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
Features
oDrives Six Parallel Strings with Multiple Series-
Connected LEDs per String
o±1.5% Current Regulation Accuracy Between
Strings
oLow 450mV Feedback Voltage at Full Current
Improves Efficiency
oStep-Up Controller Regulates the Output Just
Above the Highest LED String Voltage
oFull-Scale LED Current Adjustable from 15mA to
27mA, or Preset 20mA
oWide 100:1 Dimming Range
oProgrammable Dimming Control: Direct DPWM or
Analog Dimming
oBuilt-In PLL for Synchronized Dimming Control
oOpen and Short LED Protections
oOutput Overvoltage Protection
oWide Input Voltage Range from 4.5V to 26V
oExternal MOSFET Allows a Large Number of LEDs
per String
o500kHz/750kHz/1MHz Switching Frequency
oSmall, 20-Pin, 4mm x 4mm Thin QFN Package
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
________________________________________________________________
Maxim Integrated Products
1
FB1
N1
BRT
CCV
GND
IN
VIN
EXT
CS
OV
FB2
FB3
FB4
FB5
FB6
L1 D1
VCC
ISET
CIN
FSET
CPLL
EP
N.C.
OSC
N.C.
ENA
R2
R1
Rs
SHDN
0.1μF
MAX8790A
VOUT
Simplified Operating Circuit
Ordering Information
19-0989; Rev 0; 10/07
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.
+
Denotes a lead-free package.
EVALUATION KIT
AVAILABLE
PART
TEMP RANGE
PIN-PACKAGE
PKG
CODE
MAX8790AETP+
-40°C to +85°C
20 Thin QFN
(4mm x 4mm)
T2044-3
Pin Configuration appears at end of data sheet.
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1. VIN = 12V, VSHDN = VIN, CCV = 0.1µF, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
IN, SHDN, to GND .................................................-0.3V to +28V
FB_ to GND ............................................................-0.3V to +28V
VCC, BRT, ENA, OSC, OV to GND ...........................-0.3V to +6V
ISET, CCV, CS, FSET, CPLL, EXT to GND .-0.3V to (VCC + 0.3V)
Continuous Power Dissipation (TA= +70°C)
20-Pin Thin QFN (derate 16.9mW/°C above +70°C) ...1349mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range .............................-60°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
2 _______________________________________________________________________________________
PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN = VCC 4.5 5.5
IN Input Voltage Range VCC = bypassed to GND through 1µF capacitor 5.5 26.0
V
VIN = 26V 1 2
V
SHDN = high,
BRT = GND VIN = VCC = 5V 1 2
mA
IN Quiescent Current
SHDN = GND 10 µA
VCC Output voltage V
SHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA 4.7 5.0 5.3 V
VCC Short-Circuit Current 15 56 130 mA
VCC UVLO Threshold Rising edge, hysteresis = 20mV 4.00 4.25 4.45 V
STEP-UP CONVERTER
EXT High Level 10mA from EXT to GND VCC -
0.1 VCC V
EXT Low Level -10mA from EXT to VCC 0 0.1 V
EXT On-Resistance EXT high or low 2 5 Ω
EXT Sink/Source Current EXT forced to 2V 1 A
OSC High-Level Threshold VCC -
0.4 V
OSC Midlevel Threshold 1.5
VCC -
2.0 V
OSC Low-Level Threshold 0.4 V
VOSC = VCC 0.9 1.0 1.1 MHz
VOSC = open 675 750 825
Operating Frequency
VOSC = GND 450 500 550
kHz
PWM mode 10
Minimum Duty Cycle Pulse skipping, no load 0
%
Maximum Duty Cycle 94 95 %
CS Trip Voltage Duty cycle = 75% 85 100 115 mV
CONTROL INPUT
SHDN Logic-Input High Level 2.1 V
SHDN Logic-Input Low Level 0.8 V
BRT, ENA Logic-Input High Level 2.1 V
BRT, ENA Logic-Input Low Level 0.8 V
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
_______________________________________________________________________________________ 3
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VIN = 12V, VSHDN = VIN, CCV = 0.1µF, TA= 0°C to +85°C, unless otherwise noted. Typical values are at TA= +25°C.)
PARAMETER CONDITIONS MIN TYP MAX UNITS
INPUT LEAKAGE
SHDN Leakage Current SHDN = 26V +42 μA
CS Leakage Current VCS = GND +40 +50 μA
OSC Leakage Current -3 +3 μA
BRT, ENA Leakage Current -1 +1 μA
FSET, ISET Leakage Current FSET = ISET = VCC -1 +1 μA
OV Leakage Current -0.1 +0.1 μA
LED CURRENT
ISET = VCC, BRT = 100% 19.40 20.00 20.60
RISET = 80k to GND, BRT = 100% 24.25 25.00 25.75
Full-Scale FB_ Output Current
RISET = 133k to GND, BRT = 100% 14.40 15.00 15.60
mA
ISET High-Level Threshold Default setting for 20mA full-scale LED current VCC -
0.4 V
ISET Voltage 1.12 1.19 1.26 V
20% Output Current ISET = VCC, BRT = 20% 3.84 4.00 4.16 mA
ISET = VCC, BRT = 100% -1.5 +1.5 %
Current Regulation Between
Strings ISET = VCC, BRT = 20% -2.0 +2.0 %
RISET = 80k to GND, BRT = 100% 300 500 800
ISET = VCC, BRT = 100% 270 450 720
Minimum FB_ Regulation
Voltage
ISET = VCC, 12.5% 150 275 500
mV
Maximum FB_ Ripple ISET = VCC ,C
OUT = 1μF, OSC = VCC (Note 1) 120 200 mVP-P
FB_ On-Resistance VFB_ = 50mV 13 20
SHDN = GND, VFB_ = 26V 1
FB_ Leakage Current SHDN = VIN, BRT = GND, VFB_ = 15V 10 28 μA
BRT Input Frequency 100 500 Hz
Minimum BRT Duty Cycle PLL active 12.5 %
FAULT PROTECTION
OV Threshold Voltage 1.16 1.23 1.30 V
FB_ Overvoltage Threshold VCC +
0.20
VCC +
0.6
VCC +
1.45 V
FAULT Shutdown Timer VFB_ > 5.6V (typ) 50 65 80 ms
Thermal-Shutdown Threshold (Note 1) 170 °C
PHASE-LOCKED LOOP
FSET High-Level Threshold PLL disabled VCC -
0.4 V
RFSET = 500k 150 200 250
BRT Frequency Capture Range RFSET = 250k 300 400 500 Hz
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
4 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1. VIN = 12V, VSHDN = VIN, CCV = 0.1µF, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER CONDITIONS MIN TYP MAX UNITS
VIN = VCC 4.5 5.5
IN Input Voltage Range VCC bypassed to GND through 1μF cap 5.5 26.0 V
VIN = 26V 2
V = high
BRT = GND VIN = VCC = 5V 2 mA
IN Quiescent Current
= GND 10 μA
VCC Output Voltage V = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA 4.7 5.3 V
VCC Short-Circuit Current 12 130 mA
VCC UVLO Threshold Rising edge, hysteresis = 20mV 4.00 4.45 V
STEP-UP CONVERTER
EXT High Level 10mA from EXT to GND VCC -
0.1 V
EXT Low Level -10mA from EXT to VCC 0.1 V
EXT On-Resistance EXT high or low 5
OSC High-Level Threshold VCC -
0.4 V
OSC Midlevel Threshold 1.5 VCC -
2.0 V
OSC Low-Level Threshold 0.4 V
VOSC = VCC 0.9 1.1 MHz
VOSC = open 675 825
Operating Frequency
VOSC = GND 450 550 kHz
Maximum Duty Cycle 94 %
CS Trip Voltage Duty cycle = 75% 85 115 mV
CONTROL INPUT
Logic-Input High Level 2.1 V
Logic-Input Low Level 0.8 V
BRT, ENA Logic-Input High 2.1 V
BRT, ENA Logic-Input Low Level 0.8 V
INPUT LEAKAGE
Leakage Current = 26V +42 μA
CS Leakage Current VCS = GND +50 μA
OSC Leakage Current -3 +3 μA
BRT, ENA Leakage Current -1 +1 μA
FSET, ISET Leakage Current FSET = ISET = VCC -1 +1 μA
OV Leakage Current -0.1 +0.1 μA
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
_______________________________________________________________________________________ 5
Note 1: Specifications are guaranteed by design, not production tested.
