ATION
EVALU
AVAIL
19-0658; Rev 0; 11/06
KIT
ABLE
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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
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
Simplified Operating Circuit
power loss. The MAX8790 features selectable switching
V IN
L1
D1
V OUT
frequency (500kHz, 750kHz, or 1MHz), which allows
trade-offs between external component size and ope-
rating efficiency.
C IN
0.1 μ F
The MAX8790 is available in a thermally enhanced,
SHDN
IN
EXT
N1
lead-free, 20-pin, 4mm x 4mm, Thin QFN package.
Applications
Notebook, Subnotebook, and Tablet Computer
Displays
V CC
FSET
ISET
MAX8790
BRT
CS
GND
Rs
R1
Automotive Systems
Handy Terminals
N.C.
N.C.
OSC
CPLL
OV
R2
CCV
Ordering Information
FB1
FB2
PART
MAX8790ETP+
TEMP RANGE PIN-PACKAGE
20 Thin QFN
-40°C to +85°C
(4mm x 4mm)
PKG
CODE
T2044-3
ENA
EP
FB3
FB4
FB5
FB6
+Denotes a lead-free package.
Pin Configuration appears at end of data sheet.
________________________________________________________________ Maxim Integrated Products
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.
1
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