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19-0989; Rev 0; 10/07
KIT ATION EVALU ABLE AVAIL
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. The MAX8790A 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 27mA, 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
MAX8790A
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
VIN CIN 0.1F L1 D1 VOUT
SHDN VCC FSET ISET
IN
EXT CS
N1
Applications
Notebook, Subnotebook, and Tablet Computer Displays Automotive Systems Handy Terminals
N.C. N.C.
Rs
MAX8790A
BRT
GND R1
OSC CPLL CCV
OV R2
Ordering Information
PART TEMP RANGE PIN-PACKAGE 20 Thin QFN (4mm x 4mm) PKG CODE T2044-3
ENA EP
FB1 FB2 FB3 FB4 FB5 FB6
MAX8790AETP+ -40C to +85C
+Denotes a lead-free package.
Pin Configuration appears at end of data sheet.
1
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Six-String White LED Driver with Active Current Balancing for LCD Panel Applications MAX8790A
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 = +70C) 20-Pin Thin QFN (derate 16.9mW/C above +70C) ...1349mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-60C to +150C Lead Temperature (soldering, 10s) .................................+300C
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, V SHDN = VIN, CCV = 0.1F, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER IN Input Voltage Range VIN = VCC VCC = bypassed to GND through 1F capacitor V SHDN = high, BRT = GND SHDN = GND VCC Output voltage VCC Short-Circuit Current VCC UVLO Threshold STEP-UP CONVERTER EXT High Level EXT Low Level EXT On-Resistance EXT Sink/Source Current OSC High-Level Threshold OSC Midlevel Threshold OSC Low-Level Threshold VOSC = VCC Operating Frequency VOSC = open VOSC = GND Minimum Duty Cycle Maximum Duty Cycle CS Trip Voltage CONTROL INPUT SHDN Logic-Input High Level SHDN Logic-Input Low Level BRT, ENA Logic-Input High Level BRT, ENA Logic-Input Low Level 2.1 0.8 2.1 0.8 V V V V Duty cycle = 75% PWM mode Pulse skipping, no load 94 85 0.9 675 450 1.0 750 500 10 0 95 100 115 10mA from EXT to GND -10mA from EXT to VCC EXT high or low EXT forced to 2V VCC 0.4 1.5 VCC 2.0 0.4 1.1 825 550 VCC 0.1 VCC 0 2 1 0.1 5 V V A V V V MHz kHz % % mV Rising edge, hysteresis = 20mV V SHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA 4.7 15 4.00 5.0 56 4.25 VIN = 26V VIN = VCC = 5V CONDITIONS MIN 4.5 5.5 1 1 TYP MAX 5.5 26.0 2 2 10 5.3 130 4.45 UNITS V mA A V mA V
IN Quiescent Current
2
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Six-String White LED Driver with Active Current Balancing for LCD Panel Applications
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VIN = 12V, V SHDN = VIN, CCV = 0.1F, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER INPUT LEAKAGE SHDN Leakage Current CS Leakage Current OSC Leakage Current BRT, ENA Leakage Current FSET, ISET Leakage Current OV Leakage Current LED CURRENT ISET = VCC, BRT = 100% Full-Scale FB_ Output Current RISET = 80k RISET = 133k ISET High-Level Threshold ISET Voltage 20% Output Current Current Regulation Between Strings Minimum FB_ Regulation Voltage Maximum FB_ Ripple FB_ On-Resistance FB_ Leakage Current BRT Input Frequency Minimum BRT Duty Cycle FAULT PROTECTION OV Threshold Voltage FB_ Overvoltage Threshold FAULT Shutdown Timer Thermal-Shutdown Threshold PHASE-LOCKED LOOP FSET High-Level Threshold BRT Frequency Capture Range PLL disabled RFSET = 500k RFSET = 250k VCC 0.4 150 300 200 400 250 500 V Hz VFB_ > 5.6V (typ) (Note 1) 1.16 VCC + 0.20 50 1.23 VCC + 0.6 65 170 1.30 VCC + 1.45 80 V V ms C PLL active ISET = VCC, BRT = 20% ISET = VCC, BRT = 100% ISET = VCC, BRT = 20% RISET = 80k to GND, BRT = 100% ISET = VCC, BRT = 100% ISET = VCC, 12.5% ISET = VCC , C OUT = 1F, OSC = VCC (Note 1) VFB_ = 50mV SHDN = GND, VFB_ = 26V SHDN = VIN, BRT = GND, VFB_ = 15V 100 12.5 10 to GND, BRT = 100% to GND, BRT = 100% 19.40 24.25 14.40 VCC 0.4 1.12 3.84 -1.5 -2.0 300 270 150 500 450 275 120 13 1.19 4.00 1.26 4.16 +1.5 +2.0 800 720 500 200 20 1 28 500 A Hz % mVP-P mV 20.00 25.00 15.00 20.60 25.75 15.60 V V mA % % mA FSET = ISET = VCC SHDN = 26V VCS = GND -3 -1 -1 -0.1 +40 +42 +50 +3 +1 +1 +0.1 A A A A A A CONDITIONS MIN TYP MAX UNITS
MAX8790A
Default setting for 20mA full-scale LED current
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3
Six-String White LED Driver with Active Current Balancing for LCD Panel Applications MAX8790A
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1. VIN = 12V, V SHDN = VIN, CCV = 0.1F, TA = -40C to +85C, unless otherwise noted.) (Note 2)
PARAMETER IN Input Voltage Range VIN = VCC VCC bypassed to GND through 1F cap V = high BRT = GND = GND VCC Output Voltage VCC Short-Circuit Current VCC UVLO Threshold STEP-UP CONVERTER EXT High Level EXT Low Level EXT On-Resistance OSC High-Level Threshold OSC Midlevel Threshold OSC Low-Level Threshold Operating Frequency Maximum Duty Cycle CS Trip Voltage CONTROL INPUT Logic-Input High Level Logic-Input Low Level BRT, ENA Logic-Input High BRT, ENA Logic-Input Low Level INPUT LEAKAGE Leakage Current CS Leakage Current OSC Leakage Current BRT, ENA Leakage Current FSET, ISET Leakage Current OV Leakage Current FSET = ISET = VCC = 26V VCS = GND -3 -1 -1 -0.1 +42 +50 +3 +1 +1 +0.1 A A A A A A 2.1 0.8 2.1 0.8 V V V V Duty cycle = 75% VOSC = VCC VOSC = open VOSC = GND 0.9 675 450 94 85 115 10mA from EXT to GND -10mA from EXT to VCC EXT high or low VCC 0.4 1.5 VCC 2.0 0.4 1.1 825 550 VCC 0.1 0.1 5 V V V MHz kHz % mV V V Rising edge, hysteresis = 20mV V = 5V, 6V < VIN < 26V, 0 < I VCC < 10mA 4.7 12 4.00 VIN = 26V VIN = VCC = 5V CONDITIONS MIN 4.5 5.5 TYP MAX 5.5 26.0 2 2 10 5.3 130 4.45 UNITS V mA A V mA V
IN Quiescent Current
4
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Six-String White LED Driver with Active Current Balancing for LCD Panel Applications
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VIN = 12V, V SHDN = VIN, CCV = 0.1F, TA = -40C to +85C, unless otherwise noted.) (Note 2)
PARAMETER LED CURRENT ISET = VCC, BRT = 100% Full-Scale FB_ Output Current RISET = 80k to GND, BRT = 100% RISET = 133k to GND, BRT = 100% ISET High-Level Threshold ISET Voltage 20% Output Current Current Regulation Between Strings Minimum FB_ Regulation Voltage Maximum FB_ Ripple FB_ On-Resistance FB_ Leakage Current BRT Input Frequency FAULT PROTECTION OV Threshold Voltage FB_ Overvoltage Threshold FAULT Shutdown Timer PHASE-LOCKED LOOP FSET High-Level Threshold BRT Frequency Capture Range PLL disabled RFSET = 500k RFSET = 250k VCC 0.4 150 300 250 500 V Hz Hz VFB_ > 5.6V (typ) 1.16 VCC + 0.2 50 1.30 VCC + 1.45 80 V V ms ISET = VCC, BRT = 20% ISET = VCC, BRT = 100% ISET = VCC, BRT = 20% RISET = 80k to GND, BRT = 100% ISET= VCC, BRT = 100% ISET = VCC, BRT = 12.5% ISET= VCC, COUT = 1F, OSC = VCC (Note 1) VFB_ = 50mV SHDN = GND, VFB_ = 26V SHDN = VIN, BRT = GND, VFB_ = 15V 100 Default setting for 20mA full-scale LED current 19.2 24.0 14.4 VCC 0.4 1.12 3.8 -2 -3 280 250 140 1.26 4.2 +2 +3 840 760 530 200 20 1 28 500 mVP-P A Hz mV 20.8 26.0 15.6 V V mA % mA CONDITIONS MIN TYP MAX UNITS
MAX8790A
Note 1: Specifications are guaranteed by design, not production tested. Note 2: Specifications to -40C are guaranteed by design, not production tested.