Note 2: Specifications to -40°C are guaranteed by design, not production tested.
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VIN = 12V, VSHDN = VIN, CCV = 0.1µF, TA= -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETER CONDITIONS MIN TYP MAX UNITS
LED CURRENT
ISET = VCC, BRT = 100% 19.2 20.8
RISET = 80kΩ to GND, BRT = 100% 24.0 26.0Full-Scale FB_ Output Current
RISET = 133kΩ to GND, BRT = 100% 14.4 15.6
mA
ISET High-Level Threshold Default setting for 20mA full-scale LED current VCC -
0.4 V
ISET Voltage 1.12 1.26 V
20% Output Current ISET = VCC, BRT = 20% 3.8 4.2 mA
ISET = VCC, BRT = 100% -2 +2
Current Regulation Between
Strings ISET = VCC, BRT = 20% -3 +3
%
RISET = 80kΩ to GND, BRT = 100% 280 840
ISET= VCC, BRT = 100% 250 760Minimum FB_ Regulation Voltage
ISET = VCC, BRT = 12.5% 140 530
mV
Maximum FB_ Ripple ISET= VCC, COUT = 1µF, OSC = VCC (Note 1) 200 mVP-P
FB_ On-Resistance VFB_ = 50mV 20 Ω
SHDN = GND, VFB_ = 26V 1
FB_ Leakage Current SHDN = VIN, BRT = GND, VFB_ = 15V 28
µA
BRT Input Frequency 100 500 Hz
FAULT PROTECTION
OV Threshold Voltage 1.16 1.30 V
FB_ Overvoltage Threshold
VCC +
0.2
VCC +
1.45 V
FAULT Shutdown Timer VFB_ > 5.6V (typ) 50 80 ms
PHASE-LOCKED LOOP
FSET High-Level Threshold PLL disabled VCC -
0.4 V
RFSET = 500kΩ 150 250 Hz
BRT Frequency Capture Range RFSET = 250kΩ 300 500 Hz
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
6 _______________________________________________________________________________________
Typical Operating Characteristics
(Circuit configuration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA= +25°C, unless otherwise noted.)
BOOST CONVERTER EFFICIENCY
vs. INPUT VOLTAGE (BRT = 100%)
MAX8790A toc01
BOOST CONVERTER EFFICIENCY (%)
INPUT VOLTAGE (V)
94
93
92
91
90
89
88
87
86 71217
500kHz
750kHz
1MHz
NORMALIZED POWER vs. TOTAL LED CURRENT
(ANALOG AND DPWM DIMMING)
MAX8790A toc02
NORMALIZED POWER
TOTAL LED CURRENT (mA)
1.2
1.0
0.8
0.6
0.4
0.2
01 10 100 1000
NORMALIZED TO VIN = 20V, AND ILED = 20mA
VIN = 7V
TOTAL LED
POWER, ANALOG
TOTAL INPUT
POWER, DPWM
TOTAL INPUT
POWER, ANALOG
TOTAL LED
POWER, DPWM
LED CURRENT vs. BRT DUTY CYCLE
(BRT AT 200Hz)
MAX8790A toc03
LED CURRENT (mA)
BRT DUTY CYCLE (%)
25
20
15
10
5
0110100
IDENTICAL FOR DPWM DIMMING
AND ANALOG DIMMING
LED CURRENT
vs. AMBIENT TEMPERATURE (BRT = 100%)
MAX8790A toc04
LED CURRENT (mA)
AMBIENT TEMPERATURE (°)
21.0
20.8
20.6
20.4
20.2
20.0
19.8
19.6
19.4
19.2
19.0 020 6040 80
LED CURRENT REGULATION
vs. INPUT VOLTAGE
MAX8790A toc05
LED CURRENT REGULATION (%)
INPUT VOLTAGE (V)
0.05
0.04
0.03
0.02
0.01
0
-0.01
-0.02
-0.03
-0.04
-0.05
71217
ANALOG DIMMING
BRT = 10%
DPWM DIMMING
BRT = 100%
DPWM DIMMING
BRT = 10%
FB_ VOLTAGE vs. LED CURRENT
(ANALOG DIMMING)
MAX8790A toc06
FB_ REGULATION VOLTAGE (V)
LED STRING CURRENT (mA)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 5 10 15 20 25 30
SUPPLY CURRENT vs. INPUT VOLTAGE
(DPWM DIMMING)
MAX8790A toc07
SUPPLY CURRENT (mA)
INPUT VOLTAGE (V)
7
6
5
4
3
2
1
0
71217
BRT = 100%
BRT = 0%
SHUTDOWN CURRENT vs. INPUT VOLTAGE
MAX8790A toc08
SHUTDOWN CURRENT (μA)
INPUT VOLTAGE (V)
7
6
5
4
3
2
1
0
71217
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
_______________________________________________________________________________________
7
SWITCHING WAVEFORMS
(BRT = 15%, ANALOG DIMMING)
MAX8790A toc10
1μs/div
IL
500mA/div
0mA
VLX
10V/div
0V
STARTUP WAVEFORMS
(BRT = 100%, DPWM DIMMING)
4ms/div
0A
SHDN
5V/div
VOUT
20V/div
VCCV
2V/div
IL
1A/div
0V
0V
0V
MAX8790A toc11
LED CURRENT WAVEFORMS
(BRT = 50% AT 200Hz, DPWM DIMMING)
2ms/div
0A
BRT
5V/div
VFB1
5V/div
ILED
100mA/div
IL
1A/div
0mA
0V
0V
MAX8790A toc12
LED CURRENT WAVEFORMS
(BRT = 1% AT 200Hz, DPWM DIMMING)
1ms/div
0A
BRT
5V/div
VFB1
5V/div
ILED
100mA/div
IL
1A/div
0mA
0V
0V
MAX8790A toc13
LED CURRENT WAVEFORMS
(BRT = 50% AT 200Hz, ANALOG DIMMING)
1ms/div
0A
BRT
5V/div
VFB1
1V/div
ILED
50mA/div
IL
1A/div
0mA
0V
0V
MAX8790A toc14
LED CURRENT WAVEFORMS
(BRT = 1% AT 200Hz, ANALOG DIMMING)
1ms/div
0mA
BRT
5V/div
VFB1
2V/div
ILED
50mA/div
IL
500mA/div
0mA
0V
0V
MAX8790A toc15
LED-OPEN FAULT PROTECTION
(BRT = 100%, LED OPEN ON FB3)
20ms/div
0A
VFB3
1V/div
VFB1
10V/div
VOUT
20V/div
IL
1A/div
0V
0V
0V
MAX8790A toc16
Typical Operating Characteristics (continued)
(Circuit configuration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA= +25°C, unless otherwise noted.)
