MAXIM MAX8790

19-0658; Rev 0; 11/06
KIT
ATION
EVALU
E
L
B
A
AVAIL
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
The MAX8790 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 MAX8790 has a wide input-voltage
range from 4.5V to 26V, and provides a fixed 20mA or
adjustable 15mA to 25mA full-scale LED current.
The MAX8790 has two dimming control modes to
enable a wide variety of applications. In direct DPWM
mode, the LED current is directly turned on and off by a
PWM signal. In analog dimming mode, an internal
phase-locked loop (PLL) circuit translates the PWM 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 MAX8790 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 MAX8790 features selectable switching
frequency (500kHz, 750kHz, or 1MHz), which allows
trade-offs between external component size and operating efficiency.
The MAX8790 is available in a thermally enhanced,
lead-free, 20-pin, 4mm x 4mm, Thin QFN package.
Features
o Drives Six Parallel Strings with Multiple SeriesConnected LEDs per String
o ±1.5% Current Regulation Accuracy Between
Strings
o Low 450mV Feedback Voltage at Full Current
Improves Efficiency
o Step-Up Controller Regulates the Output Just
Above the Highest LED String Voltage
o Full-Scale LED Current Adjustable from 15mA to
25mA, or Preset 20mA
o Wide 100:1 Dimming Range
o Programmable Dimming Control: Direct DPWM or
Analog Dimming
o
o
o
o
Built-In PLL for Synchronized Dimming Control
Open and Short LED Protections
Output Overvoltage Protection
Wide Input Voltage Range from 4.5V to 26V
o External MOSFET Allows a Large Number of LEDs
per String
o 500kHz/750kHz/1MHz Switching Frequency
o Small, 20-Pin, 4mm x 4mm Thin QFN Package
Simplified Operating Circuit
VOUT
0.1μF
SHDN
IN
VCC
FSET
ISET
EXT
N1
CS
Rs
MAX8790
Notebook, Subnotebook, and Tablet Computer
Displays
Handy Terminals
D1
CIN
Applications
Automotive Systems
L1
VIN
GND
BRT
R1
N.C.
OSC
N.C.
CPLL
OV
R2
CCV
FB1
Ordering Information
PART
MAX8790ETP+
TEMP RANGE PIN-PACKAGE
-40°C to +85°C
+Denotes a lead-free package.
20 Thin QFN
(4mm x 4mm)
PKG
CODE
FB2
ENA
FB3
FB4
EP
FB5
FB6
T2044-3
Pin Configuration appears at end of data sheet.
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX8790
General Description
MAX8790
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)
20-Pin Thin QFN (derate 16.9mW/°C above +70°C) ...1349mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-60°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
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
VIN = VCC
4.5
5.5
VCC = bypassed to GND through 1µF cap
5.5
26.0
VSHDN = high
VIN = 26V
1
2
VIN = VCC = 5V
1
2
SHDN = GND
VCC Output voltage
MAX
VSHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA
10
UNITS
V
mA
µA
4.7
5.0
5.3
V
15
56
130
mA
Rising edge, hysteresis = 20mV
4.00
4.25
4.45
V
EXT High Level
10mA from EXT to GND
VCC 0.1
VCC
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
VCC Short-Circuit Current
VCC UVLO Threshold
STEP-UP CONVERTER
OSC Midlevel Threshold
V
VCC 2.0
1.5
OSC Low-Level Threshold
Minimum Duty Cycle
V
0.4
V
VOSC = VCC
0.9
1.0
1.1
MHz
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
2.1
BRT, ENA Logic-Input Low Level
2
V
0.8
_______________________________________________________________________________________
V
V
0.8
V
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
(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
+35
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
4.00
1.26
V
20% Output Current
ISET = VCC, BRT = 20%
3.84
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
ISET = VCC, BRT = 100%
270
450
720
ISET = VCC, 12.5%
150
275
500
Minimum FB_ Regulation Voltage
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
65
80
ms
50
170
°C
PHASE-LOCKED LOOP
FSET High-Level Threshold
BRT Frequency Capture Range
PLL disabled
VCC 0.4
V
RFSET = 500kΩ
150
200
250
RFSET = 250kΩ
300
400
500
Hz