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5
Six-String White LED Driver with Active Current Balancing for LCD Panel Applications MAX8790A
Typical Operating Characteristics
(Circuit configuration 1, VIN = 12V, V SHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25C, unless otherwise noted.)
BOOST CONVERTER EFFICIENCY vs. INPUT VOLTAGE (BRT = 100%)
MAX8790A toc01
NORMALIZED POWER vs. TOTAL LED CURRENT (ANALOG AND DPWM DIMMING)
MAX8790A toc02
LED CURRENT vs. BRT DUTY CYCLE (BRT AT 200Hz)
IDENTICAL FOR DPWM DIMMING AND ANALOG DIMMING
MAX8790A toc03
94 BOOST CONVERTER EFFICIENCY (%) 93 500kHz 92 91 90 89 1MHz 88 87 86 7 12 17 INPUT VOLTAGE (V) 750kHz
1.2 1.0 NORMALIZED POWER 0.8 0.6 0.4 0.2 0 1
NORMALIZED TO VIN = 20V, AND ILED = 20mA VIN = 7V
25
20 LED CURRENT (mA)
TOTAL INPUT POWER, ANALOG TOTAL INPUT POWER, DPWM TOTAL LED POWER, DPWM
15
10
5 TOTAL LED POWER, ANALOG 10 100 TOTAL LED CURRENT (mA) 1000
0 1 10 BRT DUTY CYCLE (%) 100
LED CURRENT vs. AMBIENT TEMPERATURE (BRT = 100%)
MAX8790A toc04
LED CURRENT REGULATION vs. INPUT VOLTAGE
0.04 LED CURRENT REGULATION (%) 0.03 0.02 0.01 0 -0.01 -0.02 -0.03 -0.04 -0.05 DPWM DIMMING BRT = 100% ANALOG DIMMING BRT = 10% 7 12 17 INPUT VOLTAGE (V) DPWM DIMMING BRT = 10%
MAX8790A toc05
FB_ VOLTAGE vs. LED CURRENT (ANALOG DIMMING)
MAX8790A toc06
21.0 20.8 20.6 LED CURRENT (mA) 20.4 20.2 20.0 19.8 19.6 19.4 19.2 19.0 0 20 40 60 AMBIENT TEMPERATURE ()
0.05
0.7 FB_ REGULATION VOLTAGE (V) 0.6 0.5 0.4 0.3 0.2 0.1 0 0 5 10 15 20 25 LED STRING CURRENT (mA)
80
30
SUPPLY CURRENT vs. INPUT VOLTAGE (DPWM DIMMING)
MAX8790A toc07
SHUTDOWN CURRENT vs. INPUT VOLTAGE
MAX8790A toc08
SWITCHING WAVEFORMS (BRT = 100%)
MAX8790A toc09
7 6 SUPPLY CURRENT (mA) 5 4 3 2 1 0 7 12 17 INPUT VOLTAGE (V) BRT = 0%
7 6 SHUTDOWN CURRENT (A) 5 4
BRT = 100%
VLX 10V/div 0V
3 2 1 0 7 12 17 INPUT VOLTAGE (V) 200ns/div IL 500mA/div 0mA
6
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Six-String White LED Driver with Active Current Balancing for LCD Panel Applications
Typical Operating Characteristics (continued)
(Circuit configuration 1, VIN = 12V, V SHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25C, unless otherwise noted.)
