EVALUATION KIT AVAILABLE
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
General Description
The MAX8790A is a high-efficiency driver for white lightemitting diodes (LEDs). It is designed for large liquidcrystal 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 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 controller 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 provide consistent operation and soft-start capability. A thermal-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 frequency (500kHz, 750kHz, or 1MHz), which allows trade-offs
between external component size and ope-rating efficiency.
Features
●● Drives Six Parallel Strings with Multiple SeriesConnected 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
Simplified Operating Circuit
CIN
SHDN
VCC
FSET
ISET
BRT
19-0989; Rev 1; 10/14
EXT
N.C.
OSC
N.C.
CPLL
N1
CS
RS
GND
R1
OV
R2
CCV
FB2
TEMP RANGE
PIN-PACKAGE
-40°C to +85°C
20 TQFN-EP*
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
VOUT
FB1
Ordering Information
MAX8790AETP+
IN
MAX8790A
Applications
●● Notebook, Subnotebook, and Tablet Computer
Displays
●● Handy Terminals
D1
0.1µF
The MAX8790A is available in a thermally enhanced,
lead-free, 20-pin, 4mm x 4mm, thin QFN package.
PART
L1
VIN
ENA
FB3
EP
FB5
FB6
FB4
Pin Configuration appears at end of data sheet.
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Absolute Maximum Ratings
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)
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
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
IN Input Voltage Range
IN Quiescent Current
CONDITIONS
MIN
TYP
4.5
5.5
VCC = bypassed to GND through 1µF capacitor
5.5
26.0
VSHDN = high,
BRT = GND
V
1
2
VIN = VCC = 5V
1
2
10
µA
4.7
5.0
5.3
V
15
56
130
mA
4.00
4.25
4.45
V
VCC 0.1
VCC
VSHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA
VCC Short-Circuit Current
VCC UVLO Threshold
UNITS
VIN = 26V
SHDN = GND
VCC Output voltage
MAX
VIN = VCC
Rising edge, hysteresis = 20mV
mA
STEP-UP CONVERTER
EXT High Level
10mA from EXT to GND
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
OSC Midlevel Threshold
V
1.5
OSC Low-Level Threshold
Minimum Duty Cycle
VCC 2.0
V
0.4
V
MHz
VOSC = VCC
0.9
1.0
1.1
VOSC = open
675
750
825
VOSC = GND
450
500
550
PWM mode
10
Pulse skipping, no load
0
Maximum Duty Cycle
CS Trip Voltage
A
VCC 0.4
OSC High-Level Threshold
Operating Frequency
V
Duty cycle = 75%
94
95
85
100
kHz
%
%
115
mV
CONTROL INPUT
SHDN Logic-Input High Level
2.1
SHDN Logic-Input Low Level
BRT, ENA Logic-Input High Level
BRT, ENA Logic-Input Low Level
www.maximintegrated.com
V
0.8
2.1
V
V
0.8
V
Maxim Integrated │ 2
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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
+42
µA
+50
µA
INPUT LEAKAGE
SHDN Leakage Current
SHDN = 26V
CS Leakage Current
VCS = GND
+40
OSC Leakage Current
-3
+3
µA
BRT, ENA Leakage Current
-1
+1
µA
-1
+1
µA
-0.1
+0.1
µA
FSET, ISET Leakage Current
FSET = ISET = VCC
OV Leakage Current
LED CURRENT
Full-Scale FB_ Output Current
ISET High-Level Threshold
ISET = VCC, BRT = 100%
19.40
20.00
20.60
RISET = 80kΩ to GND, BRT = 100%
24.25
25.00
25.75
RISET = 133kΩ to GND, BRT = 100%
14.40
15.00
15.60
Default setting for 20mA full-scale LED current
VCC 0.4
ISET Voltage
mA
V
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