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
8 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit configuration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA= +25°C, unless otherwise noted.)
LED-SHORT FAULT PROTECTION
(BRT = 100%, 2 LEDs SHORT ON FB3)
10ms/div
0A
VFB3
1V/div
VFB1
10V/div
VOUT
20V/div
IL
1A/div
0V
0V
0V
MAX8790A toc17
LED CURRENT BALANCING
vs. INPUT VOLTAGE (BRT = 100%)
MAX8790A toc18
LED CURRENT BALANCING ACCURACY (%)
INPUT VOLTAGE (V)
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
071217
750kHz
500kHz
1MHz
Pin Description
PIN NAME FUNCTION
1 OSC O sci l l ator Fr eq uency S el ecti on P i n. C onnect OS C to V
C C
to set the step - up conver ter ’ s osci l l ator fr eq uency to
1M H z. C onnect O S C to G N D to set the fr eq uency to 500kH z. Fl oat OS C to set the fr eq uency to 750kH z.
2 ENA
Anal og D i m m i ng E nab l e. E N A sets the P WM contr ol m od e. S et E N A LOW to enab l e d i r ect D P WM d i m m i ng .
S et E N A H IGH to enab l e anal og d i m m i ng . In b oth m od es, the d uty cycl e of the P WM si g nal at the BRT i np ut
contr ol s the LE D cur r ent char acter i sti cs. S ee the D i m m i ng C ontr ol secti on for a com p l ete d escr i p ti on.
3 BRT
Brightness Control Input. The duty cycle of this digital input signal controls the LED current characteristics.
The allowable frequency range is 100Hz to 500Hz in analog dimming mode. The duty cycle can be 100%
to 1%. The BRT frequency can go above 500Hz in direct DPWM mode as long as the BRT pulse width is
greater than 50µs minimum. See the Dimming Control section for a complete description.
4SHDN Shutdown Control Input. The MAX8790A shuts down when SHDN is less than 0.8V. Pulling SHDN above
2.1V enables the MAX8790A. SHDN can be connected to the input voltage if desired.
5 FB1 LED String 1 Cathode Connection. FB1 is the open-drain output of an internal regulator, which controls
current through FB1. FB1 can sink up to 27mA. If unused, connect FB1 to GND.
6 FB2 LED String 2 Cathode Connection. FB2 is the open-drain output of an internal regulator, which controls
current through FB2. FB2 can sink up to 27mA. If unused, connect FB2 to GND.
7 FB3 LED String 3 Cathode Connection. FB3 is the open-drain output of an internal regulator, which controls
current through FB3. FB3 can sink up to 27mA. If unused, connect FB3 to GND.
8 GND Ground
9 FB4 LED String 4 Cathode Connection. FB4 is the open-drain output of an internal regulator, which controls
current through FB4. FB4 can sink up to 27mA. If unused, connect FB4 to GND.
10 FB5 LED String 5 Cathode Connection. FB5 is the open-drain output of an internal regulator, which controls
current through FB5. FB5 can sink up to 27mA. If unused, connect FB5 to GND.
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
_______________________________________________________________________________________ 9
Pin Description (continued)
PIN NAME FUNCTION
11 FB6 LED String 6 Cathode Connection. FB6 is the open-drain output of an internal regulator, which controls
current through FB6. FB6 can sink up to 27mA. If unused, connect FB6 to GND.
12 CS Step-Up Controller Current-Sense Input. Connect the CS input to a ground-referenced sense resistor to
measure the current in the external MOSFET switch.
13 EXT External MOSFET Gate-Drive Output
14 OV Overvoltage Sense. Connect OV to the center tap of a resistive voltage-divider from VOUT to ground. The
detection threshold for voltage limiting at OV is 1.23V (typ).
15 VCC
5V Linear Regulator Output. VCC provides power to the MAX8790A and is also used to bias the gate driver
for the external MOSFET. Bypass VCC to GND with a ceramic capacitor of 1µF or greater. If VIN is less
than or equal to 5.5V, connect VCC to IN to the disable the internal LDO and use the external 5V supply to
VCC. When SHDN is low, the internal linear regulator is disabled.
16 IN Supply Input. VIN biases the internal 5V linear regulator that powers the device. Bypass IN to GND directly
at the pin with a 0.1µF or greater ceramic capacitor.
17 CCV Step-Up Converter Compensation Pin. Connect a 0.1µF ceramic capacitor and 1.2kΩ resistor from CCV to
GND. When the MAX8790A shuts down, CCV is discharged to 0V through an internal 20kΩ resistor.
18 ISET
Full-Scale LED Current Adjustment Pin. The resistance from ISET to GND controls the full-scale current in
each LED string:
ILEDmax = 20mA x 100kΩ/RISET
The acceptable resistance range is 74kΩ < RISET < 133kΩ, which corresponds to full-scale LED current of
27mA > ILEDmax > 15mA. Connect ISET to VCC for a default full-scale LED current of 20mA.
19 FSET
PLL Free-Running Frequency Control Pin. The resistance from FSET to GND controls the PLL oscillator’s
free-running frequency, fPLL:
fPLL = 1 / (10 x RFSET x 800pF)
The capture range is 0.6 x fPLL to fPLL. The acceptable resistance range for FSET is 250kΩ < RFSET <
754kΩ, which corresponds to a frequency range of 500Hz > fPLL > 166Hz. The resulting capture
frequency range is 100Hz to 500Hz.
20 CPLL Phase-Locked Loop-Compensation Capacitor Pin. The capacitance at CPLL compensates the PLL loop
response. Connect a 0.1µF ceramic capacitor from CPLL to GND.
EP EP Exposed Backside Pad. Solder to the circuit board ground plane with sufficient copper connection to
ensure low thermal resistance. See the PCB Layout Guidelines section.
MAX8790A
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.
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
10 ______________________________________________________________________________________
FB1
N1
BRT
CCV
GND
IN
VIN
7V TO 21V
EXT
CS
OV
CPLL FB2
FB3
FB4
FB5
FB6
L1
4.7μHD1
VCC
ISET
FSET
CIN
ENA
OSC
N.C.