SWITCHING WAVEFORMS (BRT = 15%, ANALOG DIMMING)
MAX8790A toc10
MAX8790A
STARTUP WAVEFORMS (BRT = 100%, DPWM DIMMING)
MAX8790A toc11
LED CURRENT WAVEFORMS (BRT = 50% AT 200Hz, DPWM DIMMING)
MAX8790A toc12
0V VLX 10V/div 0V
SHDN 5V/div
0V
BRT 5V/div VFB1 5V/div
0V
VOUT 20V/div
0V
VCCV 0V 2V/div IL 500mA/div 0mA IL 1A/div
ILED 0mA 100mA/div IL 1A/div
0A 4ms/div
0A 2ms/div
1s/div
LED CURRENT WAVEFORMS (BRT = 1% AT 200Hz, DPWM DIMMING)
MAX8790A toc13
LED CURRENT WAVEFORMS (BRT = 50% AT 200Hz, ANALOG DIMMING)
MAX8790A toc14
0V
BRT 5V/div VFB1 5V/div
0V
BRT 5V/div VFB1 1V/div
0V
0V
ILED 0mA 100mA/div IL 1A/div
ILED 0mA 50mA/div IL 1A/div
0A 1ms/div
0A 1ms/div
LED CURRENT WAVEFORMS (BRT = 1% AT 200Hz, ANALOG DIMMING)
MAX8790A toc15
LED-OPEN FAULT PROTECTION (BRT = 100%, LED OPEN ON FB3)
MAX8790A toc16
0V
BRT 5V/div VFB1 2V/div
0V
VFB3 1V/div VFB1 10V/div
0V
0V
ILED 0mA 50mA/div IL 500mA/div
VOUT 0V 20V/div IL 1A/div
0mA 1ms/div
0A 20ms/div
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7
Six-String White LED Driver with Active Current Balancing for LCD Panel Applications MAX8790A
Typical Operating Characteristics (continued)
(Circuit configuration 1, VIN = 12V, V SHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25C, unless otherwise noted.)
LED-SHORT FAULT PROTECTION (BRT = 100%, 2 LEDs SHORT ON FB3)
MAX8790A toc17
LED CURRENT BALANCING vs. INPUT VOLTAGE (BRT = 100%)
LED CURRENT BALANCING ACCURACY (%) 0V VFB3 1V/div VFB1 10V/div 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0 7 12 17 INPUT VOLTAGE (V) 750kHz 500kHz 1MHz
MAX8790A toc18
1.00
0V
VOUT 0V 20V/div IL 1A/div
0A 10ms/div
Pin Description
PIN 1 NAME OSC FUNCTION 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. 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. 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 50s minimum. See the Dimming Control section for a complete description. 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. 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. 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. 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. Ground 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. 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.
2
ENA
3
BRT
4 5 6 7 8 9 10
SHDN FB1 FB2 FB3 GND FB4 FB5
8
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Six-String White LED Driver with Active Current Balancing for LCD Panel Applications
Pin Description (continued)
PIN 11 12 13 14 NAME FB6 CS EXT OV FUNCTION 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. 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. External MOSFET Gate-Drive Output 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). 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 1F 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. Supply Input. VIN biases the internal 5V linear regulator that powers the device. Bypass IN to GND directly at the pin with a 0.1F or greater ceramic capacitor. Step-Up Converter Compensation Pin. Connect a 0.1F ceramic capacitor and 1.2k resistor from CCV to GND. When the MAX8790A shuts down, CCV is discharged to 0V through an internal 20k resistor. Full-Scale LED Current Adjustment Pin. The resistance from ISET to GND controls the full-scale current in each LED string: ILEDmax = 20mA x 100k/RISET The acceptable resistance range is 74k < RISET < 133k, which corresponds to full-scale LED current of 27mA > ILEDmax > 15mA. Connect ISET to VCC for a default full-scale LED current of 20mA. PLL Free-Running Frequency Control Pin. The resistance from FSET to GND controls the PLL oscillator's free-running frequency, fPLL: fPLL = 1 / (10 x RFSET x 800pF) The capture range is 0.6 x fPLL to fPLL. The acceptable resistance range for FSET is 250k < RFSET < 754k, which corresponds to a frequency range of 500Hz > fPLL > 166Hz. The resulting capture frequency range is 100Hz to 500Hz. Phase-Locked Loop-Compensation Capacitor Pin. The capacitance at CPLL compensates the PLL loop response. Connect a 0.1F 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.
