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19-4485; Rev 0; 2/09
KIT ATION EVALU BLE AVAILA
8-String White LED Driver with SMBus for LCD Panel Applications
General Description Features
o Accurate Dimming Control Using SMBus, PWM Interface o Dimming Range with 8-Bit Resolution o Adjustable DPWM Frequency with 1.5% Accuracy o Up to Eight Parallel Strings Multiple SeriesConnected LEDs o 1.5% Current Regulation Accuracy Between Strings o Low String Feedback Voltage: 625mV at 25mA LED Current o Full-Scale LED Current Adjustable from 15mA to 30mA, or Preset 25mA o Open and Short LED Protections o Output Overvoltage Protection o 0.15 Internal HV Power MOSFET (45V max) o Wide Input-Voltage Range from 4.5V to 26V o 500kHz/750kHz/1MHz Selectable Switching Frequency o Small 28-Pin, 4mm x 4mm, Thin QFN package
MAX17061A
The MAX17061A 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. An internal switch current-mode step-up controller drives the LED array, which can be configured for up to eight strings in parallel and 10 LEDs per string. Each string is terminated with ballast that achieves 1.5% current-regulation accuracy between strings, ensuring even LED brightness. The MAX17061A has a wide inputvoltage range from 7.5V to 26V, and provides a fixed 25mA or adjustable 15mA to 30mA full-scale LED current. The MAX17061A internally generates a DPWM signal for accurate WLED dimming control. The DPWM frequency is resistor programmable, while DPWM duty cycle is controlled directly from an external PWM signal or through a control word through the MAX17061A's SMBusTM interface. This DPWM control provides a dimming range with 8-bit resolution and supports Intel display-power-saving technology (DPST) to maximize battery life. The MAX17061A has multiple features to protect the controller from fault conditions. Separate feedback loops limit the output voltage under any circumstance, ensuring safe operation. Once an open string is detected, the string is disabled while other strings operate normally. The MAX17061A also features short LED detection. The shorted strings are also disabled after a 2ms fault blanking interval. The controller features cycle-by-cycle current limit to provide stable operation and soft-start capability. If the MAX17061A is in currentlimit condition, the step-up converter is latched off after an internal timer expires. A thermal-shutdown circuit provides another level of protection. The MAX17061A's step-up controller features an internal 150m (typ), 45V (max) power MOSFET with local current-sense amplifier for accurate cycle-by-cycle current limit. This architecture greatly simplifies the external circuitry and saves PCB space. Low-feedback voltage at each LED string 625mV (typ) at 25mA LED current helps reduce power loss and improve efficiency. The MAX17061A features selectable switching frequency (500kHz, 750kHz, or 1MHz), which enables a wide variety of applications that can trade off component size for operating frequency. The MAX17061A is available in a thermally enhanced 28-pin, 4mm x 4mm Thin QFN package.
Ordering Information
PART MAX17061AETI+ TEMP RANGE -40oC to +85oC PIN-PACKAGE 28 Thin QFN
+Denotes a lead(Pb)-free/RoHS-compliant package.
Simplified Operating Circuit
L1 VIN D1 VOUT
VCC 5V
VDD IN VCC ISET
LX1, 2 PGND1, 2 R1
CCV
OV R2
MAX17061A
PWM PWMI PWMO FB1 GND N.C. CLK DATA OSC SCL SDA FSET EP FB2 FB3 FB4 FB5 FB6 FB7 FB8
Applications
Notebook, Subnotebook, and Tablet Computer Displays Automotive Systems Handy Terminals
Pin Configuration appears at end of data sheet. SMBus is a trademark of Intel Corp.
________________________________________________________________ Maxim Integrated Products
1
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.