%
RISET = 80kΩ to GND, BRT = 100%
300
500
800
270
450
720
150
275
500
Minimum FB_ Regulation Voltage ISET = VCC, BRT = 100%
ISET = VCC, 12.5%
mV
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
SHDN = VIN, BRT = GND, VFB_ = 15V
BRT Input Frequency
Minimum BRT Duty Cycle
1
10
100
PLL active
28
500
12.5
µA
Hz
%
FAULT PROTECTION
OV Threshold Voltage
FB_ Overvoltage Threshold
FAULT Shutdown Timer
VFB_ > 5.6V (typ)
Thermal-Shutdown Threshold
(Note 1)
1.16
1.23
1.30
V
VCC +
0.20
VCC +
0.6
VCC +
1.45
V
50
65
80
ms
170
°C
PHASE-LOCKED LOOP
FSET High-Level Threshold
BRT Frequency Capture Range
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PLL disabled
VCC 0.4
V
RFSET = 500kΩ
150
200
250
RFSET = 250kΩ
300
400
500
Hz
Maxim Integrated │ 3
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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
IN Input Voltage Range
IN Quiescent Current
CONDITIONS
VIN = VCC
VCC bypassed to GND through 1µF cap
VSHDN = high
BRT = GND
MIN
MAX
UNITS
4.5
5.5
V
5.5
26.0
VIN = 26V
2
VIN = VCC = 5V
2
SHDN = GND
VCC Output Voltage
mA
10
µA
5.3
V
VSHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA
4.7
12
130
mA
Rising edge, hysteresis = 20mV
4.00
4.45
V
VCC Short-Circuit Current
VCC UVLO Threshold
TYP
STEP-UP CONVERTER
EXT High Level
10mA from EXT to GND
EXT Low Level
-10mA from EXT to VCC
EXT On-Resistance
EXT high or low
VCC 0.1
OSC Midlevel Threshold
1.5
OSC Low-Level Threshold
V
5
Ω
V
VCC
-2.0
V
0.4
V
MHz
VOSC = VCC
0.9
1.1
VOSC = open
675
825
VOSC = GND
450
550
Maximum Duty Cycle
CS Trip Voltage
0.1
VCC 0.4
OSC High-Level Threshold
Operating Frequency
V
94
Duty cycle = 75%
85
kHz
%
115
mV
CONTROL INPUT
SHDN Logic-Input High Level
2.1
SHDN Logic-Input Low Level
V
0.8
BRT, ENA Logic-Input High Level
2.1
BRT, ENA Logic-Input Low Level
V
V
0.8
V
+42
µA
INPUT LEAKAGE
SHDN Leakage Current
SHDN = 26V
CS Leakage Current
VCS = GND
+50
µA
OSC Leakage Current
-3
+3
µA
BRT, ENA Leakage Current
-1
+1
µA
-1
+1
µA
-0.1
+0.1
µA
FSET, ISET Leakage Current
OV Leakage Current
www.maximintegrated.com
FSET = ISET = VCC
Maxim Integrated │ 4
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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
Full-Scale FB_ Output Current
ISET = VCC, BRT = 100%
19.2
20.8
RISET = 80kΩ to GND, BRT = 100%
24.0
26.0
14.4
15.6
RISET = 133kΩ to GND, BRT = 100%
ISET High-Level Threshold
Default setting for 20mA full-scale LED current
ISET Voltage
VCC 0.4
V
1.12
1.26
V
mA
20% Output Current
ISET = VCC, BRT = 20%
3.8
4.2
Current Regulation Between
Strings
ISET = VCC, BRT = 100%
-2
+2
ISET = VCC, BRT = 20%
-3
+3
280
840
250
760
140
530
RISET = 80kΩ to GND, BRT = 100%
Minimum FB_ Regulation Voltage ISET= VCC, BRT = 100%
ISET = VCC, BRT = 12.5%
mA
%
mV
Maximum FB_ Ripple
ISET= VCC, COUT = 1µF, OSC = VCC (Note 1)
200
mVP-P
FB_ On-Resistance
VFB_ = 50mV
20
Ω
FB_ Leakage Current
SHDN = GND, VFB_ = 26V
1
SHDN = VIN, BRT = GND, VFB_ = 15V
28
BRT Input Frequency
µA
100
500
Hz
1.16
1.30
V
VCC +
0.2
VCC +
1.45
V
50
80
ms
FAULT PROTECTION
OV Threshold Voltage
FB_ Overvoltage Threshold
FAULT Shutdown Timer
VFB_ > 5.6V (typ)
PHASE-LOCKED LOOP
FSET High-Level Threshold
BRT Frequency Capture Range
PLL disabled
VCC 0.4
V
RFSET = 500kΩ
150
250
Hz
RFSET = 250kΩ
300
500
Hz
Note 1: Specifications are guaranteed by design, not production tested.