MAX8790A
15
VCC
16 17
IN CCV
18
ISET
19
FSET
20 EP
CPLL EP
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9
Six-String White LED Driver with Active Current Balancing for LCD Panel Applications MAX8790A
Detailed Description
The MAX8790A is a high-efficiency driver for arrays of white LEDs. It contains a fixed-frequency, currentmode, PWM step-up controller, 5V linear regulator, dimming control circuit, and six regulated current sources (see Figure 2). When enabled, the step-up controller boosts the output voltage to provide sufficient headroom for the current sources to regulate their respective string currents. The MAX8790A features selectable switching frequency (500kHz, 750kHz, or 1MHz), which allows trade-offs between external component size and 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 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 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 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.
VOUT UP TO 35V
VIN 7V TO 21V CIN 0.1F
L1 4.7H
D1 COUT
SHDN 1F VCC ENA ISET
IN
EXT
N1
CS RS 56m GND R1 1M OV
BRT FSET 511k N.C. OSC
1.2k CCV 0.1F
MAX8790A
R2 37.4k
FB1 CPLL 0.1F FB2 FB3 FB4 EP FB5 FB6
Figure 1. Typical Operating Circuit
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Six-String White LED Driver with Active Current Balancing for LCD Panel Applications MAX8790A
OUTPUT OVERVOLTAGE COMPARATOR IN 5V LINEAR REGULATOR ERROR COMPARATOR
1.25V OV
VCC VCC OSCILLATOR
CLOCK
CONTROL AND DRIVER LOGIC
EXT
SLOPE COMPENSATION FB OVERVOLTAGE COMPARATOR VCC + 0.6V 65ms TIMER SHUTDOWN LATCH ERROR AMPLIFIER HVC
CURRENT SENSE
CS
OSC
TRI-LEVEL COMPARATOR
FB6 FB5 FB4
SHDN LVC gm
FB3 FB2
CCV
ISET FSET
REF ADJ OSC 256 x fBRT CLK
REF
SAT FB1
8-BIT DAC
EN
N
8 VCC - 0.4V DIGITAL CONTROL CURRENT SOURCE 8 CPLL PLL 8-BIT COUNTER 8 8-BIT LATCH CURRENT SOURCE 5 MSBs 5 LSBs DIGITAL COMPARATOR BRT CURRENT SOURCE ENA FB6 CURRENT SOURCE FB4 FB3 GND 10
CURRENT SOURCE
FB2
CURRENT SOURCE
FB5
Figure 2. Control Circuit Block Diagram
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Six-String White LED Driver with Active Current Balancing for LCD Panel Applications MAX8790A
Fixed-Frequency Step-Up Controller
The MAX8790A's fixed-frequency, current-mode, stepup controller automatically chooses the lowest active FB_ voltage to regulate the feedback voltage. Specifically, the difference between the lowest FB_ voltage and the current source-control signal plus an offset (VSAT) is integrated at the CCV output. The resulting error signal is compared to the external switch current plus slope compensation to terminate the switch ontime. As the load changes, the error amplifier sources or sinks current to the CCV output to adjust the required peak inductor current. The slope-compensation signal is added to the current-sense signal to improve stability at high duty cycles. At light loads, the MAX8790A automatically skips pulses to improve efficiency and prevent overcharging the 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 (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 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 1F or greater ceramic capacitor.
