MAX8790A Datasheet, PDF (1/23 Pages) Maxim Integrated Products – Six-String White LED Driver with Active Current Balancing for LCD Panel Applications

19-0989; Rev 0; 10/07
EVAALVUAAILTAIOBNLEKIT
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
General Description
The MAX8790A 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 MAX8790A has a wide input-volt-
age range from 4.5V to 26V, and provides a fixed 20mA
or adjustable 15mA to 27mA full-scale LED current.
The MAX8790A 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 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.
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 MAX8790A features selectable switch-
ing frequency (500kHz, 750kHz, or 1MHz), which
allows trade-offs between external component size and
ope-rating efficiency.
The MAX8790A 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
Ordering Information
PART
TEMP RANGE PIN-PACKAGE ^PKG
CODE
MAX8790AETP+ -40°C to +85°C ^{20 Thin QFN}
T2044-3
(4mm x 4mm)
+Denotes a lead-free package.
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
27mA, 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
Simplified Operating Circuit
VIN
CIN
0.1μF
L1
D1 VOUT
SHDN ^IN EXT
VCC
FSET
CS
ISET
MAX8790A
GND
BRT
N.C. OSC
OV
N.C. CPLL
CCV
FB1
FB2
ENA
FB3
FB4
EP
FB5
FB6
N1
Rs
R1
R2
Pin Configuration appears at end of data sheet.
________________________________________________________________ Maxim Integrated Products 1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.