0.1μF
0.1μF
1μF
EP
511kΩ
1.2kΩ
R2
37.4kΩ
R1
1MΩ
RS
56mΩ
SHDN
0.1μF
MAX8790A
COUT
VOUT
UP TO 35V
Figure 1. Typical Operating Circuit
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
______________________________________________________________________________________ 11
5V LINEAR
REGULATOR
OSCILLATOR SLOPE
COMPENSATION
CLOCK
FB OVERVOLTAGE
COMPARATOR
ERROR
COMPARATOR
OUTPUT OVERVOLTAGE
COMPARATOR
OV
EXT
CS
ERROR
AMPLIFIER
CURRENT SOURCE
IN
FB1
GND
N
FB2
65ms TIMER
SHUTDOWN
LATCH
CONTROL AND
DRIVER LOGIC
CURRENT SENSE
HVC FB6
FB5
FB4
FB3
FB2
LVC
TRI-LEVEL
COMPARATOR
gm
EN
VCC
OSC
REF ADJ
CLK
REF
VCC - 0.4V
VCC + 0.6V
1.25V
ISET
PLL
8
8
5 LSBs
5 MSBs
CPLL
BRT
ENA
OSC
256 x fBRT
DIGITAL
COMPARATOR
8-BIT
LATCH
8-BIT DAC
SAT
DIGITAL CONTROL
CURRENT SOURCE
8-BIT
COUNTER
FSET
CCV
VCC
SHDN
FB3
CURRENT SOURCE
FB4
CURRENT SOURCE
FB5
CURRENT SOURCE
FB6
CURRENT SOURCE
8
10Ω
Figure 2. Control Circuit Block Diagram
MAX8790A
Fixed-Frequency Step-Up Controller
The MAX8790A’s fixed-frequency, current-mode, step-
up controller automatically chooses the lowest active
FB_ voltage to regulate the feedback voltage.
Specifically, the difference between the lowest FB_ volt-
age and the current source-control signal plus an offset
(VSAT) is integrated at the CCV output. The resulting
error signal is compared to the external switch current
plus slope compensation to terminate the switch on-
time. As the load changes, the error amplifier sources
or sinks current to the CCV output to adjust the
required peak inductor current. The slope-compensa-
tion signal is added to the current-sense signal to
improve stability at high duty cycles.
At light loads, the MAX8790A automatically skips pulses
to improve efficiency and prevent overcharging the out-
put capacitor. In SKIP mode, the inductor current ramps
up for a minimum on-time of approximately 150ns, then
discharges the stored energy to the output. The switch
remains off until another pulse is needed to boost the
output voltage.
Internal 5V Linear Regulator
VCC and UVLO
The MAX8790A includes an internal low-dropout linear
regulator (VCC). When VIN is higher than 5.5V and
SHDN is high, this linear regulator generates a 5V sup-
ply to power an internal PWM controller, control logic,
and MOSFET driver. This linear regulator can deliver at
least 10mA of total additional load current. If VIN is less
than or equal to 5.5V, VCC and IN can be connected
together and powered from an external 5V supply.
There is an internal diode from VCC to IN, so VIN must
be greater than VCC (see Figure 2).
The MAX8790A includes UVLO protection. The controller
is disabled until VCC exceeds the UVLO threshold of
4.25V (typ). Hysteresis on UVLO is approximately 20mV.
The VCC pin should be bypassed to GND with a 1µF or
greater ceramic capacitor.
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
12 ______________________________________________________________________________________
FB1
N1
BRT
CCV
GND
IN
VIN
2.8V TO 5.5V
EXTERNAL
5V SUPPLY
EXT
CS
OV
CPLL FB2
FB3
FB4
FB5
FB6
L1
0.9μHD1
VCC
ISET
FSET
CIN
ENA
OSC
N.C.
0.1μF
0.1μF
1μF
EP
511kΩ
1.2kΩ
R2
59kΩ
R1
1MΩ
RS
30mΩ
SHDN
MAX8790A
COUT
VOUT
UP TO 22V
Figure 3. Low-Input-Voltage Application Circuit
Startup
At startup, the MAX8790A checks each FB_ pin to
determine if the respective current string is enabled.
Each FB_ pin is internally pulled up with a 180µA cur-
rent source. If an FB_ pin is connected to GND, the cor-
responding string current source is disabled. This
feedback scan takes approximately 4.2ms, after which
the step-up converter begins switching.
Shutdown
When the SHDN pin is less than 0.8V, the MAX8790A
shuts down the internal LDO, the reference, current
sources, and all control circuitry. The resulting supply
current is less than 10µA. While the n-channel MOSFET
is turned off, the step-up regulator’s output is connected
to IN through the external inductor and rectifier diode.
Frequency Selection
A tri-level OSC input sets the internal oscillator frequency
for the step-up converter, as shown in Table 1. High-fre-
quency (1MHz) operation optimizes the regulator for the
smallest component size, at the expense of efficiency
due to increased switching losses. Low-frequency
(500kHz) operation offers the best overall efficiency, but
requires larger components and PCB area.
Overvoltage Protection
To protect the step-up converter when the load is open,
or the output voltage becomes excessive for any rea-
son, the MAX8790A features a dedicated overvoltage
feedback input (OV). The OV pin is connected to the
center tap of a resistive voltage-divider from the high-
voltage output (see Figure 1). When the MAX8790A is
powered up, if none of the LED strings on FB1–FB6 are
connected to the step-up converter output, the step-up
converter regulates the output voltage to VOUT =
1.23V(1 + R1 / R2). When VOV exceeds 1.23V, a com-
parator turns off N1. The step-up converter switch is
reenabled after the output voltage drops below the pro-
tection threshold.
LED Current Sources
Maintaining uniform LED brightness and dimming
capability are critical for LCD backlight applications.
The MAX8790A is equipped with a bank of six matched
current sources. These specialized current sources are
accurate to within ±1.5% and can be switched on and
off within 10µs, enabling PWM frequencies of up to
2kHz. All LED full-scale currents are identical and are
set through the ISET pin (15mA < ILED < 27mA).
The minimum voltage drop across each current source
is approximately 450mV at 20mA. The low voltage drop
helps reduce dissipation while maintaining sufficient
compliance to control the LED current within the
required tolerances.
The LED current sources can be disabled by grounding
the respective FB_ pin at startup. When the IC is pow-
ered up, the controller scans settings for all FB_ pins. If
an FB_ pin is not grounded, an internal circuit pulls this
pin high, and the controller enables the corresponding
current source to regulate the string current. If the FB_ pin
is grounded, the controller disables the corresponding
current regulator. The current regulator cannot be dis-
abled by grounding any of the FB_ pins after the IC is
powered up.
All FB_ pins in use are measured and the highest signal
(HVC) and the lowest signal (LVC) are extracted for two
feedback loops. HVC is used to identify excessive dis-
sipation across the current-source inputs. When HVC is
greater than VCC + 0.6V (typ) for greater than 65ms
(see the
Current-Source Fault Protection
section), a
fault latch is set and the MAX8790A is shut down. The
LDO output is not affected by the fault latch. LVC is fed
into the step-up converter’s error amplifier to regulate
the step-up converter’s output voltage.
Current-Source Fault Protection
The LED current sources are protected against string
open, short, and gross mismatch faults, using overvolt-
age detection circuitry on each FB_ pin. If any of these
three fault conditions persists for a preset duration, the
MAX8790A is latched off. The duration of the fault time
depends on the dimming mode and the duty cycle of
the BRT input (DBRT). In the DPWM mode, the timeout
interval is:
tTIMEOUT_DPWM = 65ms/DBRT
In analog dimming mode, the fault time is fixed at 65ms
for DBRT greater than 12.5%. When DBRT is less than
12.5%, the timeout interval is:
tTIMEOUT_ANALOG = 8.125ms/DBRT
The fault latch can be cleared by cycling the power or
toggling the shutdown pin SHDN.