8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
ABSOLUTE MAXIMUM RATINGS
IN to GND ..............................................................-0.3V to +28V FB_, LX_ to GND ....................................................-0.3V to +45V PGND_ to GND......................................................-0.3V to +0.3V VCC, VDD, PWMI, SDA, SCL to GND ........................-0.3V to +6V ISET, CCV, PWMO, FSET, OSC, OV to GND ...............................................-0.3V to VCC + 0.3V Continuous Power Dissipation (TA = +70C) 28-Pin Thin QFN (derate 16.9mW/C above +70C) .1667mW 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, CCCV = 0.022F, RCCV = 5.1k, VISET = VOSC = VDD = VCC = 5V, RFSET = 464k, VPWMI = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER IN Input Voltage Range IN UVLO Threshold IN Input Current VCC Input Voltage VCC UVLO Threshold VCC Quiescent Current VCC Shutdown Current BOOST CONVERTER LX On-Resistance LX Leakage Current Operating Frequency 20mA from LX_ to PGND 45V on LX_, TA = +25C VOSC = VCC VOSC = open VOSC = GND OSC High-Level Threshold OSC Midlevel Threshold OSC Low-Level Threshold Minimum Duty Cycle Maximum Duty Cycle LX Current Limit CONTRL INPUT SDA, SCL Logic Input-High Level SDA, SCL Logic Input-Low Level PWMI Logic Input-High Level PWMI Logic Input-Low Level 2.1 0.8 2.1 0.8 V V V Duty cycle = 75% (Note 1) PWM mode (Note 1) 94 1.6 10 95.5 97 0.9 675 450 VCC 0.4 1.5 VCC 2.0 0.4 1.0 750 500 0.15 0.3 1 1.1 825 550 A MHz kHz V V V % % A Rising edge, typical hysteresis = 10mV MAX17061A is enabled MAX17061A is disabled Rising edge Falling edge VIN = 26V MAX17061A is enabled MAX17061A is disabled, TA = +25C +1 4.5 4.00 5 4.25 1.24 CONDITIONS MIN 7.5 7.08 6.86 7.2 6.98 70 TYP MAX 26 7.32 7.1 110 +1 5.5 4.45 2 10 UNITS V V A V V mA A
2
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8-String White LED Driver with SMBus for LCD Panel Applications
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VIN = 12V, CCCV = 0.022F, RCCV = 5.1k, VISET = VOSC = VDD = VCC = 5V, RFSET = 464k, VPWMI = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER INPUT LEAKAGE PWMI Leakage Current ISET, FSET Leakage Current OV Leakage Current SDA, SCL Input Bias Current SDA Output Low Sink Current OSC Leakage Current LED CURRENT ISET = VCC Full-Scale FB_ Output Current RISET = 133k RISET = 200k RISET = 266k ISET High-Level Threshold ISET Output Voltage Current Regulation Between Strings ISET = VCC IFB_ = 25mA Minimum FB_ Regulation Voltage Maximum FB_ Ripple FB_ On-Resistance FB_ Leakage Current FAULT PROTECTION OV Threshold Voltage FB_ Overvoltage Threshold FB_ Undervoltage Threshold Boost Global Fail Thermal-Shutdown Threshold Overcurrent FAULT Shutdown Timer FB_ Overvoltage Fault Timer (Note 1) I PEAK > 3.3A at duty = 0% or 1.8A at duty = 90% (typical) 88 1.7 Rising edge, typical hysteresis = 60mV 1.166 5.2 130 48 1.236 5.6 175 84 160 128 2 168 2.3 1.306 6.0 220 120 V V mV mV C s ms IFB_ = 30mA IFB_ = 20mA IFB_ = 15mA IFB_ = 20mA (COUT = 1F, OSC = VCC) (Note 1) VFB_ = 50mV (includes 10 VFB_ = 26V, TA = +25C VFB_ = 45V, TA = +25C 2.5 sense resistor) Default setting 24.5 28.8 19.3 14.4 VCC 0.4 1.166 -1.5 475 575 380 285 625 750 500 375 120 17.5 1.236 1.306 +1.5 910 1100 740 560 200 28.4 1 4 A mVP-P mV 25.0 30.0 20.0 15.0 25.5 31.2 20.7 15.6 V V % mA TA = +25C ISET, FSET to VCC, TA = +25C TA = +25C TA = +25C VSDA = 0.4V TA = +25C -0.1 -1 4 -3 +3 -0.3 +0.3 +1 +0.1 +1 A A A A mA A CONDITIONS MIN TYP MAX UNITS
