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MAX8790ETP+-MAX8790ETP+T
High-Efficiency, 6-String, White-LED (WLED) Driver with Active Current Balancing for LCD Panel Applications
General DescriptionThe MAX8790 is a high-efficiency driver for white light-
emitting diodes (LEDs). It is designed for large liquid-
crystal displays (LCDs) that employ an array of LEDs as
the light source. A current-mode step-up controller drives
up to six parallel strings of multiple series-connected
LEDs. Each string is terminated with ballast that achieves
±1.5% current regulation accuracy, ensuring even bright-
ness for all LEDs. The 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 control-
ler 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 frequen-
cy (500kHz, 750kHz, or 1MHz), which allows trade-offs
between external component size and operating efficiency.
The MAX8790 is available in a thermally enhanced,
lead(Pb)-free, 20-pin, 4mm x 4mm, TQFN package.
Applications●Notebook, Subnotebook, and Tablet Computer Displays●Handy Terminals
Features●Drives Six Parallel Strings with Multiple Series-
Connected LEDs per String●±1.5% Current Regulation Accuracy Between Strings●Low 450mV Feedback Voltage at Full Current
Improves Efficiency●Step-Up Controller Regulates the Output Just Above
the Highest LED String Voltage●Full-Scale LED Current Adjustable from 15mA to
25mA, or Preset 20mA●Wide 100:1 Dimming Range●Programmable Dimming Control: Direct DPWM or
Analog Dimming●Built-In PLL for Synchronized Dimming Control●Open and Short LED Protections●Output-Overvoltage Protection●Wide Input Voltage Range from 4.5V to 26V●External MOSFET Allows a Large Number of LEDs
per String●500kHz/750kHz/1MHz Switching Frequency●Small, 20-Pin, 4mm x 4mm TQFN Package
Pin Configuration appears at end of data sheet.+Denotes a lead(Pb)-free/RoHS-compliant package.
PARTTEMP RANGEPIN-PACKAGEPKG
CODEMAX8790ETP+-40°C to +85°C20 Thin QFN
(4mm x 4mm)T2044-3
FB1
BRT
CCV
GND
VIN
EXT
FB2
FB3
FB4
FB5
FB6D1
VCC
ISET
CIN
FSET
CPLL
N.C.
OSCN.C.
ENA
SHDN
0.1µF
VOUT
MAX8790
MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Simpliied Operating Circuit
Ordering Information
EVALUATION KIT AVAILABLE
IN, SHDN, to GND..................................................-0.3V to +28V
FB_ to GND............................................................-0.3V to +28V
VCC, BRT, ENA, OSC, OV to GND...........................-0.3V to +6V
ISET, CCV, CS, FSET, CPLL, EXT to GND...-0.3V to (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
(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.)
PARAMETERCONDITIONSMINTYPMAXUNITSIN Input Voltage RangeVIN = VCC4.55.5VVCC = bypassed to GND through 1µF capacitor5.526.0
IN Quiescent CurrentVSHDN = high
VIN = 26V12VIN = VCC = 5V12
SHDN = GND10µA
VCC Output VoltageVSHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA4.75.05.3V
VCC Short-Circuit Current1556130mA
VCC UVLO ThresholdRising edge, hysteresis = 20mV4.004.254.45V
STEP-UP CONVERTEREXT High Level10mA from EXT to GNDVCC -
0.1VCCV
EXT Low Level-10mA from EXT to VCC00.1V
EXT On-ResistanceEXT high or low25Ω
EXT Sink/Source CurrentEXT forced to 2V1A
OSC High-Level ThresholdVCC -
0.4V
OSC Midlevel Threshold1.5VCC -
2.0V
OSC Low-Level Threshold0.4V
Operating Frequency
VOSC = VCC0.91.01.1MHz
VOSC = open675750825kHzVOSC = GND450500550
Minimum Duty CyclePWM mode10%Pulse skipping, no load0
Maximum Duty Cycle9495%
CS Trip VoltageDuty cycle = 75% 85100115mV
CONTROL INPUTSHDN Logic-Input High Level2.1V
SHDN Logic-Input Low Level0.8V
BRT, ENA Logic-Input High Level2.1V
MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Absolute Maximum RatingsStresses 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.)