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
______________________________________________________________________________________ 13
Table 1. Frequency Selection
OSC SWITCHING FREQUENCY (kHz)
GND 500
Open 750
VCC 1000
MAX8790A
Open-Current Source Protection
The MAX8790A step-up converter output voltage is reg-
ulated according to the minimum value of the enable
FB_ voltages. If an individual LED string is open, the
respective FB_ is pulled down to near ground. In this sit-
uation, the step-up converter output voltage increases
but is clamped to a level set with the OV feedback
input. When this elevated output voltage is applied to
the undamaged strings, excessive voltage drop devel-
ops across the FB_ pins. If the resulting HVC signal
exceeds VCC + 0.6V for greater than 65ms, the fault
latch is triggered to protect the circuit.
LED-Short and String Mismatch Protection
Normally, white LEDs have variations in forward-voltage
drop of 3.1V to 3.6V. The MAX8790A can tolerate slight
mismatches between LED strings. When the sum of the
LED forward voltages creates a mismatch in the strings
so the HVC signal exceeds VCC + 0.6V for greater than
65ms, the fault latch is triggered in much the same way
as the circuit responds to open string faults. Similar pro-
tection is activated when an LED is shorted.
The larger the number of series-connected LEDs (N),
the smaller the tolerable mismatch between LEDs:
VSAT ≈450mV and VCC = 5V
For N = 8, the average error per LED = 644mV.
For N = 10, the average error per LED = 510mV.
The larger the total mismatch, the larger the voltage
drop required across each current source to correct for
the error, and therefore the larger the dissipation within
the MAX8790A.
Dimming Control
The MAX8790A features both analog and digital dim-
ming control. Analog dimming can provide potentially
higher converter efficiency because of low voltage drop
across each WLED when the current is low. Digital dim-
ming (DPWM) provides less WLED color distortion
since the WLED current is held at full scale when the
WLED is on.
The MAX8790A’s dimming control circuit consists of a
PLL, a digital comparator, and a DAC. The controller
provides 100:1 dimming range through either analog or
digital control methods. Both methods translate the
duty cycle of the BRT input into a control signal for the
LED current sources. In analog dimming mode, the cur-
rent-source outputs are DC and the BRT duty cycle
(12.5% < DBRT < 100%) modulates the amplitude of
the currents. For DBRT < 12.5%, the LED current is digi-
tally modulated to reduce the average LED current
down to 1% of full scale. The PLL detects the BRT fre-
quency and phase, and adjusts the current-source
amplitude and duty cycle synchronously (see Figure 4).
Average Error Per LED V
N
=5 150.
Error V
N
∑<5 150.
Error V V V
N
CC SAT
∑<+−06.
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
14 ______________________________________________________________________________________
BRT
0A
ILEDMAX
ILED
ANALOG DIMMING MODE
D = 50% D = 30% D = 12.5% D = 6.25%
D = tON
tON
tBRT
tBRT
Figure 4. LED Current Control Using Analog Dimming Mode
In digital dimming mode, the step-up controller and
current source are directly turned on and off by the
PWM signal. The current pulse magnitude, or full-scale
current, is set by ISET and is independent of PWM duty
factor. The current-source outputs are PWM signals
synchronized to the BRT input signal (see Figure 5).
The full-scale current in both methods is specified by
resistance from the ISET pin to ground:
The acceptable resistance range is 74kΩ< RISET <
133kΩ, which corresponds to full-scale LED current of
27mA > ILEDmax > 15mA. Connect ISET to VCC for a
default full-scale LED current of 20mA. When ENA is
high, the analog dimming is enabled, when ENA is low,
digital dimming is enabled.
When the current-source output is pulse-width modulated,
current-source turn-on is synchronized with the BRT
signal. Synchronization and low jitter in the PWM sig-
nals help reduce flicker noise in the display. The cur-
rent through each FB_ pin is controlled only during the
step-up converter’s on-time. During the converter’s off-
time, the current sources are turned off. The output volt-
age does not discharge and stays high. Each FB_ pin can
withstand 28V, which is the pin’s maximum rated voltage.
Table 2 summarizes the characteristics of both analog
and digital dimming methods.
A PLL translates the duty cycle of the BRT input into a
reference for the MAX8790A’s current sources. A resis-
tor from the FSET pin to ground controls the PLL’s free-
running frequency:
The PLL’s loop filter bandwidth is set with a capacitor
from the CPLL pin to ground. This filter integrates the
phase difference between the BRT input signal and the
PLL oscillator. The filter bandwidth determines the
PLL’s dynamic response to frequency changes in the
BRT signal. For most applications, a 0.1µF capacitor is
adequate for oscillator frequencies in the 166Hz < fPLL
< 500Hz range. The PLL frequency capture window is
0.6 x fPLL to fPLL.
fRpF
PLL FSET
=××
1
10 800
ImA k
R
LED ISET
max =×20 100 Ω
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
______________________________________________________________________________________ 15
D = 30% D = 12.5% D = 6.25%
BRT
0A
ILEDMAX
ILED
D = 50%
D = tON
tON
tBRT
tBRT
DPWM DIMMING MODE
Figure 5. LED Current Control Using DPWM Dimming Mode
Table 2. Dimming Mode
MODE ENA PLL FREQUENCY CPLL DESCRIPTION
Analog + DPWM > 2.1V 250kΩ < RFSET < 754kΩ0.1µF
Analog dimming from 100% to 12.5% brightness. From 12.5% to
1% brightness, DPWM dimming is employed. BRT frequency
range is 100Hz to 500Hz.
Direct DPWM < 0.8V VFSET > VCC - 0.4V,
disables PLL OPEN Direct dimming by BRT signal. BRT frequency can be 100Hz to
2kHz; 50µs minimum BRT on-time limits the minimum brightness.
MAX8790A
The PLL is disabled in DPWM mode; consequently, the
BRT frequency is not limited by fPLL. The maximum
BRT frequency is determined by the minimum BRT on-
time of 50µs and the minimum acceptable dimming
factor. If a 1% dimming factor is needed, the maximum
BRT frequency is 200Hz. If a 10% dimming factor is
acceptable, the maximum BRT frequency is 2kHz.
In analog dimming mode, load-current transients can
occur when the BRT frequency abruptly changes on the
fly. Large regulation transients induce a flash on the LED
load that is observable with the naked eye and should
therefore be avoided. Such annoying flashes can be
eliminated by dynamically changing the ENA pin setting.
When a capacitor is connected to the CPLL pin and the
ENA pin is grounded, the PLL continues to run but does
not affect the dimming. When fast PLL lockup transitions
are required, the ENA pin can be momentarily pulled to
ground; after the PLL is locked up, ENA can be pulled
high to reenable PLL in dimming control.
Thermal Shutdown
The MAX8790A includes a thermal-protection circuit.
When the local IC temperature exceeds +170°C (typ),
the controller and current sources shut down and do
not restart until the die temperature drops by 15°C.
Design Procedure
All MAX8790A designs should be prototyped and tested
prior to production. Table 3 provides a list of power
components for the typical applications circuit. Table 4
lists component suppliers. External component value
choice is primarily dictated by the output voltage and the
maximum load current, as well as maximum and minimum
input voltages. Begin by selecting an inductor value.