MAX17061A
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1. VIN = 12V, CCCV = 0.022F, RCCV = 5.1k, VISET = VOSC = VDD = VCC = 5V, RFSET = 464k, VPWMI = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER PWM FILTER PWM Output Impedance RFSET = 464k DPWM Oscillator Frequency RFSET = 113k RFSET = 65k RFSET = 42k PWMI Input Frequency Range PWMI Full-Range Accuracy PWMI duty cycle = 100% PWMI Brightness Setting SMBus TIMING SPECIFICATION SMBus Frequency Bus Free Time START Condition Hold Time from SCL START Condition Setup Time from SCL STOP Condition Setup Time from SCL SDA Hold Time from SCL SDA Setup Time from SCL SCL Low Period SCL High Period FSMB TBUF THD:STA TSU:STA TSU:STO THD:DAT TSU:DAT TLOW THIGH 10 4.7 4 4.7 4 300 250 4.7 4 100 kHz s s s s s s s s PWMI duty cycle = 50% PWMI duty cycle = 0% 98 48 2.6 100 50 2.7 52 2.8 % 5 20 197 750 1.270 40 200 785 1.335 2 10 100 5 60 203 820 1.400 k Hz kHz kHz LSB CONDITIONS MIN TYP MAX UNITS
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 1, VIN = 12V, CCCV = 0.022F, RCCV = 5.1k, VISET = VOSC = VDD = VCC, 5V = RFSET = 464k, VPWMI = GND, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER IN Input Voltage Range IN UVLO Threshold IN Pin Input Current VCC Input Voltage VCC UVLO Threshold VCC Quiescent Current VCC Shutdown Current Rising edge, typical hysteresis = 10mV MAX17061A is enabled MAX17061A is disabled Rising edge Falling edge VIN = 26V MAX17061A is enabled 4.5 4.00 CONDITIONS MIN 7.5 7.08 6.86 TYP MAX 26 7.32 7.1 110 5.5 4.45 2 10 UNITS V V A V V mA A
4
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8-String White LED Driver with SMBus for LCD Panel Applications
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VIN = 12V, CCCV = 0.022F, RCCV = 5.1k, VISET = VOSC = VDD = VCC, 5V = RFSET = 464k, VPWMI = GND, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER BOOST CONVERTER LX On-Resistance Operating Frequency 20mA from LX_ to PGND VOSC = VCC VOSC = open VOSC = GND OSC High-Level Threshold OSC Midlevel Threshold OSC Low-Level Threshold Maximum Duty Cycle LX Current Limit CONTRL INPUT SDA, SCL Logic Input-High Level SDA, SCL Logic Input-Low Level PWMI Logic Input-High Level PWMI Logic Input-Low Level INPUT LEAKAGE SDA Output Low Sink Current LED CURRENT ISET = VCC Full-Scale FB_ Output Current RISET = 133k RISET = 200k RISET = 266k ISET High-Level Threshold ISET Output Voltage Current Regulation Between Strings ISET = VCC IFB_ = 25mA Minimum FB_ Regulation Voltage IFB_ = 30mA IFB_ = 20mA IFB_ = 15mA Maximum FB_ Ripple FB_ On-Resistance IFB_ = 20mA (COUT = 1F, OSC = VCC) (Note1) VFB_ = 50mV (includes 10 sense resistor) Default setting 24.5 28.6 19.0 14.4 VCC 0.4 1.166 -1.5 425 575 380 285 1.306 1.5 910 1100 740 560 200 28.4 mVP-P mV 25.5 31.4 21.0 15.6 V V % mA VSDA = 0.4V 4 mA 2.1 0.8 2.1 0.8 V V V Duty cycle = 75% (Note 1) 94 1.6 0.89 675 450 VCC 0.4 1.5 VCC 2.0 0.4 97 0.3 1.10 825 560 MHz kHz V V V % A CONDITIONS MIN TYP MAX UNITS
