consistent opera-

tion 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

D1

COUT

VOUT

UP TO 35V

SHDN

IN

EXT

VCC

1μF

ENA

CS

ISET

BRT

FSET

GND

511kΩ

N.C.

OSC

OV

1.2kΩ

CCV

MAX8790A

0.1μF

FB1

CPLL

FB2

0.1μF

FB3

FB4

EP

FB5

FB6

Figure 1. Typical Operating Circuit

N1

RS

56mΩ

R1

1MΩ

R2

37.4kΩ

10 ______________________________________________________________________________________

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