Once L is known, choose the diode and capacitors.
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
16 ______________________________________________________________________________________
Table 3. Component List
CIRCUIT FIGURE 1 FIGURE 1 FIGURE 1 FIGURE 3
Switching
Frequency 1MHz 750kHz 500kHz 750kHz
White LED
3.2V (typ), 3.5V (max) at
20mA
Nichia NSSW008C
3.2V (typ), 3.5V (max) at
20mA
Nichia NSSW008C
3.2V (typ), 3.5V (max) at
20mA
Nichia NSSW008C
3.2V (typ), 3.5V (max) at
20mA
Nichia NSSW008C
Number of
White LEDs
6 series x 6 parallel,
20mA (max)
8 series x 6 parallel,
20mA (max)
10 series x 6 parallel,
25mA (max)
6 series x 6 parallel,
20mA (max)
Input Voltage 4.5V to 5.5V, VCC = IN 7V to 21V 7V to 21V 2.8V to 5.5V, VCC = 5V
Inductor L1 2.2µH , 2.5A p ow er i nd uctor
Sumida CDRH5D16-2R2
4.7µH , 2.05A p ow er i nd uctor
Sumida CDRH5D16-4R7
4.7µH , 3.6A p ow er i nd uctor
Sumida CDRH8D28-4R7
0.9µH , 4.7A p ow er i nd uctor
Sumida CDRH5D16-0R9
Input
Capacitors
10µF ±10%, 10V X5R
ceramic capacitor (1206)
Murata GRM31MR61A106K
10µF ±10%, 25V X5R
ceramic capacitor (1206)
M ur ata GRM 31C R61E 106KA
10µF ±10%, 25V X5R
ceramic capacitor (1206)
M ur ata GRM 31C R61E 106KA
10µF ±10%, 10V X5R
ceramic capacitor (1206)
M ur ata GRM 31M R61A106K
COUT Output
Capacitor
2.2µF ±10%, 50V X7R
ceramic capacitor (1x)
Murata GRM31CR71H225K
2.2µF ±10%, 50V X7R
ceramic capacitor (1206)
(1x)
Murata GRM31CR71H225K
4.7µF ±10%, 50V X7R
ceramic capacitor (1210)
(1x)
Murata GRM32ER71H475K
2.2µF ±10%, 50V X7R
ceramic capacitor (1x)
Murata GRM31CR71H225K
MOSFET N1
30V, 3A n-channel MOSFET
(6-pin SC70)
Vishay Si1402DH
60V, 2.8A n-channel
MOSFET (6-pin TSOP)
Fairchild Semiconductor
FDC5612
Sanyo Semiconductor
CPH6424
60V , 6A n- channel M O S FE T
( P ow er P AK 1212- 8)
Vishay Si7308DN
30V, 4.9A n-channel
MOSFET (6-pin TSOP)
Vishay Si3456BDV
Diode
Rectifier D1
2A, 30V Schottky diode
Nihon EC21QS03L
2A, 40V Schottky diode
Toshiba CMS11
Nihon EC21QS04
3A, 60V Schottky diode
Nihon EC31QS06
3A, 30V Schottky diode
Nihon EC31QS03L
Sense
Resistor
50mΩ ±1%, 1/2W
IRC LRC-LRF-1206LF-01-
R050-F
56mΩ ±1%, 1/2W
IRC LRC-LRF-1206LF-01-
R056-F
40mΩ ±1%, 1/2W
IRC LRC-LRF-1206LF-01-
R040-F
30mΩ ±1%, 1/2W
IRC LRC-LRF-1206LF-01-
R030-F
Inductor Selection
The inductance, peak current rating, series resistance,
and physical size should all be considered when
selecting an inductor. These factors affect the conver-
ter’s operating mode, efficiency, maximum output load
capability, transient response time, output voltage ripple,
and cost.
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance minimizes the current rip-
ple, and therefore reduces the peak current, which
decreases core losses in the inductor and I2R losses in
the entire power path. However, large inductor values
also require more energy storage and more turns of
wire, which increases physical size and I2R copper loss-
es in the inductor. Low inductor values decrease the
physical size, but increase the current ripple and peak
current. Finding the best inductor involves the compro-
mises among circuit efficiency, inductor size, and cost.
When choosing an inductor, the first step is to deter-
mine the operating mode: continuous conduction mode
(CCM) or discontinuous conduction mode (DCM). The
MAX8790A has a fixed internal slope compensation,
which requires a minimum inductor value. When CCM
mode is chosen, the ripple current and the peak cur-
rent of the inductor can be minimized. If a small-size
inductor is required, DCM mode can be chosen. In
DCM mode, the inductor value and size can be mini-
mized but the inductor ripple current and peak current
are higher than those in CCM. The controller can be
stable, independent of the internal slope compensation
mode, but there is a maximum inductor value require-
ment to ensure the DCM operating mode.
The equations used here include a constant LIR, which
is the ratio of the inductor peak-to-peak ripple current
to the average DC inductor current at the full-load cur-
rent. The controller operates in DCM mode when LIR is
higher than 2.0, and it switches to CCM mode when LIR
is lower than 2.0. The best trade-off between inductor
size and converter efficiency for step-up regulators
generally has an LIR between 0.3 and 0.5. However,
depending on the AC characteristics of the inductor
core material and ratio of inductor resistance to other
power-path resistances, the best LIR can shift up or
down. If the inductor resistance is relatively high, more
ripple can be accepted to reduce the number of
required turns and increase the wire diameter. If the
inductor resistance is relatively low, increasing induc-
tance to lower the peak current can reduce losses
throughout the power path. If extremely thin high-resis-
tance inductors are used, as is common for LCD panel
applications, LIR higher than 2.0 can be chosen for
DCM operating mode.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficiency
improvements in typical operating regions. The detail
design procedure can be described as follows:
Calculate the approximate inductor value using the typ-
ical input voltage (VIN), the maximum output current
(IOUT(MAX)), the expected efficiency (ηTYP) taken from
an appropriate curve in the
Typical Operating
Characteristics
, and an estimate of LIR based on the
above discussion:
The MAX8790A has a minimum inductor value limitation
for stable operation in CCM mode at low input voltage
because of the internal fixed slope compensation. The
minimum inductor value for stability is calculated by the
following equation:
where 51mV is a scale factor based on slope compen-
sation, and RSis the current-sense resistor. To deter-
mine the minimum inductor value, the RScan be
temporarily calculated using the following equation:
where 100mV is the current-limit sense voltage.
RmV
I
STMP IN DCMAX
_(, )
.
=×
100
12
LVVVR
mV f
CCM MIN OUT MAX DIODE IN MIN S
OSC MIN
() () ()
()
=+−×
()
×
×
2
51
LV
V
VV
I f LIR
IN MIN
OUT
OUT IN MIN
OUT MAX OSC
TYP
=⎛
⎝
⎜⎞
⎠
⎟−
×
⎛
⎝
⎜⎞
⎠
⎟⎛
⎝
⎜⎞
⎠
⎟
__
()
2η
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
______________________________________________________________________________________ 17
Table 4. Component Suppliers
SUPPLIER PHONE WEBSITE
Murata 770-436-1300 www.murata.com
Nichia 248-352-6575 www.nichia.com
Sumida 847-545-6700 www.sumida.com
Toshiba 949-455-2000 www.toshiba.com/taec
Vishay 203-268-6261 www.vishay.com
MAX8790A
The minimum inductor value should be recalculated
after the RSis determined (see the
Sense-Resistor
Selection
section).