MAX17061A
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 1, VIN = 12V, CCCV = 0.022F, RCCV = 5.1k, VISET = VOSC = VDD = VCC, 5V = RFSET = 464k, VPWMI = GND, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER FAULT PROTECTION OV Threshold Voltage FB_ Overvoltage Threshold FB_ Undervoltage Threshold Boost Global Fail Overcurrent FAULT Shutdown Timer FB_ Overvoltage Fault Timer PWM FILTER PWM Output Impedance RFSET = 464k DPWM Oscillator Frequency PWMI Input Frequency Range PWMI Full-Range Accuracy PWMI duty cycle = 100% PWMI Brightness Setting SMBus TIMING SPECIFICATION SMBus Frequency Bus Free Time START Condition Hold Time from SCL START Condition Setup Time from SCL STOP Condition Setup Time from SCL SDA Hold Time from SCL SDA Setup Time from SCL SCL Low Period SCL High Period FSMB TBUF THD:STA TSU:STA TSU:STO THD:DAT TSU:DAT TLOW THIGH 10 4.7 4 4.7 4 300 250 4.7 4 100 kHz s s s s s s s s PWMI duty cycle = 50% PWMI duty cycle = 0% 98 48 2.6 52 2.8 % RFSET = 113k RFSET = 65k 20 197 750 1.27 5 60 203 820 1.40 100 5 k Hz kHz kHz LSB I PEAK > 3.3A at duty = 0% or 1.8A at duty = 90% (typ) Rising edge, typical hysteresis = 60mV 1.166 5.2 130 48 88 1.6 1.306 6.0 220 120 168 2.4 V V mV mV s ms CONDITIONS MIN TYP MAX UNITS
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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8-String White LED Driver with SMBus for LCD Panel Applications
Typical Operating Characteristics
(Circuit of Figure 1, VIN = 12V, CCCV = 0.022F, RCCV = 5.1k, VISET = VOSC = VDD = VCC, RFSET = 464k, VPWMI = GND, LEDs = 10 series x 4 parallel strings, TA = +25C, unless otherwise noted.)
BOOST CONVERTER EFFICIENCY vs. INPUT VOLTAGE (BRIGHTNESS = 100%)
MAX17061A toc01
MAX17061A
LED CURRENT vs. SMBus BRIGHTNESS SETTING
MAX17061A toc02
LED CURRENT vs. PWMI DUTY CYCLE
MAX17061A toc03
96 BOOST CONVERTER EFFICIENCY (%) 95 94 93 92 91 90 5 10 15 INPUT VOLTAGE (V) 20
30 25 LED CURRENT (mA) 20 15 10 5 0
30 25 LED CURRENT (mA) 20 15 SMBus = 128 10 5 0 SMBus = 255
25
0
50
100
150
200
250
0
20
40
60
80
100
SMBus BRIGHTNESS SETTING
PWMI DUTY CYCLE (%)
LED CURRENT vs. AMBIENT TEMPERATURE (BRIGHTNESS = 100%)
MAX17061A toc04
LED CURRENT vs. INPUT VOLTAGE
MAX17061A toc05
26.0 25.8 25.6 LED CURRENT (mA) 25.4 25.2 25.0 24.8 24.6 24.4 24.2 24.0 0 20 40 60 80 AMBIENT TEMPERATURE (C)
30 25 LED CURRENT (mA) 20 15 10 5 0 5 8 11 14 INPUT VOLTAGE (V) 17 20 SMBus = 0x1F
SMBus = 0xFF
VCC SUPPLY CURRENT vs. INPUT VOLTAGE
MAX17061A toc06
VCC SHUTDOWN CURRENT vs. INPUT VOLTAGE
5.98 SHUTDOWN CURRENT (A) 5.96 5.94 5.92 5.90 5.88 5.86 5.84 5.82
MAX17061A toc07
5
6.00
4 SUPPLY CURRENT (mA) SMBus = 255 3
2
1
SMBus = 0
0 5 10 15 20 INPUT VOLTAGE (V) 25
5.80 5 10 15 20 INPUT VOLTAGE (V) 25
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 12V, CCCV = 0.022F, RCCV = 5.1k, VISET = VOSC = VDD = VCC, RFSET = 464k, VPWMI = GND, LEDs = 10 series x 4 parallel strings, TA = +25C, unless otherwise noted.)