_______________________________________________________________________________________
3
MAX8790
ELECTRICAL CHARACTERISTICS (continued)
MAX8790
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
MIN
TYP
4.5
5.5
VCC bypassed to GND through 1µF cap
5.5
26.0
VSHDN = high
VIN = 26V
2
VIN = VCC = 5V
2
SHDN = GND
VCC Output Voltage
MAX
VIN = VCC
VSHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA
10
UNITS
V
mA
µA
4.7
5.3
V
12
130
mA
Rising edge, hysteresis = 20mV
4.00
4.45
V
EXT High Level
10mA from EXT to GND
VCC 0.1
EXT Low Level
-10mA from EXT to VCC
EXT On-Resistance
EXT high or low
VCC Short-Circuit Current
VCC UVLO Threshold
STEP-UP CONVERTER
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
+35
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
4
FSET = ISET = VCC
_______________________________________________________________________________________
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
(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 High-Level Threshold
ISET = VCC, BRT = 100%
19.2
20.8
RISET = 80kΩ to GND, BRT = 100%
24.0
26.0
RISET = 133kΩ to GND, BRT = 100%
14.4
15.6
Default setting for 20mA full-scale LED current
VCC 0.4
ISET Voltage
mA
V
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
RISET = 80kΩ to GND, BRT = 100%
280
840
ISET= VCC, BRT = 100%
250
760
ISET = VCC, BRT = 12.5%
140
530
Minimum FB_ Regulation Voltage
%
mV
Maximum FB_ Ripple
ISET= VCC, COUT = 1µF, OSC = VCC (Note 1)
200
mVP-P
FB_ On-Resistance
VFB_ = 50mV
20
Ω
SHDN = GND, VFB_ = 26V
1
SHDN = VIN, BRT = GND, VFB_ = 15V
28
FB_ Leakage Current
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.
_______________________________________________________________________________________
5
MAX8790
ELECTRICAL CHARACTERISTICS (continued)
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.)
NORMALIZED POWER
750kHz
90
89
1MHz
0.8
TOTAL INPUT
POWER, ANALOG
0.6
88
1
12
17
INPUT VOLTAGE (V)
20.4
20.2
20.0
19.8
19.6
19.4
0
0.04
0.03
DPWM DIMMING
BRT = 10%
0.02
0.01
0
-0.01
DPWM DIMMING
BRT = 100%
-0.02
-0.03
ANALOG DIMMING
BRT = 10%
-0.04
20
40
60
AMBIENT TEMPERATURE (°)
80
5
BRT = 100%
4
3
BRT = 0%
2
0.5
0.4
0.3
0.2
5
10
15
20
25
LED STRING CURRENT (mA)
SWITCHING WAVEFORMS
(BRT = 100%)
6
VLX
10V/div
5
4
0V
3
2
IL
500mA/div
0mA
1
1
0
0
7
12
17
INPUT VOLTAGE (V)
30
MAX8790 toc09
7
SHUTDOWN CURRENT (μA)
6
0.6
0
12
17
INPUT VOLTAGE (V)
SHUTDOWN CURRENT vs. INPUT VOLTAGE
MAX8790 toc07
7
0.7
0
7
SUPPLY CURRENT vs. INPUT VOLTAGE
(DPWM DIMMING)
100
0.1
-0.05
19.0
10
BRT DUTY CYCLE (%)
FB_ VOLTAGE vs. LED CURRENT
(ANALOG DIMMING)
MAX8790 toc05
0.05
19.2
0
1
1000
MAX8790 toc08
LED CURRENT (mA)
20.6
10
LED CURRENT REGULATION
vs. INPUT VOLTAGE
LED CURRENT REGULATION (%)
MAX8790 toc04
20.8
10
100
TOTAL LED CURRENT (mA)
FB_ REGULATION VOLTAGE (V)
LED CURRENT
vs. AMBIENT TEMPERATURE (BRT = 100%)
21.0
15
5
TOTAL LED
POWER, ANALOG
0
7
IDENTICAL FOR DPWM DIMMING
AND ANALOG DIMMING
20
0.2
87
86
6
TOTAL LED
POWER, DPWM
TOTAL INPUT
POWER, DPWM
0.4
MAX8790 toc03
500kHz
92
1.0
25
MAX8790 toc06
93
NORMALIZED TO VIN = 20V, AND ILED = 20mA
VIN = 7V
LED CURRENT (mA)
1.2
MAX8790 toc01
BOOST CONVERTER EFFICIENCY (%)
94
91
LED CURRENT vs. BRT DUTY CYCLE
(BRT AT 200Hz)
NORMALIZED POWER vs. TOTAL LED CURRENT
(ANALOG AND DPWM DIMMING)
MAX8790 toc02
BOOST CONVERTER EFFICIENCY
vs. INPUT VOLTAGE (BRT = 100%)
SUPPLY CURRENT (mA)
MAX8790
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
7
12
17
INPUT VOLTAGE (V)
200ns/div
_______________________________________________________________________________________
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
(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)
LED CURRENT WAVEFORMS
(BRT = 50% AT 200Hz, DPWM DIMMING)
STARTUP WAVEFORMS
(BRT = 100%, DPWM DIMMING)
MAX8790 toc11
MAX8790 toc10
MAX8790 toc12
0V
VLX
10V/div
0V
SHDN
5V/div
0V
VOUT
20V/div
0V
0V
0A
1μs/div
0mA 100mA/div
IL
1A/div
0A
LED CURRENT WAVEFORMS
(BRT = 50% AT 200Hz, ANALOG DIMMING)
MAX8790 toc14