Note 2: Specifications to -40°C are guaranteed by design, not production tested.
www.maximintegrated.com
Maxim Integrated │ 5
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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.)
1MHz
88
0.6
7
0
12
17
INPUT VOLTAGE (V)
20.4
20.2
20.0
19.8
19.6
19.4
1
20
40
60
AMBIENT TEMPERATURE (°)
5
BRT = 100%
4
3
BRT = 0%
2
1
0
1000
1
DPWM DIMMING
BRT = 10%
0.02
0.01
0
-0.01
DPWM DIMMING
BRT = 100%
-0.02
-0.03
ANALOG DIMMING
BRT = 10%
7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
10
15
20
25
LED STRING CURRENT (mA)
12
17
INPUT VOLTAGE (V)
6
MAX8790A toc09
VLX
10V/div
5
4
0V
3
2
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0
30
SWITCHING WAVEFORMS
(BRT = 100%)
IL
500mA/div
0mA
1
7
100
0.7
0
12
17
INPUT VOLTAGE (V)
7
10
BRT DUTY CYCLE (%)
FB_ VOLTAGE vs. LED CURRENT
(ANALOG DIMMING)
SHUTDOWN CURRENT vs. INPUT VOLTAGE
SHUTDOWN CURRENT (µA)
MAX8790A toc07
SUPPLY CURRENT (mA)
6
0
0.03
-0.05
80
SUPPLY CURRENT vs. INPUT VOLTAGE
(DPWM DIMMING)
7
10
100
TOTAL LED CURRENT (mA)
0.04
-0.04
0
MAX8790A toc03
5
0.05
19.2
19.0
10
MAX8790A toc08
LED CURRENT (mA)
20.6
15
LED CURRENT REGULATION
vs. INPUT VOLTAGE
LED CURRENT REGULATION (%)
MAX8790A toc04
20.8
20
TOTAL LED
POWER, ANALOG
LED CURRENT
vs. AMBIENT TEMPERATURE (BRT = 100%)
21.0
TOTAL LED
POWER, DPWM
TOTAL INPUT
POWER, DPWM
0.4
0.2
87
86
TOTAL INPUT
POWER, ANALOG
IDENTICAL FOR DPWM DIMMING
AND ANALOG DIMMING
MAX8790A toc06
89
25
LED CURRENT (mA)
90
0.8
LED CURRENT vs. BRT DUTY CYCLE
(BRT AT 200Hz)
FB_ REGULATION VOLTAGE (V)
750kHz
91
MAX8790A toc05
500kHz
92
NORMALIZED TO VIN = 20V, AND
ILED = 20mA VIN = 7V
1.0
NORMALIZED POWER
93
1.2
MAX8790A toc01
BOOST CONVERTER EFFICIENCY (%)
94
NORMALIZED POWER vs. TOTAL LED CURRENT
(ANALOG AND DPWM DIMMING)
MAX8790A toc02
BOOST CONVERTER EFFICIENCY
vs. INPUT VOLTAGE (BRT = 100%)
7
12
17
INPUT VOLTAGE (V)
200ns/div
Maxim Integrated │ 6
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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.)