SWITCHING WAVEFORMS (VIN = 7V, BRIGHTNESS = 100%)
MAX17061A toc08
SWITCHING WAVEFORMS (VIN = 20V, BRIGHTNESS = 100%)
MAX17061A toc09
VLX 20V/div 0V
VLX 20V/div 0V
IL 200mA/div IL 200mA/div
0mA
0mA
1s/div
1s/div
STARTUP WAVEFORMS (BRIGHTNESS = 100%)
MAX17061A toc10
STARTUP WAVEFORMS (SMBus = 0x04)
MAX17061A toc11
VLX 20V/div 0V 0A IL 1A/div VOUT 20V/div 0V VCCV 2V/div 0V 400s/div 0V 400s/div 0V 0V 0A
VLX 20V/div IL 1A/div VOUT 20V/div
VCCV 2V/div
LED CURRENT WAVEFORMS (SMBus = 0x80)
MAX17061A toc12
LED CURRENT WAVEFORMS (SMBus = 0x04)
MAX17061A toc13
VFB1 10V/div 0V ILED 20mA/div 0mA IL 500mA/div 0mA VOUT 20V/div 0V 2ms/div 0V 2ms/div 0mA 0mA 0V
VFB1 10V/div
ILED 20mA/div IL 500mA/div VOUT 20V/div
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8-String White LED Driver with SMBus for LCD Panel Applications
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VIN = 12V, CCCV = 0.022F, RCCV = 5.1k, VISET = VOSC = VDD = VCC, RFSET = 464k, VPWMI = GND, LEDs = 10 series x 4 parallel strings, TA = +25C, unless otherwise noted.)
LED-OPEN FAULT PROTECTION (BRIGHTNESS = 100%, LED OPEN ON FB1)
MAX17061A toc14
MAX17061A
LED-SHORT FAULT PROTECTION (BRT = 100%, 2 LEDs SHORT ON FB1)
MAX17061A toc15
VFB1 1V/div 0V IFB2 10V/div 0V VOUT 20V/div 0V IFB2 20mA/div 0mA 400s/div 0mA 1ms/div 0mA IFB2 20mA/div 0V 0V VFB1 5V/div VFB2 10V/div IFB1 20mA/div
LINE TRANSIENT RESPONSE (VIN = 12V19V, BRIGHTNESS = 100%)
MAX17061A toc16
LINE TRANSIENT RESPONSE (VIN = 19V12V, BRIGHTNESS = 100%)
MAX17061A toc17
VOUT 20V/div 0V 0V VIN 10V/div IL 1A/div 0A IFB1 20mA/div 0mA 100s/div 0mA 100s/div 0A 0V 0V
VOUT 20V/div
VIN 10V/div IL 1A/div IFB1 20mA/div
LED CURRENT BALANCE vs. INPUT VOLTAGE
MAX17061A toc18
0.30 0.25 0.20 0.15 0.10 0.05 0 5 8 11 14 INPUT VOLTAGE (V) 17 20
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LED CURRENT BALANCE (%)
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
Pin Description
PIN 1 2 3 4, 6, 18 5 7 8 9 10 NAME FB3 FB4 GND N.C. FB5 FB6 FB7 FB8 OSC FUNCTION 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 30mA. If unused, connect FB3 to VCC. 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 30mA. If unused, connect FB4 to VCC. Analog Ground No Connection 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 30mA. If unused, connect FB5 to VCC. 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 30mA. If unused, connect FB6 to VCC. LED String 7 Cathode Connection. FB7 is the open-drain output of an internal regulator, which controls current through FB7. FB7 can sink up to 30mA. If unused, connect FB7 to VCC. LED String 8 Cathode Connection. FB8 is the open-drain output of an internal regulator, which controls current through FB8. FB8 can sink up to 30mA. If unused, connect FB8 to VCC. 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. PWM Signal Input. This PWM signal is used for brightness control in PWM mode or DPST mode. This signal is filtered and its duty cycle is converted into a digital signal to calculate DPWM duty cycle. In PWM mode, the DPWM duty cycle equals the input PWM duty cycle. In DPST mode, the DPWM duty cycle is the input PWM duty cycle multiplied by the SMBus brightness command. Filtered PWM Signal Output. Connect a capacitor between PWMO and GND. The capacitor forms a lowpass filter with an internal 40k (typ) resistor to filter the PWM signal into an analog signal whose level represents the duty-cycle information of the input PWM signal. DPWM Frequency Adjustment Pin. Connect a resistor from FSET to GND to set the internal DPWM frequency:
11
PWMI
12
PWMO
13
FSET where:
fDPWM =
109 x R[] +
= 10.638 = 58509 This DPWM signal directly chops WLED current with the calculated duty cycle for brightness control. SMBus Serial-Data Input
14 15 16 17 19 20 21
SDA SCL LX2 LX1 PGND2 PGND1 IN
SMBus Serial-Clock Input Boost Regulator Internal MOSFET Drain. Connect the inductor and the Schottky diode to LX2 node. LX2 should always be shorted to LX1 externally. Boost Regulator Internal MOSFET Drain. Connect the inductor and the Schottky diode to LX1 node. LX1 should always be shorted to LX2 externally. Boost Regulator Power Ground Boost Regulator Power Ground Supply Input, 7.5V to 26V. Bypass IN to GND directly at the pin with a 0.1F or greater ceramic capacitor. When IN voltage is below its UVLO threshold, the MAX17061A turns off the output.
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8-String White LED Driver with SMBus for LCD Panel Applications
Pin Description (continued)
PIN 22 23 24 25 NAME VDD VCC CCV OV FUNCTION Boost Regulator MOSFET Gate Drive Supply. Bypass VDD to GND with a ceramic capacitor of 1F or greater. 5V Control Supply Input. VCC provides power to the MAX17061A. Bypass VCC to GND with a ceramic capacitor of 1F or greater. Step-Up Converter Compensation Pin. Connect a 0.022F ceramic capacitor and 5.1k resistor from CCV to GND. When the MAX17061A shuts down, CCV is discharged to 0V through an internal 20k resistor. 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 1.236V (typ). Full-Scale LED Current Adjustment Pin. The resistance from ISET to GND controls the full-scale current in each LED string: ILEDMAX = 20mA x 200k /RISET The acceptable resistance range is 133k < RISET < 266k , which corresponds to full-scale LED current of 30mA > ILEDMAX > 15mA. Connect ISET to VCC for a default full-scale LED current of 25mA. 27 28 -- FB1 FB2 EP 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 30mA. If unused, connect FB1 to VCC. 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 30mA. If unused, connect FB2 to VCC. 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.
MAX17061A
26
ISET
Detailed Description
The MAX17061A is a high-efficiency driver for arrays of white LEDs. It contains a fixed-frequency current- mode PWM step-up controller, dimming control circuit, SMBus interface, internal power MOSFET, and eight regulated current sources (see Figure 2). When enabled, the stepup controller boosts the output voltage to provide sufficient headroom for the current sources to regulate their respective string currents. The MAX17061A features selectable switching frequency (500kHz, 750kHz, or 1MHz), which allows trade-offs between external component size and operating efficiency. WLED brightness is controlled by turning the WLEDs on and off with a DPWM signal. The DPWM frequency can be accurately adjusted with a resistor. The brightness of the LEDs is proportional to the duty cycle of the DPWM signal, which is controlled externally through either a PWM or 2-wire SMBus-compatible interface, or both. When both interfaces are used at the same time, the product of the PWM duty cycle and SMBus command value is used for the dimming control. This DPWM control scheme provides a full dimming range with 8-bit resolution.
