General Description
The MAX8790A is a high-efficiency driver for white light-
emitting diodes (LEDs). It is designed for large liquidcrys-
tal 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 brightness for
all LEDs. The MAX8790A has a wide input-voltage 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 signal into an analog sig-
nal 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 switching fre-
quency (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
Handy Terminals
Features
Drives Six Parallel Strings with Multiple Series-
Connected LEDs per String
±1.5% Current Regulation Accuracy Between Strings
Low 450mV Feedback Voltage at Full Current
Improves Efficiency
Step-Up Controller Regulates the Output Just Above
the Highest LED String Voltage
Full-Scale LED Current Adjustable from 15mA to
27mA, or Preset 20mA
Wide 100:1 Dimming Range
Programmable Dimming Control: Direct DPWM or
Analog Dimming
Built-In PLL for Synchronized Dimming Control
Open and Short LED Protections
Output Overvoltage Protection
Wide Input Voltage Range from 4.5V to 26V
External MOSFET Allows a Large Number of LEDs
per String
500kHz/750kHz/1MHz Switching Frequency
Small, 20-Pin, 4mm x 4mm Thin QFN Package
Pin Configuration appears at end of data sheet.
19-0989; Rev 1; 10/14
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
PART TEMP RANGE PIN-PACKAGE
MAX8790AETP+ -40°C to +85°C 20 TQFN-EP*
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
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Simplied Operating Circuit
Ordering Information
EVALUATION KIT AVAILABLE
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 (V
CC
+ 0.3V)
Continuous Power Dissipation (TA = +70°C)
TQFN (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
(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
IN Input Voltage Range VIN = VCC 4.5 5.5 V
VCC = bypassed to GND through 1µF capacitor 5.5 26.0
IN Quiescent Current
VSHDN = high,
BRT = GND
VIN = 26V 1 2 mA
VIN = VCC = 5V 1 2
SHDN = GND 10 µA
VCC Output voltage VSHDN = 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
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
Operating Frequency
VOSC = VCC 0.9 1.0 1.1 MHz
VOSC = open 675 750 825 kHz
VOSC = GND 450 500 550
Minimum Duty Cycle PWM mode 10 %
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
www.maximintegrated.com Maxim Integrated
2
Absolute Maximum Ratings
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.
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.)
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
Full-Scale FB_ Output Current
ISET = VCC, BRT = 100% 19.40 20.00 20.60
mARISET = 80kΩ to GND, BRT = 100% 24.25 25.00 25.75
RISET = 133kΩ to GND, BRT = 100% 14.40 15.00 15.60
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
Current Regulation Between
Strings
ISET = VCC, BRT = 100% -1.5 +1.5 %
ISET = VCC, BRT = 20% -2.0 +2.0 %
Minimum FB_ Regulation Voltage
RISET = 80kΩ to GND, BRT = 100% 300 500 800
mVISET = VCC, BRT = 100% 270 450 720
ISET = VCC, 12.5% 150 275 500
Maximum FB_ Ripple ISET = VCC , COUT = 1µF, OSC = VCC (Note 1) 120 200 mVP-P
FB_ On-Resistance VFB_ = 50mV 13 20 Ω
FB_ Leakage Current
SHDN = GND, VFB_ = 26V 1 µA
SHDN = VIN, BRT = GND, VFB_ = 15V 10 28
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
BRT Frequency Capture Range RFSET = 500kΩ 150 200 250 Hz
RFSET = 250kΩ 300 400 500
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
3
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
IN Input Voltage Range VIN = VCC 4.5 5.5 V
VCC bypassed to GND through 1µF cap 5.5 26.0
IN Quiescent Current
VSHDN = high
BRT = GND
VIN = 26V 2 mA
VIN = VCC = 5V 2
SHDN = GND 10 µA
VCC Output Voltage VSHDN = 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
Operating Frequency
VOSC = VCC 0.9 1.1 MHz
VOSC = open 675 825 kHz
VOSC = GND 450 550
Maximum Duty Cycle 94 %
CS Trip Voltage Duty cycle = 75% 85 115 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
INPUT LEAKAGE
SHDN Leakage Current SHDN = 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
www.maximintegrated.com Maxim Integrated
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)
Note 1: Specifications are guaranteed by design, not production tested.
Note 2: Specifications to -40°C are guaranteed by design, not production tested.
PARAMETER CONDITIONS MIN TYP MAX UNITS
LED CURRENT
Full-Scale FB_ Output Current
ISET = VCC, BRT = 100% 19.2 20.8
mARISET = 80kΩ to GND, BRT = 100% 24.0 26.0
RISET = 133kΩ to GND, BRT = 100% 14.4 15.6
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
Current Regulation Between
Strings
ISET = VCC, BRT = 100% -2 +2 %
ISET = VCC, BRT = 20% -3 +3
Minimum FB_ Regulation Voltage
RISET = 80kΩ to GND, BRT = 100% 280 840
mVISET= VCC, BRT = 100% 250 760
ISET = VCC, BRT = 12.5% 140 530
Maximum FB_ Ripple ISET= VCC, COUT = 1µF, OSC = VCC (Note 1) 200 mVP-P
FB_ On-Resistance VFB_ = 50mV 20
FB_ Leakage Current
SHDN = GND, VFB_ = 26V 1 µA
SHDN = VIN, BRT = GND, VFB_ = 15V 28
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