VIN 2.8V TO 5.5V CIN
L1 0.9H
D1 COUT
VOUT UP TO 22V
SHDN EXTERNAL 5V SUPPLY 1F VCC ENA ISET
IN
EXT
N1
CS RS 30m GND R1 1M OV
BRT FSET 511k N.C. OSC
1.2k CCV 0.1F
MAX8790A
R2 59k
FB1 CPLL 0.1F 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
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 180A 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. The MAX8790A is equipped with a bank of six matched current sources. These specialized current sources are accurate to within 1.5% and can be switched on and off within 10s, 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.
MAX8790A
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 10A. 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 GND Open VCC SWITCHING FREQUENCY (kHz) 500 750 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 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.
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: t TIMEOUT_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: t TIMEOUT_ANALOG = 8.125ms/DBRT The fault latch can be cleared by cycling the power or toggling the shutdown pin SHDN.
LED Current Sources
Maintaining uniform LED brightness and dimming capability are critical for LCD backlight applications.
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13
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
MAX8790A
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 MAX8790A.
Dimming Control
The MAX8790A features both analog and digital dimming control. Analog dimming can provide potentially higher converter efficiency because of low voltage drop across each WLED when the current is low. Digital dimming (DPWM) provides less WLED color distortion since the WLED current is held at full scale when the WLED is on. The MAX8790A's dimming control circuit consists of a PLL, a digital comparator, and a DAC. The controller provides 100:1 dimming range through either analog or digital control methods. Both methods translate the duty cycle of the BRT input into a control signal for the LED current sources. In analog dimming mode, the current-source outputs are DC and the BRT duty cycle (12.5% < DBRT < 100%) modulates the amplitude of the currents. For DBRT < 12.5%, the LED current is digitally modulated to reduce the average LED current down to 1% of full scale. The PLL detects the BRT frequency and phase, and adjusts the current-source amplitude and duty cycle synchronously (see Figure 4).
VSAT 450mV and VCC = 5V Error < 5.150V
N
D=
tON tBRT D = 50% D = 30% tON tBRT
ANALOG DIMMING MODE D = 12.5% D = 6.25%
BRT
ILEDMAX
ILED
0A
Figure 4. LED Current Control Using Analog Dimming Mode
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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). The full-scale current in both methods is specified by resistance from the ISET pin to ground: ILEDmax = 20mA x 100k RISET step-up converter's on-time. During the converter's offtime, the current sources are turned off. The output voltage does not discharge and stays high. Each FB_ pin can withstand 28V, which is the pin's maximum rated voltage. Table 2 summarizes the characteristics of both analog and digital dimming methods. A PLL translates the duty cycle of the BRT input into a reference for the MAX8790A's current sources. A resistor from the FSET pin to ground controls the PLL's freerunning frequency: 1 fPLL = 10 x RFSET x 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 0.1F capacitor is adequate for oscillator frequencies in the 166Hz < fPLL < 500Hz range. The PLL frequency capture window is 0.6 x fPLL to fPLL.
MAX8790A
The acceptable resistance range is 74k < RISET < 133k, which corresponds to full-scale LED current of 27mA > ILEDmax > 15mA. Connect ISET to VCC for a default full-scale LED current of 20mA. 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
D=
tON tBRT D = 50% D = 30% tON tBRT
DPWM DIMMING MODE D = 6.25%
D = 12.5%
BRT
ILEDMAX
ILED
0A
Figure 5. LED Current Control Using DPWM Dimming Mode
Table 2. Dimming Mode
MODE Analog + DPWM ENA > 2.1V PLL FREQUENCY 250k < RFSET < 754k VFSET > VCC - 0.4V, disables PLL CPLL 0.1F DESCRIPTION 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 dimming by BRT signal. BRT frequency can be 100Hz to 2kHz; 50s minimum BRT on-time limits the minimum brightness.