The MAX17061A has multiple features to protect the controller from fault conditions. Separate feedback loops limit the output voltage in all circumstances. The MAX17061A checks each FB_ voltage during the operation. If one or more strings are open, the corresponding FB_ voltages are pulled below 175mV (typ), and opencircuit fault is detected. As a result, the respective current sources are disabled. When one or more LEDs are shorted and the FB_ voltage exceeds 1.1 x VCC, short fault is detected and the respective current source is disabled. In either LED open or short conditions, the fault strings are disabled while other strings can still operate normally. The controller features cycle-bycycle current limit to provide stable operation and softstart protection. In a current-limit condition, the controller shuts down after a 128s overcurrent fault timer expires. A thermal-shutdown circuit provides another level of protection. The MAX17061A requires an external 5V supply to provide the internal bias and gate drive for the step-up controller.
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
Fixed-Frequency Step-Up Controller
The MAX17061A's fixed-frequency, current-mode, step-up controller automatically chooses the lowest active FB_ voltage to regulate the output 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 determine the switch ontime. As the load changes, the error amplifier sources or sinks current to the CCV output to deliver the required peak-inductor current. The slope-compensation signal is added to the current-sense signal to improve stability at high duty cycles. At high input voltages when the on-time is pushed to be lower than the minimum on-time (90ns typ), the boost converter runs with minimum on-time, boosting the output voltage higher than the input voltage.
5V Supply VCC and UVLO The MAX17061A requires an external 5V VCC supply for internal control voltage.
The MAX17061A includes power-on reset (POR) and undervoltage lockout (UVLO) features. POR resets the fault latch and sets all the SMBus registers to their POR values. POR occurs when VCC rises above 2.8V (typ). The controller is disabled until VCC exceeds the UVLO threshold of 4.25V (typ). Hysteresis on UVLO is approximately 85mV. The VCC and VDD pins should be bypassed to GND with a minimum 1F ceramic capacitor.
VIN 7V TO 21V 4.7F 0.1F
L1 10H
D1 2F COUT
VOUT UP TO 45V
+5V VCC 1F
VDD VCC OSC ISET FSET 5.1k
IN LX1 LX2 PGND1 PGND2
464k 220pF 0.022F
CCV
MAX17061A
OV
R1 2.21M
R2 61.9k
PWM 1F
PWMI PWMO
FB1 FB2 FB3 FB4
CLK DATA
SCL SDA GND EP
FB5 FB6 FB7 FB8
VCC
Figure 1. Typical Operating Circuit
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
OUTPUT OVERVOLTAGE COMPARATOR
MAX17061A
IN IN UVLO ERROR COMPARATOR VCC VCC
1.236V OV VDD LX1, 2
CONTROL AND DRIVER LOGIC CLOCK SLOPE COMPENSATION FB OVERVOLTAGE COMPARATOR VCC + 0.6V HVC CURRENT SENSE PGND1, 2
OSCILLATOR
FB8 FB7 FB6 FB5
OSC
3-LEVEL COMPARATOR
OV FAULT
ERROR AMPLIFIER CCV gm
FB4 LVC FB3 FB2 VSAT FB1 REF EN
ISET
REF ADJ
8-BIT D/A
FSET
DPWM SIGNAL GENERATOR DPWM SETTING WLED ON/OFF
10 CURRENT SOURCE CURRENT SOURCE GND
FB2 FB3 FB4 FB5 FB6 FB7 FB8
PWMO PWMI A "< = 1" D "1" MUX PWM_SEL DIMMING BLOCK DIAGRAM PWM_MD 0x03 0x02 0x01 0x00 MUX
CURRENT SOURCE CURRENT SOURCE CURRENT SOURCE CURRENT SOURCE CURRENT SOURCE
SDA SCL
SMBus INTERFACE
CURRENT SOURCE
Figure 2. Control Circuit Block Diagram
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
IN UVLO
The MAX17061A implements an IN UVLO function to safeguard the operation in the specified range. When IN pin voltage drops below its UVLO threshold (6.98V typ), the converter is shut off and the BL_Stat bit of the fault/status register is cleared. The BL_CTL bit of control register is not altered. When IN pin voltage rises above the UVLO rising edge threshold (7.2V typ) the backlight resumes automatically if the BL_CTL bit is set to 1.