SWITCHING WAVEFORMS
(BRT = 15%, ANALOG DIMMING)
STARTUP WAVEFORMS
(BRT = 100%, DPWM DIMMING)
LED CURRENT WAVEFORMS
(BRT = 50% AT 200Hz, DPWM DIMMING)
MAX8790A toc11
MAX8790A toc10
0V
VLX
10V/div
0V
0V
MAX8790A toc12
SHDN
5V/div
0V
VOUT
20V/div
0V
0A
1µs/div
0mA 100mA/div
IL
1A/div
0A
4ms/div
LED CURRENT WAVEFORMS
(BRT = 1% AT 200Hz, DPWM DIMMING)
0V
0V
2ms/div
MAX8790A toc14
BRT
5V/div
0V
VFB1
5V/div
0V
ILED
0A
IL
1A/div
0A
1ms/div
LED CURRENT WAVEFORMS
(BRT = 1% AT 200Hz, ANALOG DIMMING)
LED-OPEN FAULT PROTECTION
(BRT = 100%, LED OPEN ON FB3)
MAX8790A toc16
0V
0V
BRT
5V/div
0V
VFB1
2V/div
0V
IL
1A/div
VFB3
1V/div
VFB1
10V/div
VOUT
0V 20V/div
ILED
0mA 50mA/div
0mA
VFB1
1V/div
0mA 50mA/div
1ms/div
MAX8790A toc15
BRT
5V/div
ILED
0mA 100mA/div
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IL
1A/div
LED CURRENT WAVEFORMS
(BRT = 50% AT 200Hz, ANALOG DIMMING)
MAX8790A toc13
1ms/div
VFB1
5V/div
ILED
VCCV
0V 2V/div
IL
500mA/div
0mA
BRT
5V/div
IL
500mA/div
0A
IL
1A/div
20ms/div
Maxim Integrated │ 7
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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)
0V
0V
VFB3
1V/div
VFB1
10V/div
VOUT
0V 20V/div
0A
10ms/div
IL
1A/div
1.00
MAX8790A toc18
MAX8790A toc17
LED CURRENT BALANCING ACCURACY (%)
LED CURRENT BALANCING
vs. INPUT VOLTAGE (BRT = 100%)
0.90
0.80
0.70
0.60
1MHz
0.50
0.40
0.30
500kHz
0.20
750kHz
0.10
0
7
12
17
INPUT VOLTAGE (V)
Pin Description
PIN
NAME
FUNCTION
1
OSC
Oscillator Frequency Selection Pin. Connect OSC to VCC to set the step-up converter’s 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.
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.
4
SHDN
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.
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Maxim Integrated │ 8
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Pin Description (continued)
PIN
NAME
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
FUNCTION
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
20
CPLL
EP
EP
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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.
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.
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.
Maxim Integrated │ 9
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Detailed Description
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 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 operating efficiency. The control architecture automatically skips
pulses at light loads to improve efficiency and prevents
overcharging the output capacitor.
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) protection threshold, the
controller shuts down and latches off after an internal
timer expires. The controller features cycle-by-cycle current 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 provides 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.
A PWM logic input signal, BRT, controls the LED brightness. 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-
L1
4.7µH
VIN
7V TO 21V
CIN
IN
VCC
ENA
ISET
EXT
511kΩ
N.C.
RS
56mΩ
MAX8790A
GND
OSC
OV
1.2kΩ
N1
CS
BRT
FSET
VOUT
UP TO 35V
COUT
0.1µF
SHDN
1µF
D1
R1
1MΩ
R2
37.4kΩ
CCV
0.1µF
FB1
CPLL
0.1µF
FB2
FB3
FB4
EP
FB5
FB6
Figure 1. Typical Operating Circuit
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Maxim Integrated │ 10
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
OUTPUT OVERVOLTAGE
COMPARATOR
IN
1.25V
ERROR
COMPARATOR
5V LINEAR
REGULATOR
VCC
OV
CLOCK
CONTROL AND
DRIVER LOGIC
EXT
CURRENT SENSE
CS
VCC
SLOPE
COMPENSATION
OSCILLATOR
TRI-LEVEL
COMPARATOR
OSC
65ms TIMER
SHUTDOWN
LATCH
SHDN
ERROR
AMPLIFIER
CCV
ISET
REF ADJ
FSET
OSC
256 x fBRT
FB OVERVOLTAGE
COMPARATOR
VCC + 0.6V
FB6
HVC
FB5
FB4
LVC
FB3
gm
FB2
SAT
REF
CLK
∑
8-BIT DAC
FB1
N
EN
8
VCC - 0.4V
DIGITAL CONTROL
10Ω
CURRENT SOURCE
8
CPLL
PLL
8-BIT
COUNTER
8
8-BIT
LATCH
GND
CURRENT SOURCE
FB2
CURRENT SOURCE
FB3
CURRENT SOURCE
FB4
CURRENT SOURCE
FB5
CURRENT SOURCE
FB6
5 MSBs
5 LSBs
DIGITAL
COMPARATOR
BRT
ENA
Figure 2. Control Circuit Block Diagram
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Maxim Integrated │ 11
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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 slopecompensation 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 output 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 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.