Direct DPWM
< 0.8V
OPEN
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15
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 50s 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
MAX8790A
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 +170C (typ), the controller and current sources shut down and do not restart until the die temperature drops by 15C.
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 Switching Frequency White LED Number of White LEDs 1MHz 3.2V (typ), 3.5V (max) at 20mA Nichia NSSW008C 6 series x 6 parallel, 20mA (max) FIGURE 1 750kHz 3.2V (typ), 3.5V (max) at 20mA Nichia NSSW008C 8 series x 6 parallel, 20mA (max) 7V to 21V 4.7H, 2.05A power inductor Sumida CDRH5D16-4R7 FIGURE 1 500kHz 3.2V (typ), 3.5V (max) at 20mA Nichia NSSW008C 10 series x 6 parallel, 25mA (max) 7V to 21V 4.7H, 3.6A power inductor Sumida CDRH8D28-4R7 FIGURE 1 750kHz 3.2V (typ), 3.5V (max) at 20mA Nichia NSSW008C 6 series x 6 parallel, 20mA (max) 2.8V to 5.5V, VCC = 5V 0.9H, 4.7A power inductor Sumida CDRH5D16-0R9 10F 10%, 10V X5R ceramic capacitor (1206) Murata GRM31MR61A106K 2.2F 10%, 50V X7R ceramic capacitor (1x) Murata GRM31CR71H225K FIGURE 3
Input Voltage 4.5V to 5.5V, VCC = IN Inductor L1 Input Capacitors 2.2H, 2.5A power inductor Sumida CDRH5D16-2R2
10F 10%, 10V X5R 10F 10%, 25V X5R 10F 10%, 25V X5R ceramic capacitor (1206) ceramic capacitor (1206) ceramic capacitor (1206) Murata GRM31MR61A106K Murata GRM31CR61E106KA Murata GRM31CR61E106KA 2.2F 10%, 50V X7R ceramic capacitor (1x) Murata GRM31CR71H225K 2.2F 10%, 50V X7R ceramic capacitor (1206) (1x) Murata GRM31CR71H225K 4.7F 10%, 50V X7R ceramic capacitor (1210) (1x) Murata GRM32ER71H475K
COUT Output Capacitor
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 2A, 30V Schottky diode Nihon EC21QS03L 50m 1%, 1/2W IRC LRC-LRF-1206LF-01R050-F 2A, 40V Schottky diode Toshiba CMS11 Nihon EC21QS04 56m 1%, 1/2W IRC LRC-LRF-1206LF-01R056-F
60V, 6A n-channel MOSFET (PowerPAK 1212-8) Vishay Si7308DN
30V, 4.9A n-channel MOSFET (6-pin TSOP) Vishay Si3456BDV
Diode Rectifier D1 Sense Resistor
3A, 60V Schottky diode Nihon EC31QS06 40m 1%, 1/2W IRC LRC-LRF-1206LF-01R040-F
3A, 30V Schottky diode Nihon EC31QS03L 30m 1%, 1/2W IRC LRC-LRF-1206LF-01R030-F
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Six-String White LED Driver with Active Current Balancing for LCD Panel Applications
Table 4. Component Suppliers
SUPPLIER Murata Nichia Sumida Toshiba Vishay PHONE 770-436-1300 248-352-6575 847-545-6700 949-455-2000 203-268-6261 WEBSITE www.murata.com www.nichia.com www.sumida.com www.toshiba.com/taec www.vishay.com
MAX8790A
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 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 (TYP) 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 TYP L= VOUT IOUT(MAX) x fOSC 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 x VIN(MIN) ) x RS
51mV x fOSC(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 R S can be temporarily calculated using the following equation: RS _ TMP = 100mV 1.2 x IIN(DCMAX) ,
where 100mV is the current-limit sense voltage.