MAX8790 toc13
0V
0V
BRT
5V/div
0V
VFB1
5V/div
0V
BRT
5V/div
VFB1
1V/div
ILED
ILED
0mA 100mA/div
0mA 50mA/div
0A
IL
1A/div
1ms/div
0A
IL
1A/div
1ms/div
LED CURRENT WAVEFORMS
(BRT = 1% AT 200Hz, ANALOG DIMMING)
LED-OPEN FAULT PROTECTION
(BRT = 100%, LED OPEN ON FB3)
MAX8790 toc15
MAX8790 toc16
0V
0V
BRT
5V/div
0V
VFB1
2V/div
0V
ILED
0mA
VFB3
1V/div
VFB1
10V/div
VOUT
0V 20V/div
0mA 50mA/div
1ms/div
IL
1A/div
2ms/div
4ms/div
LED CURRENT WAVEFORMS
(BRT = 1% AT 200Hz, DPWM DIMMING)
VFB1
5V/div
ILED
VCCV
0V 2V/div
IL
500mA/div
0mA
BRT
5V/div
IL
500mA/div
0A
IL
1A/div
20ms/div
_______________________________________________________________________________________
7
MAX8790
Typical Operating Characteristics (continued)
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)
LED CURRENT BALANCING
vs. INPUT VOLTAGE (BRT = 100%)
0V
0V
VFB3
1V/div
VFB1
10V/div
VOUT
0V 20V/div
0A
10ms/div
IL
1A/div
1.00
MAX8790 toc18
MAX8790 toc17
LED CURRENT BALANCING ACCURACY (%)
MAX8790
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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
8
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 MAX8790 shuts down when SHDN is less than 0.8V. Pulling SHDN above
2.1V enables the MAX8790. 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 25mA. 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 25mA. 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 25mA. 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 25mA. 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 25mA. If unused, connect FB5 to GND.
_______________________________________________________________________________________
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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 25mA. 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 MAX8790 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 MAX8790 shuts down, CCV is discharged to 0V through an internal 20kΩ resistor.
ISET
Full-Scale LED Current Adjustment Pin. The resistance from ISET to GND controls the full-scale current in
each LED string:
ILEDmax = 20mA x 100kΩ/RISET
The acceptable resistance range is 80kΩ < RISET < 133kΩ, which corresponds to full-scale LED current of
25mA > ILEDmax > 15mA. Connect ISET to VCC for a default full-scale LED current of 20mA.
19
FSET
PLL Free-Running Frequency Control Pin. The resistance from FSET to GND controls the PLL oscillator’s
free-running frequency, fPLL:
fPLL = 1 / (10 x RFSET x 800pF)
The capture range is 0.6 x fPLL to fPLL. The acceptable resistance range for FSET is 250kΩ < RFSET <
754kΩ, which corresponds to a frequency range of 500Hz > fPLL > 166Hz. The resulting capture
frequency range is 100Hz to 500Hz.
20
CPLL
Phase-Locked Loop-Compensation Capacitor Pin. The capacitance at CPLL compensates the PLL loop
response. Connect a 0.1µF ceramic capacitor from CPLL to GND.
EP
EP
18
FUNCTION
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.
_______________________________________________________________________________________
9
MAX8790
Pin Description (continued)
MAX8790
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Detailed Description
are directly turned on and off by the PWM signal. In analog 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 MAX8790 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 V CC 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 MAX8790 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.
The MAX8790 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 MAX8790 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.
A PWM logic input signal, BRT, controls the LED brightness. The MAX8790 supports both analog and digital
control of the LED current, and achieves 100:1 dimming
range. The MAX8790’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
L1
4.7μH
VIN
7V TO 21V
D1
CIN
VOUT
UP TO 35V
COUT
0.1μF
SHDN
IN
VCC
ENA
ISET
1μF
EXT
CS
RS
56mΩ
BRT
GND
FSET
511kΩ
N.C.