BRT Frequency Capture Range RFSET = 500kΩ 150 250 Hz
RFSET = 250kΩ 300 500 Hz
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
5
Electrical Characteristics (continued)
(Circuit conguration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.)
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
0
1 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
0
1 10 100
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
0 20 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
7 12 17
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
7 12 17
BRT = 100%
BRT = 0%
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
7 12 17
500kHz
750kHz
1MHz
SHUTDOWN CURRENT vs. INPUT VOLTAGE
MAX8790A toc08
SHUTDOWN CURRENT (µA)
INPUT VOLTAGE (V)
7
6
5
4
3
2
1
0
7 12 17
SWITCHING WAVEFORMS
(BRT = 100%)
MAX8790A toc09
200ns/div
IL
500mA/div
0mA
VLX
10V/div
0V
Maxim Integrated
6
www.maximintegrated.com
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Typical Operating Characteristics
(Circuit conguration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.)
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
Maxim Integrated
7
www.maximintegrated.com
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Typical Operating Characteristics (continued)
(Circuit conguration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.)
PIN NAME FUNCTION
1 OSC Oscillator Frequency Selection Pin. Connect OSC to VCC to set the step-up converters oscillator frequency to
1MHz. Connect OSC to GND to set the frequency to 500kHz. Float OSC to set the frequency to 750kHz.
2 ENA
Analog Dimming Enable. ENA sets the PWM control mode. Set ENA LOW to enable direct DPWM dimming.
Set ENA HIGH to enable analog dimming. In both modes, the duty cycle of the PWM signal at the BRT input
controls the LED current characteristics. See the Dimming Control section for a complete description.
3BRT
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.
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
0
7 12 17
750kHz
500kHz
1MHz
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
8
Pin Description
Typical Operating Characteristics (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 100k/RISET
The acceptable resistance range is 74k < RISET < 133k, 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 250k < RFSET <
754k, 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 sufcient copper connection to
ensure low thermal resistance. See the PCB Layout Guidelines section.
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
9
Pin Description (continued)
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.
Figure 1. Typical Operating Circuit
FB1
N1
BRT
CCV
GND
IN
VIN
7V TO 21V
EXT
CS
OV
CPLL FB2
FB3
FB4
FB5
FB6
L1
4.7µH D1
VCC
ISET
FSET
CIN
ENA
OSC
N.C.
0.1µF
0.1µF
1µF
EP
511k
1.2k
R2
37.4k
R1
1M
RS
56m
SHDN
0.1µF
MAX8790A
COUT
VOUT
UP TO 35V
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
10
Figure 2. Control Circuit Block Diagram
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
10
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
11
Fixed-Frequency Step-Up Controller
The MAX8790A’s fixed-frequency, current-mode, stepup
controller automatically chooses the lowest active FB_
voltage to regulate the feedback voltage. Specifically, the
difference between the lowest FB_ voltage 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 ontime. As the load changes, the
error amplifier sources or sinks current to the CCV output
to adjust the required peak inductor current. The slope-
compensation 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 supply
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 control-
ler 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.
Figure 3. Low-Input-Voltage Application Circuit
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
0.9µH D1
VCC
ISET
FSET
CIN
ENA
OSC
N.C.
0.1µF
0.1µF
1µF
EP
511k
1.2k
R2
59k
R1
1M
RS
30m
SHDN
MAX8790A
COUT
VOUT
UP TO 22V
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
12
Startup
At startup, the MAX8790A checks each FB_ pin to deter-
mine if the respective current string is enabled. Each FB_
pin is internally pulled up with a 180μA current source. If
an FB_ pin is connected to GND, the corresponding 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 cur-
rent 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-
frequency (1MHz) operation optimizes the regulator for
the smallest component size, at the expense of effi-
ciency 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 reason, 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 highvoltage 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 comparator turns off N1. The step-up
converter switch is reenabled after the output voltage
drops below the protection threshold.
LED Current Sources
Maintaining uniform LED brightness and dimming capa-
bility 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 correspond-
ing current regulator. The current regulator cannot be