PARAMETERCONDITIONSMINTYPMAXUNITS
INPUT LEAKAGESHDN Leakage CurrentVSHDN = 26V+35µA
CS Leakage Current VCS = GND+40+50µA
OSC Leakage Current-3+3µA
BRT, ENA Leakage Current-1+1µA
FSET, ISET Leakage CurrentFSET = ISET = VCC-1+1µA
OV Leakage Current-0.1+0.1µA
LED CURRENT Full-Scale FB_ Output Current
ISET = VCC, BRT = 100%19.4020.0020.60RISET = 80kΩ to GND, BRT = 100%24.2525.0025.75
RISET = 133kΩ to GND, BRT = 100%14.4015.0015.60
ISET High-Level ThresholdDefault setting for 20mA full-scale LED currentVCC -
0.4V
ISET Voltage1.121.191.26V
20% Output Current ISET = VCC, BRT = 20% 3.844.004.16mA
Current Regulation Between
Strings
ISET = VCC, BRT = 100% -1.5+1.5%
ISET = VCC, BRT = 20% -2.0+2.0%
Minimum FB_ Regulation Voltage
RISET = 80kΩ to GND, BRT = 100% 300500800ISET = VCC, BRT = 100% 270450720
ISET = VCC, 12.5% 150275500
Maximum FB_ RippleISET = VCC, COUT = 1µF, OSC = VCC (Note 1)120200mVP-P
FB_ On-ResistanceVFB_ = 50mV 1320Ω
FB_ Leakage CurrentSHDN = GND, VFB_ = 26V1µA
VSHDN = VIN, BRT = GND, VFB_ = 15V1028
BRT Input Frequency100500Hz
Minimum BRT Duty CyclePLL active 12.5%
FAULT PROTECTIONOV Threshold Voltage1.161.231.30V
FB_ Overvoltage ThresholdVCC +
VCC +
VCC +
1.45V
FAULT Shutdown TimerVFB_ > 5.6V (typ)506580ms
Thermal-Shutdown Threshold(Note 1)170°C
PHASE-LOCKED LOOPFSET High-Level ThresholdPLL disabledVCC -
0.4V
BRT Frequency Capture RangeRFSET = 500kΩ150200250HzRFSET = 250kΩ300400500
MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Electrical Characteristics (continued)
(Circuit of Figure 1. VIN = 12V, VSHDN = VIN, CCV = 0.1μF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2)
PARAMETERCONDITIONSMINTYPMAXUNITSIN Input Voltage Range
VIN = VCC4.55.5VCC bypassed to GND through 1µF cap5.526.0
IN Quiescent Current
VSHDN = highVIN = 26V2mA
VIN = VCC = 5V2
SHDN = GND10µA
VCC Output VoltageVSHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA4.75.3V
VCC Short-Circuit Current12130mA
VCC UVLO ThresholdRising edge, hysteresis = 20mV4.004.45V
STEP-UP CONVERTEREXT High Level10mA from EXT to GNDVCC -
0.1V
EXT Low Level-10mA from EXT to VCC0.1V
EXT On-ResistanceEXT high or low5Ω
OSC High-Level ThresholdVCC -
0.4V
OSC Midlevel Threshold1.5VCC
-2.0V
OSC Low-Level Threshold0.4V
Operating Frequency
VOSC = VCC0.91.1MHz
VOSC = open675825
kHzVOSC = GND450550
Maximum Duty Cycle94%
CS Trip VoltageDuty cycle = 75%85115mV
CONTROL INPUTSHDN Logic-Input High Level2.1V
SHDN Logic-Input Low Level0.8V
BRT, ENA Logic-Input High Level2.1V
BRT, ENA Logic-Input Low Level 0.8V
INPUT LEAKAGESHDN Leakage CurrentVSHDN = 26V+35µA
CS Leakage Current VCS = GND+50µA
OSC Leakage Current-3+3µA
BRT, ENA Leakage Current-1+1µA
FSET, ISET Leakage CurrentFSET = ISET = VCC-1+1µA
OV Leakage Current-0.1+0.1µA
MAX8790Six-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)
Note 1: Specifications are guaranteed by design, not production tested.