L1
0.9µH
VIN
2.8V TO 5.5V
D1
CIN
EXTERNAL
5V SUPPLY
COUT
SHDN
1µF
IN
VCC
ENA
ISET
EXT
RS
30mΩ
GND
FSET
N.C.
1.2kΩ
N1
CS
BRT
511kΩ
VOUT
UP TO 22V
OSC
MAX8790A
OV
R1
1MΩ
R2
59kΩ
CCV
0.1µF
FB1
CPLL
0.1µF
FB2
FB3
FB4
EP
FB5
FB6
Figure 3. Low-Input-Voltage Application Circuit
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Maxim Integrated │ 12
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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 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 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. Highfrequency (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.
Table 1. Frequency Selection
OSC
SWITCHING FREQUENCY (kHz)
GND
500
Open
750
VCC
1000
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 stepup 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 capability are critical for LCD backlight applications. The
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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 powered 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
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 dissipation 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 overvoltage 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.
Maxim Integrated │ 13
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Open-Current Source Protection
The MAX8790A step-up converter output voltage is regulated 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 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 protection is activated when an LED is shorted.
The larger the number of series-connected LEDs (N), the
smaller the tolerable mismatch between LEDs:
∑ Error < VCC + 0.6V − VSAT
N
VSAT ≈ 450mV and VCC = 5V
∑ Error < 5.150V
N
D=
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 currentsource 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).
ANALOG DIMMING MODE
tON
tBRT
D = 50%
BRT
5.150V
N
For N = 8, the average error per LED = 644mV.
Average Error Per LED =
D = 30%
D = 12.5%
D = 6.25%
tON
tBRT
ILEDMAX
ILED
0A
Figure 4. LED Current Control Using Analog Dimming Mode
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Maxim Integrated │ 14
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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).
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.
The full-scale current in both methods is specified by
resistance from the ISET pin to ground:
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 freerunning frequency:
ILEDmax =
20mA × 100kΩ
R ISET
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 signals help reduce flicker noise in the display. The current
through each FB_ pin is controlled only during the step-up
D=
f PLL =
1
10 × R FSET × 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 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.
DPWM DIMMING MODE
tON
tBRT
D = 6.25%
D = 12.5%
D = 30%
D = 50%
BRT
Table 2 summarizes the characteristics of both analog
and digital dimming methods.
tON
tBRT
ILEDMAX
ILED
0A
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.
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Maxim Integrated │ 15
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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 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.