disabled 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 tog-
gling the shutdown pin SHDN.
Table 1. Frequency Selection
OSC SWITCHING FREQUENCY (kHz)
GND 500
Open 750
VCC 1000
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
13
Open-Current Source Protection
The MAX8790A step-up converter output voltage is regu-
lated according to the minimum value of the enable FB_
voltages. If an individual LED string is open, the respec-
tive FB_ is pulled down to near ground. In this situation,
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 develops 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 protec-
tion is activated when an LED is shorted.
The larger the number of series-connected LEDs (N), the
smaller the tolerable mismatch between LEDs:
CC SAT
N
Error V 0.6V V<+
VSAT ≈ 450mV and VCC = 5V
N
Error 5.150V<
5.150V
Average Error Per LED
N
=
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 dimming
control. Analog dimming can provide potentially higher
converter efficiency because of low voltage drop across
each WLED when the current is low. Digital dimming
(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 current-
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 digitally modulated
to reduce the average LED current down to 1% of full
scale. The PLL detects the BRT frequency and phase,
and adjusts the current-source amplitude and duty cycle
synchronously (see Figure 4).
Figure 4. LED Current Control Using Analog Dimming Mode
BRT
0A
ILEDMAX
ILED
ANALOG DIMMING MODE
D = 50% D = 30% D = 12.5% D = 6.25%
D = tON
tON
tBRT
tBRT
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
14
In digital dimming mode, the step-up controller and cur-
rent source are directly turned on and off by the PWM sig-
nal. 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:
LEDmax
ISET
20mA 100k
IR
×Ω
=
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 current
through each FB_ pin is controlled only during the step-up
converter’s on-time. During the converter’s offtime, the
current sources are turned off. The output voltage 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 resistor
from the FSET pin to ground controls the PLL’s freerun-
ning frequency:
PLL
FSET
1
f10 R 800pF
=××
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 oscil-
lator. 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 oscil-
lator frequencies in the 166Hz < fPLL < 500Hz range. The
PLL frequency capture window is 0.6 x fPLL to fPLL.
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 < 754kΩ 0.1µF
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.
D = 30% D = 12.5% D = 6.25%
BRT
0A
ILEDMAX
ILED
D = 50%
D = tON
tON
tBRT
tBRT
DPWM DIMMING MODE
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
15
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 ontime 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 maxi-
mum 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 com-
ponents 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.
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 power inductor
Sumida CDRH5D16-2R2
4.7µH, 2.05A power inductor
Sumida CDRH5D16-4R7
4.7µH, 3.6A power inductor
Sumida CDRH8D28-4R7
0.9µH, 4.7A power inductor
Sumida CDRH5D16-0R9
Input
Capacitors
10µF ±10%, 10V X5R
ceramic capacitor (1206)
Murata GRM31MR61A106K
10µF ±10%, 25V X5R
ceramic capacitor (1206)
Murata GRM31CR61E106KA
10µF ±10%, 25V X5R
ceramic capacitor (1206)
Murata GRM31CR61E106KA
10µF ±10%, 10V X5R
ceramic capacitor (1206)
Murata GRM31MR61A106K
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 MOSFET
(PowerPAK 1212-8)
Vishay Si7308DN
30V, 4.9A n-channel
MOSFET (6-pin TSOP)
Vishay Si3456BDV
Diode
Rectier 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
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
16
Inductor Selection
The inductance, peak current rating, series resistance,
and physical size should all be considered when selecting
an inductor. These factors affect the converter’s operat-
ing 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 ripple,
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 losses in the
inductor. Low inductor values decrease the physical size,
but increase the current ripple and peak current. Finding
the best inductor involves the compromises 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 current
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 minimized 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 requirement 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 current. 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 con-
verter 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 diam-
eter. If the inductor resistance is relatively low, increasing
inductance 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
typical input voltage (VIN), the maximum output cur-
rent (IOUT(MAX)), the expected efficiency TYP) taken
from an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
2
IN_MIN OUT IN_MIN TYP
OUT OUT(MAX) OSC
V VV
LV I f LIR