Note 2: Specifications to -40°C are guaranteed by design, not production tested.
PARAMETERCONDITIONSMINTYPMAXUNITS
LED CURRENT Full-Scale FB_ Output Current
ISET = VCC, BRT = 100%19.220.8RISET = 80kΩ to GND, BRT = 100%24.026.0
RISET = 133kΩ to GND, BRT = 100%14.415.6
ISET High-Level ThresholdDefault setting for 20mA full-scale LED currentVCC -
0.4V
ISET Voltage1.121.26V
20% Output Current ISET = VCC, BRT = 20% 3.84.2mA
Current Regulation Between
Strings
ISET = VCC, BRT = 100% -2+2ISET = VCC, BRT = 20% -3+3
Minimum FB_ Regulation Voltage
RISET = 80kΩ to GND, BRT = 100% 280840ISET= VCC, BRT = 100% 250760
ISET = VCC, BRT = 12.5% 140530
Maximum FB_ RippleISET= VCC, COUT = 1µF, OSC = VCC (Note 1)200mVP-P
FB_ On-ResistanceVFB_ = 50mV20Ω
FB_ Leakage CurrentSHDN = GND, VFB_ = 26V1µA
SHDN = VIN, BRT = GND, VFB_ = 15V28
BRT Input Frequency100500Hz
FAULT PROTECTIONOV Threshold Voltage1.161.30V
FB_ Overvoltage ThresholdVCC +
VCC +
1.45V
FAULT Shutdown TimerVFB_ > 5.6V (typ)5080ms
PHASE-LOCKED LOOPFSET High-Level ThresholdPLL disabledVCC -
0.4V
BRT Frequency Capture RangeRFSET = 500kΩ150250Hz
RFSET = 250kΩ300500Hz
MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Electrical Characteristics (continued)
(Circuit coniguration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.)
NORMALIZED POWER vs. TOTAL LED CURRENT
(ANALOG AND DPWM DIMMING)MAX8790 toc02
NORMALIZED POWER
TOTAL LED CURRENT (mA)
NORMALIZED TO VIN = 20V, AND ILED = 20mA
VIN = 7V
TOTAL LED
POWER, ANALOG
TOTAL INPUT
POWER, DPWM
TOTAL INPUT
POWER, ANALOG
TOTAL LED
POWER, DPWM
LED CURRENT vs. BRT DUTY CYCLE
(BRT AT 200Hz)MAX8790 toc03
LED CURRENT (mA)
BRT DUTY CYCLE (%)10100
IDENTICAL FOR DPWM DIMMING
AND ANALOG DIMMING
LED CURRENT
vs. AMBIENT TEMPERATURE (BRT = 100%)MAX8790 toc04
LED CURRENT (mA)
AMBIENT TEMPERATURE (°)
LED CURRENT REGULATION
vs. INPUT VOLTAGE
MAX8790 toc05
LED CURRENT REGULATION (%)
INPUT VOLTAGE (V)
ANALOG DIMMING
BRT = 10%
DPWM DIMMING
BRT = 100%
DPWM DIMMING
BRT = 10%
FB_ VOLTAGE vs. LED CURRENT
(ANALOG DIMMING)MAX8790 toc06
FB_ REGULATION VOLTAGE (V)
LED STRING CURRENT (mA)
SUPPLY CURRENT vs. INPUT VOLTAGE
(DPWM DIMMING)
MAX8790 toc07
SUPPLY CURRENT (mA)1217
BRT = 100%
BRT = 0%
BOOST CONVERTER EFFICIENCY
vs. INPUT VOLTAGE (BRT = 100%)MAX8790 toc01
BOOST CONVERTER EFFICIENCY (%)
INPUT VOLTAGE (V)1217
500kHz
750kHz
1MHz
SHUTDOWN CURRENT vs. INPUT VOLTAGEMAX8790 toc08
SHUTDOWN CURRENT (A)1217
SWITCHING WAVEFORMS
(BRT = 100%)MAX8790 toc09
200ns/div
500mA/div
0mA
VLX
10V/div
MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Typical Operating Characteristics
(Circuit coniguration 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)MAX8790 toc10
1µs/div
500mA/div
0mA
VLX
10V/div
STARTUP WAVEFORMS
(BRT = 100%, DPWM DIMMING)4ms/div
SHDN
5V/div
VOUT
20V/div