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Six-String White LED Driver with Active Current Balancing for LCD Panel Applications MAX8790A
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 (MIN) taken from an appropriate curve in the Typical Operating Characteristics: IIN(DCMAX) = , IOUT(MAX) x VOUT VIN(MIN) x MIN 7V LDCM(MAX) = 1 - x 28.72V + 0.4V (7V)2 x 0.9 = 5.8H 2 x 0.825MHz x 28.72V x 120mA An inductance less than LDCM(MAX) is required, so a 4.7H inductor is chosen. The peak inductor current at minimum input voltage is calculated as follows:
IPEAK = 120mA x 2 x 28.72V x (28.72V + 0.4V - 7V ) 4.7H x 0.675MHz x 0.9 x (28.72V + 0.4V ) = 1.35A
Calculate the ripple current at that operating point and the peak current required for the inductor: IRIPPLE = VIN(MIN) x (VOUT(MAX) - VIN(MIN) ) L x VOUT(MAX) x fOSC
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 x (0.75 - DMAX ) IPEAK
I IPEAK = IIN(DCMAX) + 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) LDCM(MAX) = 1 - x VOUT(MAX) + VDIODE VIN(MIN)2 x 2 x fOSC(MAX) x VOUT(MAX) x IOUT(MAX) The peak inductor current in DCM mode is calculated using the following equation:
IPEAK = IOUT(max) x 2 x VOUT(MAX) x (VOUT(MAX) + VDIODE - VIN(MIN) ) L x fOSC(MIN) x x (VOUT(MAX) + VDIODE )
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 x ILIM x fOSC VIN(MIN)
For the typical operating circuit as Figure 1: DMAX = 4.7H x 1.35A x 0.75MHz = 0.68 7V = 64m
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 0.1 series resistance. Considering the typical operating circuit, the maximum load current (IOUT(MAX)) is 120mA with a 28.72V output and a minimal input voltage of 7V. Choosing a DCM operating mode and estimating efficiency of 90% at this operating point:
RS <
85mV + 25.6mV x (0.75 - 0.68) 1.35A
Again, RS is calculated as a maximum, so a 56m current-sense resistor is chosen.
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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) 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. 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) x L x fOSC x IPEAK 3 3 x VIN(MIN)
MAX8790A
For the above Si3458DV, the estimated conduction loss is:
PDRES(MAX) = 0.1 x 4.7H x 750kHz x 1.35A 3 = 0.04W 3 x 7V
The approximate maximum switching loss can be calculated as: t xI x VOUT x fOSC PDSW(MAX) = turn-off PEAK 2 For the above Si3458DV, the approximate switching loss is: PDSW(MAX) = 10ns x 1.35A x 28.72V x 750kHz = 0.145W 2
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.
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 x 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: Qg(MAX) x 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.
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 x (1 + R1 ) R2
In Figure 1, the output OVP voltage is set to: VOUT(OVP) = 1.23V x (1 + 1 M ) = 34.1V 37.4k
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Six-String White LED Driver with Active Current Balancing for LCD Panel Applications MAX8790A
Input Capacitor Selection
The input capacitor (CIN) filters the current peaks drawn from the input supply and reduces noise injection into the IC. A 10F 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 10C: IRMS = dIL 2x 3 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.
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.
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 0.27F.
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Six-String White LED Driver with Active Current Balancing for LCD Panel Applications
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 MAX8790A evaluation kit for an example of proper board layout.
MAX8790A
Pin Configuration
TOP VIEW
VCC
EXT
15 IN 16 CCV 17 ISET 18 FSET 19 CPLL 20 1 OSC
14
13
12
FB6 11 10 9 FB5 FB4 GND FB3 FB2
OV
CS
MAX8790AETP+
8 7 6
2 ENA
3 BRT
4 SHDN
5 FB1
4mm x 4mm THIN QFN
Chip Information
TRANSISTOR COUNT: 12,042 PROCESS: BiCMOS
______________________________________________________________________________________
21
Six-String White LED Driver with Active Current Balancing for LCD Panel Applications MAX8790A
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information go to www.maxim-ic.com/packages.)
24L QFN THIN.EPS
22
______________________________________________________________________________________
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.)
MAX8790A
Revision History
Pages changed at Rev 1: All A added to all pages.
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 23
(c) 2007 Maxim Integrated Products is a registered trademark of Maxim Integrated Products, Inc.

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