Datasheet

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General Description

The 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.

19-0658; Rev 1; 5/14

+Denotes a lead(Pb)-free/RoHS-compliant package.

PART TEMP RANGE PIN-PACKAGE

PKG

CODE

MAX8790ETP+ -40°C to +85°C

20 Thin QFN

(4mm x 4mm)

T2044-3

FB1

BRT

CCV

GND

EXT

FB2

FB3

FB4

FB5

FB6

ISET

FSET

CPLL

N.C.

OSC

N.C.

ENA

SHDN

0.1µF

OUT

MAX8790

MAX8790 Six-String White LED Driver with Active

Current Balancing for LCD Panel Applications

Simplied Operating Circuit

Ordering Information

EVALUATION KIT AVAILABLE

Summary of content (23 pages)

PAGE 1
EVALUATION KIT AVAILABLE MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications General Description 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.
PAGE 2
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)....
PAGE 3
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications Electrical Characteristics (continued) (Circuit of Figure 1. VIN = 12V, VSHDN = VIN, CCV = 0.1μF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.
PAGE 4
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 5.5 VCC bypassed to GND through 1µF cap 5.5 26.0 VSHDN = high VIN = 26V 2 VIN = VCC = 5V 2 VSHDN = 5V, 6V < VIN < 26V, 0 < IVCC < 10mA VCC Short-Circuit Current VCC UVLO Threshold MAX 4.
PAGE 5
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications Electrical Characteristics (continued) (Circuit of Figure 1. VIN = 12V, VSHDN = VIN, CCV = 0.1μF, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER CONDITIONS MIN TYP MAX UNITS LED CURRENT Full-Scale FB_ Output Current ISET 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.
PAGE 6
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications Typical Operating Characteristics (Circuit configuration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.) 88 TOTAL INPUT POWER, DPWM 0.4 7 0 12 17 INPUT VOLTAGE (V) 20.4 20.2 20.0 19.8 19.6 19.4 1 20 40 60 AMBIENT TEMPERATURE (°) 6 5 BRT = 100% 4 3 BRT = 0% 2 1 0 1000 1 DPWM DIMMING BRT = 10% 0.02 0.01 0 -0.
PAGE 7
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications Typical Operating Characteristics (continued) (Circuit configuration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.
PAGE 8
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications Typical Operating Characteristics (continued) (Circuit configuration 1, VIN = 12V, VSHDN = VIN, LEDs = 8 series x 6 parallel strings, ISET = VCC, TA = +25°C, unless otherwise noted.) LED-SHORT FAULT PROTECTION (BRT = 100%, 2 LEDs SHORT ON FB3) 0V 0V VFB3 1V/div VFB1 10V/div VOUT 0V 20V/div 0A IL 1A/div 1.00 MAX8790 toc18 MAX8790 toc17 LED CURRENT BALANCING ACCURACY (%) LED CURRENT BALANCING vs.
PAGE 9
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications Pin Description (continued) PIN NAME FUNCTION 11 FB6 LED String 6 Cathode Connection. FB6 is the open-drain output of an internal regulator, which controls current through FB6. FB6 can sink up to 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.
PAGE 10
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications Detailed Description 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 is a high-efficiency driver for arrays of white LEDs.
PAGE 11
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications OUTPUT OVERVOLTAGE COMPARATOR IN 1.25V ERROR COMPARATOR 5V LINEAR REGULATOR VCC OV CLOCK CONTROL AND DRIVER LOGIC EXT CURRENT SENSE CS VCC SLOPE COMPENSATION OSCILLATOR TRI-LEVEL COMPARATOR OSC 65ms TIMER SHUTDOWN LATCH SHDN ERROR AMPLIFIER CCV ISET REF ADJ FSET OSC 256 x fBRT FB6 HVC FB5 FB4 LVC FB3 gm FB2 SAT REF CLK FB OVERVOLTAGE COMPARATOR VCC + 0.
PAGE 12
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.
PAGE 13
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications 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. Shutdown When the SHDN pin is less than 0.
PAGE 14
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.
PAGE 15
MAX8790 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). reduce flicker noise in the display.
PAGE 16
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications most applications, a 0.1μF capacitor is adequate for oscillator frequencies in the 166Hz < fPLL < 500Hz range. The PLL frequency capture window is 0.6 x fPLL to fPLL. 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.
PAGE 17
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications Table 4. Component Suppliers SUPPLIER PHONE WEBSITE Murata 770-436-1300 www.murata.com Nichia 248-352-6575 www.nichia.com Sumida 847-545-6700 www.sumida.com Toshiba 949-455-2000 www.toshiba.com/taec Vishay 203-268-6261 www.vishay.com Thermal Shutdown The MAX8790 includes a thermal-protection circuit.
PAGE 18
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications 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)) × R S 51mV × f OSC(MIN) where 51mV is a scale factor based on slope compensation, and RS is the current-sense resistor.
PAGE 19
MAX8790 = D MAX RS < Six-String White LED Driver with Active Current Balancing for LCD Panel Applications 4.7µH × 1.35A × 0.75MHz = 0.68 7V 85mV + 25.6mV × (0.75 − 0.68) gate charge so the average current to drive the MOSFET at maximum switching frequency is less than 10mA: Q g(MAX) × f OSC < 10mA = 64mΩ For example, the Si3458DV is specified with 16nC of max total gate charge at Vg = 10V.
PAGE 20
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications 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 Input Capacitor Selection The input capacitor (CIN) filters the current peaks drawn from the input supply and reduces noise injection into the IC.
PAGE 21
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications D1 L1 VIN EXT N1 VOUT COUT SHDN TO VIN CLED MAX8790 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.
PAGE 22
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications Chip Information Pin Configuration OV EXT CS FB6 TRANSISTOR COUNT: 12,042 VCC TOP VIEW 15 14 13 12 11 PROCESS: BiCMOS IN 16 10 FB5 CCV 17 9 FB4 8 GND FSET 19 7 FB3 CPLL 20 6 FB2 MAX8790ETP+ 1 2 3 4 5 OSC ENA BRT SHDN FB1 ISET 18 4mm x 4mm THIN QFN www.maximintegrated.
PAGE 23
MAX8790 Six-String White LED Driver with Active Current Balancing for LCD Panel Applications Revision History REVISION NUMBER REVISION DATE 0 5/14 PAGES CHANGED DESCRIPTION No /V OPNs; removed automotive reference from Applications section 1 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.