VCCV
2V/div
1A/div
MAX8790 toc11
LED CURRENT WAVEFORMS
(BRT = 50% AT 200Hz, DPWM DIMMING)2ms/div
BRT
5V/div
VFB1
5V/div
ILED
100mA/div
1A/div
0mA
MAX8790 toc12
LED CURRENT WAVEFORMS
(BRT = 1% AT 200Hz, DPWM DIMMING)1ms/div
BRT
5V/div
VFB1
5V/div
ILED
100mA/div
1A/div
0mA
MAX8790 toc13
LED CURRENT WAVEFORMS
(BRT = 1% AT 200Hz, ANALOG DIMMING)0mA
BRT
5V/div
VFB1
2V/div
ILED
50mA/div
500mA/div
0mA
MAX8790 toc15
LED CURRENT WAVEFORMS
(BRT = 50% AT 200Hz, ANALOG DIMMING)1ms/div
BRT
5V/div
VFB1
1V/div
ILED
50mA/div
1A/div
0mA
MAX8790 toc14
LED-OPEN FAULT PROTECTION
(BRT = 100%, LED OPEN ON FB3)VFB3
1V/div
VFB1
10V/div
VOUT
20V/div
1A/div
MAX8790 toc16
MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Typical Operating Characteristics (continued)
(Circuit coniguration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.)
PINNAMEFUNCTIONOSCOscillator 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. 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. 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.SHDNShutdown 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. FB1LED 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.FB2LED 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.FB3LED 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.GNDGroundFB4LED 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.FB5LED 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.
LED-SHORT FAULT PROTECTION
(BRT = 100%, 2 LEDs SHORT ON FB3)10ms/div
VFB3
1V/div
VFB1
10V/div
VOUT
20V/div
1A/div
MAX8790 toc17
LED CURRENT BALANCING
vs. INPUT VOLTAGE (BRT = 100%)MAX8790 toc18
LED CURRENT BALANCING ACCURACY (%)
INPUT VOLTAGE (V)
750kHz
500kHz
1MHz
MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Pin Description
Typical Operating Characteristics (continued)
PINNAMEFUNCTIONFB6LED 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.CSStep-Up Controller Current-Sense Input. Connect the CS input to a ground-referenced sense resistor to
measure the current in the external MOSFET switch. EXTExternal MOSFET Gate-Drive OutputOVOvervoltage 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).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.INSupply 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.CCV
Step-Up Converter Compensation Pin. Connect a 0.1µF ceramic capacitor and 1.2-free
Ω 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. 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.CPLLPhase-Locked Loop-Compensation Capacitor Pin. The capacitance at CPLL compensates the PLL loop
response. Connect a 0.1µF ceramic capacitor from CPLL to GND.EPExposed Backside Pad. Solder to the circuit board ground plane with suficient copper connection to ensure
low thermal resistance. See the PCB Layout Guidelines section.
MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Pin Description (continued)
Detailed DescriptionThe MAX8790 is a high-efficiency driver for arrays of
white LEDs. It contains a fixed-frequency, current-mode,
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 operat-
ing efficiency. The control architecture automatically skips
pulses at light loads to improve efficiency and prevents
overcharging the output capacitor.
A PWM logic input signal, BRT, controls the LED bright-
ness. The 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 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 sig-
nal that linearly controls the LED current, down to a PWM
duty factor of 12.5%.
The MAX8790 has multiple features to protect the control-
ler from fault conditions. Separate feedback loops limit
the output voltage if one or more LEDs fail open or short.
During operation, if one of the feedback string voltages
exceeds the VCC to 0.6V (typ) protection threshold, the
controller shuts down and latches off after an internal
timer expires. The controller features cycle-by-cycle cur-
rent limit to provide consistent operation and soft-start
capability. A thermal-shutdown circuit provides another
level of protection.
The 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 inter-
nal LDO is disabled when SHDN is low.
FB1
BRT
CCV
GND
VIN
7V TO 21V
EXT
CPLLFB2
FB3
FB4
FB5
FB6
4.7µHD1
VCC
ISET
FSET
CIN
ENA
OSCN.C.
0.1µF
0.1µF
1µF
511kΩ
1.2kΩ
37.4kΩ
1MΩ
56mΩ
SHDN
0.1µF
COUT
VOUT
UP TO 35V
MAX8790MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
5V LINEAR
REGULATOR
OSCILLATORSLOPE
COMPENSATION
CLOCK
FB OVERVOLTAGE
COMPARATOR
ERROR
COMPARATOR
OUTPUT OVERVOLTAGE
COMPARATOR
EXT
ERROR
AMPLIFIER
CURRENT SOURCE
FB1
GND
FB2
65ms TIMER
SHUTDOWN
LATCH
CONTROL AND
DRIVER LOGIC
CURRENT SENSE
HVCFB6
FB5
FB4
FB3
FB2
LVC
TRI-LEVEL
COMPARATOR
VCC
OSC
REF ADJ
CLK
REF
VCC - 0.4V
VCC + 0.6V
1.25V
ISET
PLL
5 LSBs
5 MSBs
CPLL
BRT
ENA
OSC
256 x fBRT
DIGITAL
COMPARATOR
8-BIT
LATCH
8-BIT DAC
SAT
DIGITAL CONTROL
CURRENT SOURCE
8-BIT
COUNTER
FSET
CCV
VCC
SHDN
FB3CURRENT SOURCE
FB4CURRENT SOURCE
FB5CURRENT SOURCE
FB6CURRENT SOURCE
10Ω
MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
Fixed-Frequency Step-Up ControllerThe 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 out-
put capacitor. In SKIP mode, the inductor current ramps
up for a minimum on-time of approximately 150ns, then
discharges the stored energy to the output. The switch
remains off until another pulse is needed to boost the
output voltage.
Internal 5V Linear Regulator VCC and UVLOThe MAX8790 includes an internal low-dropout linear
regulator (VCC). When VIN is higher than 5.5V and
SHDN is high, this linear regulator generates a 5V supply
to power an internal PWM controller, control logic, and
MOSFET driver. This linear regulator can deliver at least
10mA of total additional load current. If VIN is less than or
equal to 5.5V, VCC and IN can be connected together and
powered from an external 5V supply. There is an internal
diode from VCC to IN, so VIN must be greater than VCC
(see Figure 2).
The 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.
Figure 3. Low-Input-Voltage Application Circuit
FB1
BRT
CCV
GND
VIN
2.8V TO 5.5V
EXTERNAL
5V SUPPLY
EXT
CPLLFB2
FB3
FB4
FB5
FB6
0.9µHD1
VCC
ISET
FSET
CIN
ENA
OSCN.C.
0.1µF
0.1µF
1µF
511kΩ
1.2kΩ
59kΩ
1MΩ
30mΩ
SHDN
COUT
VOUT
UP TO 22V
MAX8790MAX8790Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