 η


= 


×


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:
( )
OUT(MAX) DIODE IN(MIN) S
CCM(MIN) OSC(MIN)
V V 2V R
L5 1m V f
+ −× ×
=×
where 51mV is a scale factor based on slope compensa-
tion, and RS is the current-sense resistor. To determine
the minimum inductor value, the RS can be temporarily
calculated using the following equation:
S _TMP IN(DC,MAX)
100mV
R1.2 I
=×
where 100mV is the current-limit sense voltage.
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 Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
17
The minimum inductor value should be recalculated after
the RS is determined (see the Sense-Resistor Selection
section).
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current
at the minimum input voltage VIN(MIN), using conserva-
tion of energy and the expected efficiency at that operat-
ing point MIN) taken from an appropriate curve in the
Typical Operating Characteristics:
MIN
OUT(MAX) OUT
IN(DC,MAX) IN(MIN)
IV
IV
×
=×η
Calculate the ripple current at that operating point and the
peak current required for the inductor:
( )
IN(MIN) OUT(MAX) IN(MIN)
RIPPLE OUT(MAX) OSC
VV V
ILV f
×−
=××
RIPPLE
PEAK IN(DC,MAX)
I
II
2
= +
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:
IN(MIN)
DCM(MAX) OUT(MAX) DIODE
2
IN(MIN)
OSC(MAX) OUT(MAX) OUT(MAX)
V
L1
VV
V
2f V I


=−×

+

×η
×× ×
The peak inductor current in DCM mode is calculated
using the following equation:
( )
( )
OUT(max) OUT(MAX) OUT(MAX) DIODE IN(MIN)
PEAK
OSC(MIN) OUT(MAX) DIODE
I 2V V V V
I
Lf V V
×× × +
=× ×η× +
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 0.1Ω series 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:
DCM(MAX)
2
7V
L1
28.72V 0.4V
(7V) 0.9 5.8 H
2 0.825MHz 28.72V 120mA

=−×

+

×= µ
× ××
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:
( )
( )
PEAK
120mA 2 28.72V 28.72V 0.4V 7V
I 1.35A
4.7 H 0.675MHz 0.9 28.72V 0.4V
×× × +
= =
µ× × × +
Sense-Resistor Selection
The detected signal is fed into the step-up converter con-
trol compensation loop through the CS pin.
The MAX8790A’s current-mode step-up converter senses
the switch current from CS to GND with an external resis-
tor, 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:
( )
CS_EC MAX
S
PEAK
V 25.6mV 0.75 D
RI
+ ×−
<
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:
LIM OSC
MAX IN(MIN)
LI f
DV
××
=
For the typical operating circuit as Figure 1:
MAX
4.7 H 1.35A 0.75MHz
D 0.68
7V
µ× ×
= =
( )
S
85mV 25.6mV 0.75 0.68
R 64m
1.35A
+ ×−
<=
Again, RS is calculated as a maximum, so a 56mΩ cur-
rent-sense resistor is chosen.
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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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):
RIPPLE RIPPLE(C) RIPPLE(ESR)
VV V= +
OUT(MAX) OUT(MAX) IN(MIN)
RIPPLE(C) OUT OUT(MAX) OSC
IV V
VC Vf