N1
R1
1MΩ
OSC
OV
1.2kΩ
CCV
MAX8790
R2
37.4kΩ
0.1μF
FB1
CPLL
0.1μF
FB2
FB3
FB4
EP
FB5
FB6
Figure 1. Typical Operating Circuit
10
______________________________________________________________________________________
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
IN
1.25V
ERROR
COMPARATOR
5V LINEAR
REGULATOR
VCC
MAX8790
OUTPUT OVERVOLTAGE
COMPARATOR
OV
CLOCK
CONTROL AND
DRIVER LOGIC
EXT
CURRENT SENSE
CS
VCC
SLOPE
COMPENSATION
OSCILLATOR
FB OVERVOLTAGE
COMPARATOR
VCC + 0.6V
TRI-LEVEL
COMPARATOR
OSC
65ms TIMER
SHUTDOWN
LATCH
FB6
HVC
FB5
SHDN
FB4
ERROR
AMPLIFIER
LVC
FB3
gm
CCV
ISET
REF ADJ
FSET
OSC
256 x fBRT
FB2
SAT
REF
FB1
CLK
8-BIT DAC
N
EN
8
VCC - 0.4V
10Ω
DIGITAL CONTROL
CURRENT SOURCE
8
CPLL
PLL
8-BIT
COUNTER
8
GND
CURRENT SOURCE
FB2
CURRENT SOURCE
FB3
CURRENT SOURCE
FB4
CURRENT SOURCE
FB5
CURRENT SOURCE
FB6
8-BIT
LATCH
5 MSBs
5 LSBs
DIGITAL
COMPARATOR
BRT
ENA
Figure 2. Control Circuit Block Diagram
______________________________________________________________________________________
11
MAX8790
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Fixed-Frequency Step-Up Controller
The MAX8790’s fixed-frequency, current-mode, step-up
controller automatically chooses the lowest active FB_
voltage to regulate the feedback voltage. Specifically,
the difference between the lowest FB_ 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 on-time. As the
load changes, the error amplifier sources or sinks current to the CCV output to adjust the required peak
inductor current. The slope-compensation signal is
added to the current-sense signal to improve stability at
high duty cycles.
At light loads, the MAX8790 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 MAX8790 includes an internal low-dropout linear
regulator (V CC ). When V IN 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 MAX8790 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
COUT
SHDN
EXTERNAL
5V SUPPLY
1μF
IN
VCC
ENA
ISET
EXT
RS
30mΩ
BRT
N.C.
N1
CS
GND
FSET
511kΩ
VOUT
UP TO 22V
R1
1MΩ
OSC
OV
1.2kΩ
CCV
MAX8790
R2
59kΩ
0.1μF
FB1
CPLL
0.1μF
FB2
FB3
FB4
EP
FB5
FB6
Figure 3. Low-Input-Voltage Application Circuit
12
______________________________________________________________________________________
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Shutdown
When the SHDN pin is less than 0.8V, the MAX8790
shuts down the internal LDO, the reference, current
sources, and all control circuitry. The resulting supply
current is less than 10µA. While the n-channel MOSFET
is turned off, the step-up regulator’s output is connected
to IN through the external inductor and rectifier diode.
Frequency Selection
A tri-level OSC input sets the internal oscillator frequency
for the step-up converter, as shown in Table 1. High-frequency (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)
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 < 25mA).
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 MAX8790 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.
GND
500
Open
750
Current-Source Fault Protection
VCC
1000
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
MAX8790 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:
t TIMEOUT_DPWM = 65ms/DBRT
Overvoltage Protection
To protect the step-up converter when the load is open,
or the output voltage becomes excessive for any reason, the MAX8790 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 MAX8790 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 V OUT =
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 MAX8790 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
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:
t TIMEOUT_ANALOG = 8.125ms/DBRT
The fault latch can be cleared by cycling the power or
toggling the shutdown pin SHDN.
Open-Current Source Protection
The MAX8790 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
______________________________________________________________________________________
13
MAX8790
Startup
At startup, the MAX8790 checks each FB_ pin to determine if the respective current string is enabled. Each
FB_ pin is internally pulled up with a 10µA current
source. If an FB_ pin is connected to GND, the corresponding string current source is disabled. This feedback scan takes approximately 264µs, after which the
step-up converter begins switching.