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)
7V to 21V
7V to 21V
2.8V to 5.5V, VCC = 5V
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 Voltage 4.5V to 5.5V, VCC = IN
Inductor L1
2.2µH, 2.5A power inductor
Sumida CDRH5D16-2R2
Input
Capacitors
10µF ±10%, 10V X5R
10µF ±10%, 25V X5R
10µF ±10%, 25V X5R
10µF ±10%, 10V X5R
ceramic capacitor (1206)
ceramic capacitor (1206)
ceramic capacitor (1206)
ceramic capacitor (1206)
Murata GRM31MR61A106K Murata GRM31CR61E106KA Murata GRM31CR61E106KA Murata GRM31MR61A106K
2.2µF ±10%, 50V X7R
2.2µF ±10%, 50V X7R
COUT Output
ceramic capacitor (1206)
ceramic capacitor (1x)
Capacitor
(1x)
Murata GRM31CR71H225K
Murata GRM31CR71H225K
4.7µF ±10%, 50V X7R
2.2µF ±10%, 50V X7R
ceramic capacitor (1210)
ceramic capacitor (1x)
(1x)
Murata GRM31CR71H225K
Murata GRM32ER71H475K
MOSFET N1
60V, 2.8A n-channel
MOSFET (6-pin TSOP)
30V, 3A n-channel MOSFET
Fairchild Semiconductor
(6-pin SC70)
FDC5612
Vishay Si1402DH
Sanyo Semiconductor
CPH6424
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-01R050-F
56mΩ ±1%, 1/2W
IRC LRC-LRF-1206LF-01R056-F
40mΩ ±1%, 1/2W
IRC LRC-LRF-1206LF-01R040-F
30mΩ ±1%, 1/2W
IRC LRC-LRF-1206LF-01R030-F
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60V, 6A n-channel MOSFET 30V, 4.9A n-channel
(PowerPAK 1212-8)
MOSFET (6-pin TSOP)
Vishay Si7308DN
Vishay Si3456BDV
Maxim Integrated │ 16
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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
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 operating mode, efficiency, maximum output load capability,
transient response time, output voltage ripple, and cost.
The maximum output current, input voltage, output voltage, 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 determine 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 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
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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
inductance to lower the peak current can reduce losses
throughout the power path. If extremely thin high-resistance 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 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:
VIN_MIN
L =
VOUT
2
VOUT − VIN_MIN η
TYP
I OUT(MAX) × f OSC LIR
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:
L CCM(MIN) =
(VOUT(MAX) + VDIODE − 2 × VIN(MIN)) × R S
51mV × f OSC(MIN)
where 51mV is a scale factor based on slope compensation, and RS is the current-sense resistor. To determine
the minimum inductor value, the RS can be temporarily
calculated using the following equation:
R S _TMP =
100mV
1.2 × IIN(DC,MAX)
where 100mV is the current-limit sense voltage.
Maxim Integrated │ 17
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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 conservation of energy and the expected efficiency at that operating point (ηMIN) taken from an appropriate curve in the
Typical Operating Characteristics:
IIN(DC,MAX) =
I OUT(MAX) × VOUT
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:
120mA × 2 × 28.72V × (28.72V + 0.4V − 7V )
MIN
(
VIN(MIN) × VOUT(MAX) − VIN(MIN)
)
L × VOUT(MAX) × f OSC
I
=
IPEAK IIN(DC,MAX) + RIPPLE
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:
VIN(MIN) 2 × η
2 × f OSC(MAX) × VOUT(MAX) × I OUT(MAX)
The peak inductor current in DCM mode is calculated
using the following equation:
(
(
I OUT(max) × 2 × VOUT(MAX) × VOUT(MAX) + VDIODE − VIN(MIN)
L × f OSC(MIN) × η × VOUT(MAX) + VDIODE
)
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 senses
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:
RS <
VCS_EC + 25.6mV × (0.75 − D MAX )
IPEAK
where 25.6mV is a scale factor from slope compensation, 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:
VIN(MIN)
1 −
×
L DCM(MAX) =
VOUT(MAX) + VDIODE
IPEAK =
(7V) 2 × 0.9
= 5.8µH
2 × 0.825MHz × 28.72V × 120mA
=
IPEAK = 1.35A
VIN(MIN)
×η
4.7µH × 0.675MHz × 0.9 × (28.72V + 0.4V )
Calculate the ripple current at that operating point and the
peak current required for the inductor:
IRIPPLE =
7V
L DCM(MAX) =
1 − 28.72V + 0.4V ×
)
D MAX =
L × ILIM × f OSC
VIN(MIN)
For the typical operating circuit as Figure 1:
4.7µH × 1.35A × 0.75MHz
=
D MAX = 0.68
The inductor’s saturation current rating should exceed
7V
IPEAK and the inductor’s DC current rating should exceed
IIN(DC,MAX). For good efficiency, choose an inductor with
85mV + 25.6mV × (0.75 − 0.68)
=
RS <
64mΩ
less than 0.1Ω series resistance.