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input cur-
rent at the minimum input voltage VIN(MIN), using con-
servation of energy and the expected efficiency at that
operating point (ηMIN) taken from an appropriate curve
in the
Typical Operating Characteristics
:
Calculate the ripple current at that operating point and
the peak current required for the inductor:
When DCM operating mode is chosen to minimize the
inductor value, the calculations are different from that in
the above CCM mode. The maximum inductor value for
DCM mode is calculated by the following equation:
The peak inductor current in DCM mode is calculated
using the following equation:
The inductor’s saturation current rating should exceed
IPEAK and the inductor’s DC current rating should
exceed IIN(DC,MAX). For good efficiency, choose an
inductor with less than 0.1Ωseries resistance.
Considering the typical operating circuit, the maximum
load current (IOUT(MAX)) is 120mA with a 28.72V output
and a minimal input voltage of 7V. Choosing a DCM
operating mode and estimating efficiency of 90% at this
operating point:
An inductance less than LDCM(MAX) is required, so a
4.7µH inductor is chosen. The peak inductor current at
minimum input voltage is calculated as follows:
Sense-Resistor Selection
The detected signal is fed into the step-up converter
control compensation loop through the CS pin.
The MAX8790A’s current-mode step-up converter sens-
es the switch current from CS to GND with an external
resistor, RS. The current-limit sense voltage is a fixed
100mV. The required resistance is calculated based
upon the peak inductor current at the end of the switch
on-time:
where 25.6mV is a scale factor from slope compensa-
tion, VCS_EC is the current-sense voltage listed in the
Electrical Characteristics
table (85mV), and the DMAX is
the maximum duty cycle at minimum input voltage and
maximum output voltage. In DCM operating mode, it is
calculated by the following equation:
For the typical operating circuit as Figure 1:
Again, RSis calculated as a maximum, so a 56mΩcur-
rent-sense resistor is chosen.
RmV mV
Am
S<+×−
()
=
85 25 6 0 75 0 68
135 64
...
.Ω
DH A MHz
V
MAX =×× =
47 135 075
7068
... .
μ
DLI f
V
MAX LIM OSC
IN MIN
=××
()
RVmVD
I
SCS EC MAX
PEAK
<+×−
()
_..25 6 0 75
ImA V V V V
H MHz V V A
PEAK =×× × + −
()
μ× × × +
()
=
120 2 2872 2872 04 7
4 7 0 675 0 9 28 72 0 4 135
...
.. ....
LV
VV
V
MHz V mA H
DCM MAX() ..
() .
.. .
=− +
⎛
⎝
⎜⎞
⎠
⎟×
×
×××
=
17
28 72 0 4
709
2 0 825 28 72 120 58
2μ
IIVVVV
Lf V V
PEAK OUT OUT MAX OUT MAX DIODE IN MIN
OSC MIN OUT MAX DIODE
=×× × + −
()
××× +
()
(max) ( ) ( ) ( )
() ( )
2
η
LV
VV
V
fVI
DCM MAX IN MIN
OUT MAX DIODE
IN MIN
OSC MAX OUT MAX OUT MAX
() ()
()
()
() () ()
=− +
⎛
⎝
⎜
⎞
⎠
⎟×
×
×× ×
1
2
2η
II I
PEAK IN DCMAX RIPPLE
=+
(, ) 2
IVV V
LV f
RIPPLE IN MIN OUT MAX IN MIN
OUT MAX OSC
=×−
()
××
() ( ) ()
()
IIV
V
IN DCMAX OUT MAX OUT
IN MIN MIN
(, ) ()
()
=×
×η
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
18 ______________________________________________________________________________________
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharging
on the output capacitor, and the ohmic ripple due to the
capacitor’s equivalent series resistance (ESR):
and:
where IPEAK is the peak inductor current (see the
Inductor Selection section
).
The output voltage-ripple voltage should be low
enough for the FB_ current-source regulation. The rip-
ple voltage should be less than 200mVP-P. For ceramic
capacitors, the output-voltage ripple is typically domi-
nated by VRIPPLE(C). The voltage rating and tempera-
ture characteristics of the output capacitor must also
be considered.
External MOSFET Selection
The MAX8790A’s step-up converter uses an external
MOSFET to enable applications with scalable output
voltage and output power. The boost switching architec-
ture is simple and ensures that the controller is never
exposed to high voltage. Only the external MOSFET,
diode, and inductor are exposed to the output voltage
plus one Schottky diode forward voltage:
The MOSFET’s breakdown ratings should be higher
than VBV with sufficient margin to ensure long-term relia-
bility. A conservative rule of thumb, a minimum 30%
margin would be recommended for MOSFET break-
down voltage. The external MOSFET should have a cur-
rent rating of no less than the IPEAK derived from the
Inductor Selection
section. To improve efficiency,
choose a MOSFET with low RDS(ON). The MAX8790A’s
gate-drive linear regulator can provide 10mA. Select the
external MOSFET with a total gate charge so the aver-
age current to drive the MOSFET at maximum switching
frequency is less than 10mA:
For example, the Si3458DV is specified with 16nC of
max total gate charge at Vg = 10V. For 5V of gate
drive, the required gate charge is 8nC, which equates
to 8mA at 1MHz.
The MOSFET conduction loss or resistive loss is
caused by the MOSFET’s on-resistance (RDS(ON)). This
power loss can be estimated as:
For the above Si3458DV, the estimated conduction loss is:
The approximate maximum switching loss can be cal-
culated as:
For the above Si3458DV, the approximate switching
loss is:
Rectifier Diode Selection
The MAX8790A’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery
time and low forward voltage. The diode should be
rated to handle the output voltage and the peak switch
current. Make sure that the diode’s peak current rating
is at least IPEAK calculated in the
Inductor Selection
section and that its breakdown voltage exceeds the
output voltage.
Setting the Overvoltage Protection Limit
The OV protection circuit should ensure the circuit safe
operation; therefore, the controller should limit the out-
put voltage within the ratings of all MOSFET, diode, and
output capacitor components, while providing sufficient
output voltage for LED current regulation. The OV pin is
connected to the center tap of a resistive voltage-
divider (R1 and R2 in Figure 1) from the high-voltage
output. When the controller detects the OV pin voltage
reaching the threshold VOV_TH, typically 1.23V, OV pro-
tection is activated. Hence, the step-up converter out-
put overvoltage protection point is:
In Figure 1, the output OVP voltage is set to:
VV
M
kV
OUT OVP()
.(
.).