MAX8790
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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 MAX8790 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
Average Error Per LED =
5.150V
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 MAX8790.
Dimming Control
The MAX8790 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 MAX8790’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).
ANALOG DIMMING MODE
tON
D=
tBRT
D = 50%
BRT
D = 30%
D = 12.5%
D = 6.25%
tON
tBRT
ILEDMAX
ILED
0A
Figure 4. LED Current Control Using Analog Dimming Mode
14
______________________________________________________________________________________
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
ILEDmax =
20mA × 100kΩ
RISET
The acceptable resistance range is 80kΩ < RISET <
133kΩ, which corresponds to full-scale LED current of
25mA > 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
D=
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 off-time, 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 MAX8790’s current sources. A resistor
from the FSET pin to ground controls the PLL’s freerunning frequency:
1
fPLL =
10 × RFSET × 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
DPWM DIMMING MODE
tON
tBRT
D = 12.5%
D = 30%
D = 50%
D = 6.25%
tON
BRT
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.
______________________________________________________________________________________
15
MAX8790
In digital dimming mode, the step-up controller and
current source are directly turned on and off by the
PWM signal. The current pulse magnitude, or full-scale
current, is set by ISET and is independent of PWM duty
factor. The current-source outputs are PWM signals
synchronized to the BRT input signal (see Figure 5).
The full-scale current in both methods is specified by
resistance from the ISET pin to ground:
MAX8790
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
adequate for oscillator frequencies in the 166Hz < fPLL
< 500Hz range. The PLL frequency capture window is
0.6 x fPLL to fPLL.
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 duty cycle 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 MAX8790 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 MAX8790 designs should be prototyped and tested
prior to production. Table 3 provides a list of power
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
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
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
ceramic capacitor (1206)
ceramic capacitor (1206)
ceramic capacitor (1206)
Murata GRM31MR61A106K Murata GRM31CR61E106KA 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
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
60V, 6A n-channel MOSFET
(PowerPAK 1212-8)
Vishay Si7308DN
30V, 4.9A n-channel
MOSFET (6-pin TSOP)
Vishay Si3456BDV
Diode
Rectifier D1
2A, 30V Schottky diode
Nihon EC21QS03L
2A, 40V Schottky diode
Toshiba CMS11
Nihon EC21QS04
3A, 60V Schottky diode
Nihon EC31QS06
3A, 30V Schottky diode
Nihon EC31QS03L
Sense
Resistor
50mΩ ±1%, 1/2W
IRC LRC-LRF-1206LF-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
16
______________________________________________________________________________________
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
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
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.
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
MAX8790 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.
MAX8790
Table 4. Component Suppliers
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 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:
2
⎛ VIN _ MIN ⎞ ⎛ VOUT − VIN _ MIN ⎞ ⎛ η TYP ⎞
L=⎜
⎟⎜
⎟
⎟ ⎜
⎝ VOUT ⎠ ⎝ IOUT(MAX) × fOSC ⎠ ⎝ LIR ⎠
The MAX8790 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) ) × RS
51mV × fOSC(MIN)
The equations used here include a constant LIR, which
is the ratio of the inductor peak-to-peak ripple current
______________________________________________________________________________________
17
MAX8790
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
where 51mV is a scale factor based on slope compensation, and RS is the current-sense resistor. To determine the minimum inductor value, the R S can be
temporarily calculated using the following equation:
RS _ TMP =
100mV
1.2 × IIN(DCMAX
,
)
where 100mV is the current-limit sense voltage.
The minimum inductor value should be recalculated
after the R S 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:
IOUT(MAX) × VOUT
IIN(DCMAX
,
)=
VIN(MIN) × η MIN
Calculate the ripple current at that operating point and
the peak current required for the inductor:
IRIPPLE =
VIN(MIN) × (VOUT(MAX) − VIN(MIN) )
L × VOUT(MAX) × fOSC
IRIPPLE
IPEAK = IIN(DCMAX
,
)+
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)
LDCM(MAX) = ⎜1 −
⎟×
⎝ VOUT(MAX) + VDIODE ⎠
VIN(MIN)2 × η
2 × fOSC(MAX) × VOUT(MAX) × IOUT(MAX)
The peak inductor current in DCM mode is calculated
using the following equation:
IPEAK =
(7V)2 × 0.9
= 5.8μH
2 × 0.825MHz × 28.72V × 120mA
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:
IPEAK =
120mA × 2 × 28.72V × (28.72V + 0.4V − 7V )
4.7μH × 0.675MHz × 0.9 × (28.72V + 0.4V )
L × fOSC(MIN) × η × (VOUT(MAX) + VDIODE )
= 1.35A
Sense-Resistor Selection
The detected signal is fed into the step-up converter
control compensation loop through the CS pin.