1.35A
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:
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Again, RS is calculated as a maximum, so a 56mΩ current-sense resistor is chosen.
Maxim Integrated │ 18
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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):
=
VRIPPLE VRIPPLE(C) + VRIPPLE(ESR)
The MOSFET conduction loss or resistive loss is caused
by the MOSFET’s on-resistance (RDS(ON)). This power
loss can be estimated as:
PD RES(MAX) =
R DS(ON) × L × f OSC × IPEAK 3
3 × VIN(MIN)
For the above Si3458DV, the estimated conduction loss is:
I OUT(MAX) VOUT(MAX) − VIN(MIN)
0.1Ω × 4.7µH × 750kHz × 1.35A 3
VRIPPLE(C) ≈
=
PD
= 0.04W
RES(MAX)
C OUT VOUT(MAX)f OSC
3 × 7V
The
approximate
maximum
switching
loss can be calcuand:
lated as:
VRIPPLE(ESR) ≈ IPEAKR ESR(COUT)
where IPEAK is the peak inductor current (see the
Inductor Selection section).
t
×I
× VOUT × f OSC
PD SW(MAX) = turn−off PEAK
2
For the above Si3458DV, the approximate switching loss
The output voltage-ripple voltage should be low enough
is:
for the FB_ current-source regulation. The ripple voltage
should be less than 200mVP-P. For ceramic capacitors, the output-voltage ripple is typically dominated by
10ns × 1.35A × 28.72V × 750kHz
VRIPPLE(C). The voltage rating and temperature
charac=
PD SW(MAX) = 0.145W
2
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 voltage 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:
VBV =
N × VF_LED + VF_SCHOTTKY + VFB_
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 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 average current to drive the MOSFET
at maximum switching frequency is less than 10mA:
Q g(MAX) × f OSC < 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.
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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 output 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 overvoltage protection point is:
VOUT(OVP)
= VOV _TH × (1 +
R1
)
R2
In Figure 1, the output OVP voltage is set to:
VOUT(OVP)
= 1.23V × (1 +
1 MΩ
=
) 34.1V
37.4kΩ
Maxim Integrated │ 19
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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 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:
IRMS =
dIL
2× 3
LED Selection and Bias
The series/parallel configuration of the LED load and the
full-scale bias current have a significant effect on regulator performance. LED characteristics vary significantly
from manufacturer to manufacturer. Consult the respective 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 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 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.
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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 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 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 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.
Maxim Integrated │ 20
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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.
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CS
FB6
TOP VIEW
EXT
Pin Configuration
15
14
13
12
11
IN 16
10
FB5
CCV 17
9
FB4
8
GND
FSET 19
7
FB3
CPLL 20
6
FB2
1
2
3
4
5
FB1
MAX8790AETP+
ISET 18
SHDN
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 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
connections between these separate ground planes.
7) Refer to the MAX8790A evaluation kit for an example
of proper board layout.
BRT
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 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.
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.
OV
1) Minimize the area of the high current-switching loop
of the rectifier diode, external MOSFET, sense resistor, 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.
ENA
Careful PCB layout is important for proper operation. Use
the following guidelines for good PCB layout:
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.
VCC
PCB Layout Guidelines
OSC
MAX8790A
4mm x 4mm THIN QFN
Chip Information
PROCESS: BiCMOS
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.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE NO.
LAND
PATTERN
NO.
20 TQFN-EP
T2044+3
21-0139
90-0037
Maxim Integrated │ 21
MAX8790A
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Revision History
REVISION
NUMBER
REVISION
DATE
0
10/07
Initial release
—
1
10/14
Removed automotive references from the Applications
1
DESCRIPTION
PAGES
CHANGED
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
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 specifications 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
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