=×+ =123 1 1
37 4 34 1
Ω
Ω
VV R
R
OUT OVP OV TH() _ ()=×+11
2
PD ns A V kHz W
SW MAX() .. .=×× × =
10 1 35 28 72 750
20 145
PD tIVf
SW MAX turn off PEAK OUT OSC
()
=×××
−
2
PD HkHz A
VW
RES MAX().. . .=×μ× ×
×=
01 47 750 135
37 004
3
Ω
PD RLfI
V
RES MAX DS ON OSC PEAK
IN MIN
() ()
()
=×× ×
×
3
3
QfmA
g MAX OSC()
×<10
VNV V V
BV F LED F SCHOTTKY FB
=× + +
__ _
VIR
RIPPLE ESR PEAK ESR COUT() ( )
≈
VI
C
VV
Vf
RIPPLE C OUT MAX
OUT
OUT MAX IN MIN
OUT MAX OSC
() () () ()
()
≈−
⎛
⎝
⎜⎞
⎠
⎟
VV V
RIPPLE RIPPLE C RIPPLE ESR
=+
() ( )
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
______________________________________________________________________________________ 19
MAX8790A
Input Capacitor Selection
The input capacitor (CIN) filters the current peaks drawn
from the input supply and reduces noise injection into
the IC. A 10µF ceramic capacitor is used in the typical
operating circuit (Figure 1) because of the high source
impedance seen in typical lab setups. Actual applica-
tions usually have much lower source impe-dance since
the step-up regulator often runs directly from the output
of another regulated supply. In some applications, CIN
can be reduced below the values used in the typical
operating circuit. Ensure a low noise supply at IN by
using adequate CIN. Alternatively, greater voltage varia-
tion can be tolerated on CIN if IN is decoupled from CIN
using an RC lowpass filter.
Select CIN’s RMS ripple current rating to ensure that its
thermal rise is less than approximately 10°C:
LED Selection and Bias
The series/parallel configuration of the LED load and the
full-scale bias current have a significant effect on regu-
lator performance. LED characteristics vary significantly
from manufacturer to manufacturer. Consult the respec-
tive LED data sheets to determine the range of output
voltages for a given brightness and LED current. In ge-
neral, brightness increases as a function of bias current.
This suggests that the number of LEDs could be
decreased if higher bias current is chosen; however,
high current increases LED temperature and reduces
operating life. Improvements in LED technology are
resulting in devices with lower forward voltage while
increasing the bias current and light output.
LED manufacturers specify LED color at a given LED
current. With lower LED current, the color of the emitted
light tends to shift toward the blue range of the spec-
trum. A blue bias is often acceptable for business appli-
cations but not for high-image-quality applications such
as DVD players. Direct DPWM dimming is a viable solu-
tion for reducing power dissipation while maintaining
LED color integrity. Careful attention should be paid to
switching noise to avoid other display quality problems.
Using fewer LEDs in a string improves step-up
converter efficiency, and lowers breakdown voltage
requirements of the external MOSFET and diode. The
minimum number of LEDs in series should always be
greater than the maximum input voltage. If the diode
voltage drop is lower than the maximum input voltage,
the voltage drop across the current-sense inputs (FB_)
increases and causes excess heating in the IC.
Between 8 and 12 LEDs in series is ideal for input volt-
ages up to 20V.
Applications Information
LED VFB_ Variation
The MAX8790A has accurate (±1.5%) matching for
each current source. However, the forward voltage of
each white LED can vary up to ±5% from part to part.
The accumulated voltage difference in each string
equates to additional power loss within the IC. For the
best efficiency, the voltage difference between strings
should be minimized. The difference between lowest
voltage string and highest voltage string should be less
than 4.5V. Otherwise, the internal LED short-circuit pro-
tection shuts the part off.
Choosing the Appropriate Dimming Mode
Analog dimming mode allows lower peak LED current
and results in higher converter efficiency and lower
noise compared to direct DPWM mode. Unfortunately,
the LED color spectrum can shift as a function of DC
current so DPWM mode is often used to achieve more
consistent display characteristics. (See the LED manu-
facturer’s data sheet to determine the extent of the
color shift.) When the MAX8790A is configured with an
FSET resistor and CPLL capacitor, the ENA signal can
toggle between modes on the fly. Care should be exer-
cised when switching between modes to prevent the
current from becoming unstable during the PLL lock-in
time. To avoid such problems, force the controller into
DPWM mode between transitions.
LCD Panel Capacitance
Some LCD panels include a capacitor in parallel with
LED string to improve ESD immunity. The MAX8790A
can start up without a problem for string capacitance
up to 0.27µF.
IdI
RMS L
=×23
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
20 ______________________________________________________________________________________
PCB Layout Guidelines
Careful PCB layout is important for proper operation. Use
the following guidelines for good PCB layout:
1) Minimize the area of the high current-switching loop
of the rectifier diode, external MOSFET, sense resis-
tor, and output capacitor to avoid excessive switching
noise. Use wide and short traces for the gate-drive
loop from the EXT pin, to the MOSFET gate, and
through the current-sense resistor, then returning to
the IC GND pin.
2) Connect high-current input and output components
with short and wide connections. The high-current
input loop goes from the positive terminal of the input
capacitor to the inductor, to the external MOSFET,
then to the current-sense resistor, and to the input
capacitor’s negative terminal. The high-current out-
put loop is from the positive terminal of the input
capacitor to the inductor, to the rectifier diode, to
the positive terminal of the output capacitors,
reconnecting between the output capacitor and
input capacitor ground terminals. Avoid using vias
in the high-current paths. If vias are unavoidable,
use multiple vias in parallel to reduce resistance
and inductance.
3) Create a ground island (PGND) consisting of the
input and output capacitor ground and negative ter-
minal of the current-sense resistor. Connect all
these together with short, wide traces or a small
ground plane. Maximizing the width of the power
ground traces improves efficiency and reduces out-
put-voltage ripple and noise spikes. Create an ana-
log ground island (AGND) consisting of the
overvoltage detection-divider ground connection,
the ISET and FSET resistor connections, CCV and
CPLL capacitor connections, and the device’s
exposed backside pad. Connect the AGND and
PGND islands by connecting the GND pins directly
to the exposed backside pad. Make no other con-
nections between these separate ground planes.
4) Place the overvoltage detection-divider resistors as
close to the OV pin as possible. The divider’s cen-
ter trace should be kept short. Placing the resistors
far away causes the sensing trace to become
antennas that can pick up switching noise. Avoid
running the sensing traces near LX.
5) Place the IN pin bypass capacitor as close to the
device as possible. The ground connection of the
IN bypass capacitor should be connected directly
to GND pins with a wide trace.
6) Minimize the size of the LX node while keeping it
wide and short. Keep the LX node away from the
feedback node and ground. If possible, avoid run-
ning the LX node from one side of the PCB to the
other. Use DC traces as shields, if necessary.
7) Refer to the MAX8790A evaluation kit for an exam-
ple of proper board layout.
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
______________________________________________________________________________________ 21
Chip Information
TRANSISTOR COUNT: 12,042
PROCESS: BiCMOS
19
20
18
17
7
6
8
ENA
FB1
9
OSC
OV
CS
FB6
VCC
1 2
ISET
45
15 14 12 11
FSET
CPLL
FB4
GND
FB3
FB2
MAX8790AETP+
BRT EXT
3
13
CCV
16 10 FB5
IN
4mm x 4mm THIN QFN
TOP VIEW
SHDN
Pin Configuration
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
22 ______________________________________________________________________________________
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
24L QFN THIN.EPS
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________
23
© 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
Revision History
Pages changed at Rev 1: All A added to all pages.