The MAX8790’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 − DMAX )
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:
DMAX =
L × ILIM × fOSC
VIN(MIN)
For the typical operating circuit as Figure 1:
DMAX =
IOUT(max) × 2 × VOUT(MAX) × (VOUT(MAX) + VDIODE − VIN(MIN) )
The inductor’s saturation current rating should exceed
I PEAK 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.
18
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:
7V
⎛
⎞
LDCM(MAX) = ⎜1 −
⎟×
⎝ 28.72V + 0.4V ⎠
RS <
4.7μH × 1.35A × 0.75MHz
= 0.68
7V
85mV + 25.6mV × (0.75 − 0.68)
1.35A
= 64mΩ
Again, RS is calculated as a maximum, so a 56mΩ current-sense resistor is chosen.
______________________________________________________________________________________
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR)
IOUT(MAX) ⎛ VOUT(MAX) − VIN(MIN) ⎞
VRIPPLE(C) ≈
⎜
⎟
COUT ⎝ VOUT(MAX)fOSC ⎠
and:
VRIPPLE(ESR) ≈ IPEAKRESR(COUT)
where I PEAK 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
capacitors, the output-voltage ripple is typically dominated by VRIPPLE(C). The voltage rating and temperature characteristics of the output capacitor must also
be considered.
External MOSFET Selection
The MAX8790’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 MAX8790’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:
Qg(MAX) × fOSC <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:
PDRES(MAX) =
RDS(ON) × L × fOSC × IPEAK 3
3 × VIN(MIN)
For the above Si3458DV, the estimated conduction loss is:
PDRES(MAX) =
0.1Ω × 4.7μH × 750kHz × 1.35A 3
= 0.04W
3 × 7V
The approximate maximum switching loss can be calculated as:
t
×I
× VOUT × fOSC
PDSW(MAX) = turn−off PEAK
2
For the above Si3458DV, the approximate switching
loss is:
PDSW(MAX) =
10ns × 1.35A × 28.72V × 750kHz
= 0.145W
2
Rectifier Diode Selection
The MAX8790’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
______________________________________________________________________________________
19
MAX8790
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):
MAX8790
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.
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
20
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 MAX8790 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 MAX8790 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. Because of the
10µA pullup current source in each FB_ input for string
detection, the MAX8790 can start up with less than
470pF capacitance on each FB_ pin. If the string
capacitance CLED is greater than 470pF, a bank of
pullup resistors to V IN should be added to prevent
startup problems (see Figure 6). A delay of 3 x 1MΩ x
CLED should be added after VIN was settled before
enabling the MAX8790 to ensure the FB_ voltage
exceeds the 3V internal threshold. A similar delay
should be added after the part is shut down to ensure
proper restart.
______________________________________________________________________________________
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
MAX8790
D1
L1
VIN
VOUT
COUT
EXT
N1
SHDN
TO VIN
MAX8790
CLED
1MΩ
FB1
FB2
FB3
FB4
FB5
FB6
Figure 6. Startup Circuit with Large Capacitors on LED Strings
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 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.
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.
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.
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 MAX8790 evaluation kit for an example
of proper board layout.
______________________________________________________________________________________
21
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
OV
EXT
CS
FB6
TOP VIEW
VCC
MAX8790
Pin Configuration
15
14
13
12
11
TRANSISTOR COUNT: 12,042
PROCESS: BiCMOS
IN 16
10
FB5
CCV 17
9
FB4
8
GND
FSET 19
7
FB3
CPLL 20
6
FB2
ISET 18
1
2
3
4
5
OSC
ENA
BRT
SHDN
FB1
MAX8790ETP+
Chip Information
4mm x 4mm THIN QFN
22
______________________________________________________________________________________
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
24L QFN THIN.EPS
PACKAGE OUTLINE,
12, 16, 20, 24, 28L THIN QFN, 4x4x0.8mm
21-0139
E
1
2
______________________________________________________________________________________
23
MAX8790
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
MAX8790
Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Package Information (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information
go to www.maxim-ic.com/packages.)
PACKAGE OUTLINE,
12, 16, 20, 24, 28L THIN QFN, 4x4x0.8mm
21-0139
E
2
2
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 24
© 2006 Maxim Integrated Products
is a registered trademark of Maxim Integrated Products, Inc.