and:
RIPPLE(ESR) PEAK ESR(COUT)
V IR
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 ripple voltage
should be less than 200mVP-P. For ceramic capaci-
tors, the output-voltage ripple is typically dominated by
VRIPPLE(C). The voltage rating and temperature charac-
teristics 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 volt-
age and output power. The boost switching architecture 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:
BV F_LED F_SCHOTTKY FB_
V NV V V=×+ +
The MOSFET’s breakdown ratings should be higher than
VBV with sufficient margin to ensure long-term reliability. A
conservative rule of thumb, a minimum 30% margin would
be recommended for MOSFET breakdown voltage. The
external MOSFET should have a current rating of no less
than the IPEAK derived from the Inductor Selection sec-
tion. 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 average current to drive the MOSFET
at maximum switching frequency is less than 10mA:
g(MAX) OSC
Q f 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:
3
DS(ON) OSC PEAK
RES(MAX) IN(MIN)
R Lf I
PD 3V
×× ×
=×
For the above Si3458DV, the estimated conduction loss is:
3
RES(MAX)
0.1 4.7µH 750kHz 1.35A
PD 0.04W
3 7V
Ω× × ×
= =
×
The approximate maximum switching loss can be calcu-
lated as:
turn off PEAK OUT OSC
SW(MAX)
t I Vf
PD
2
×××
=
For the above Si3458DV, the approximate switching loss
is:
SW(MAX)
10ns 1.35A 28.72V 750kHz
PD 0.145W
2
×× ×
= =
Rectier 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 voltagedivider
(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 protection is
activated. Hence, the step-up converter output overvolt-
age protection point is:
OUT(OVP) OV_TH
R1
V V (1 )
R2
= ×+
In Figure 1, the output OVP voltage is set to:
OUT(OVP)
1M
V 1.23V (1 ) 34.1V
37.4k
= ×+ =
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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19
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 applications
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 variation can be toler-
ated 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:
L
RMS
dI
I23
=×
LED Selection and Bias
The series/parallel configuration of the LED load and the
full-scale bias current have a significant effect on regula-
tor 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
general, brightness increases as a function of bias cur-
rent. This suggests that the number of LEDs could be
decreased if higher bias current is chosen; however, high
current increases LED temperature and reduces operat-
ing 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 spectrum.
A blue bias is often acceptable for business applications
but not for high-image-quality applications such as DVD
players. Direct DPWM dimming is a viable solution 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 maxi-
mum 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 voltages 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 accu-
mulated voltage difference in each string equates to addi-
tional 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 protection 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 manufacturer’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 exercised 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.
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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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 output
loop is from the positive terminal of the input capacitor
to the inductor, to the rectifier diode, to the positive ter-
minal of the output capacitors, reconnecting between
the output capacitor and input capacitor ground termi-
nals. 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 terminal 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 output-voltage ripple
and noise spikes. Create an analog ground island
(AGND) consisting of the overvoltage detection-divider
ground connection, the ISET and FSET resistor con-
nections, 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
connections between these separate ground planes.
4) Place the overvoltage detection-divider resistors as
close to the OV pin as possible. The divider’s center
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 running 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 example
of proper board layout.
PACKAGE
TYPE
PACKAGE
CODE OUTLINE NO.
LAND
PATTERN
NO.
20 TQFN-EP T2044+3 21-0139 90-0037
Package Information
For the latest package outline information and land patterns
(footprints), go to www.maximintegrated.com/packages. Note
that a “+”, “#”, or “-” in the package code indicates RoHS status
only. Package drawings may show a different suffix character,
but the drawing pertains to the package regardless of RoHS
status.
19
20
18
17
7
6
8
ENA
FB1
9
OSC
OV
CS
FB6
VCC
1 2
ISET
4 5
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
MAX8790A Six-String White LED Driver with Active
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21
Pin Conguration
Chip Information
PROCESS: BiCMOS
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 10/07 Initial release
1 10/14 Removed automotive references from the Applications 1
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc. © 2014 Maxim Integrated Products, Inc.
22
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Revision History
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