Table 1. Frequency Selection
OSC PIN CONNECTION 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 MAX17061A features a dedicated overvoltage feedback input (OV). The OV pin is tied to the center tap of a resistive voltage-divider from the high-voltage output. When the OV pin voltage, V OV , exceeds 1.236V, a comparator turns off the internal power MOSFET. This step-up converter switch is reenabled after the VOV drops 60mV (typ) hysteresis below the protection threshold. This overvoltage-protection feature ensures the step-up converter fail-safe operation when the LED strings are disconnected from the output.
Startup
At startup, the MAX17061A checks the OV pin to see if the Schottky diode is open. If the OV voltage is lower than 84mV (typ), the boost converter does not start. After the OV test is done, the MAX17061A performs a diagnostic test of the LED array. The test is divided to two phases; each phase takes approximately 1.024ms. In the first test phase, all FB_ inputs are quickly discharged down to 5.6V (typ) and then continuously discharged by 800A (typ) current sources. If a given FB_ voltage remains higher than 5.6V (typ), the string is considered to be shorted. Otherwise, if a given FB_ voltage is higher than 3V (typ), the string is considered to be unused. Unused strings should be tied to VCC. In the second phase, each FB_ is precharged by an internal 400A (typ) current source. If a given FB_ voltage remains lower than 1V (typ), the FB_ is considered to be a short to GND and the device is disabled. After the LED string diagnostic phases are finished, the boost converter starts. The total startup time is less than 10ms, including 4.2ms (typ) soft-start.
LED Current Sources
Maintaining uniform LED brightness and dimming capability are critical for LCD backlight applications. The MAX17061A is equipped with a bank of eight matched current sources. These specialized current sources are accurate to within 1.5% between strings and can be switched on and off within 15s, enabling PWM frequencies of up to 2kHz. All LED full-scale currents are identical and are set through the ISET pin (15mA < ILED < 30mA). When ISET is connected to VCC, the LED fullscale current is set at the 25mA default value. The minimum voltage drop across each current source is approximately 625mV when the LED current is 25mA. 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 tying the respective FB_ pin to VCC at startup. When the IC is powered up, the controller scans settings for all FB_ pins. If a FB_ pin is not tied to VCC, an internal circuit pulls this pin low, and the controller enables the corresponding current source to regulate the string current. If the FB_ pin is tied to VCC, the controller disables the corresponding current regulator. The current regulator cannot be disabled by connecting the respective FB_ pin to VCC after the IC is powered up. All FB_ pins in use are combined to extract a lowest FB_ voltage (LVC) (see Figure 2). LVC is fed into the step-up converter's error amplifier and is used to set the output voltage.
Shutdown
The MAX17061A can be placed into shutdown by clearing bit 0 of the device control register (0x01). When a critical failure is detected, the IC also enters shutdown mode. In shutdown mode, all functions of the IC are turned off. The fault/status register is set accordingly in shutdown. When bit 0 of the device control register (0x01) is recycled to 1, the MAX17061A exits shutdown mode and starts. The fault/status register is reset at startup.
Frequency Selection
A tri-level OSC input sets the internal oscillator frequency for step-up converter, as shown in Table 1. Highfrequency (1MHz) operation optimizes the regulator for the smallest component size, at the expense of efficiency due to increased switching losses. Low-frequency (500kHz) operation offers the best overall efficiency but requires larger components and PCB area.
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8-String White LED Driver with SMBus for LCD Panel Applications
Current-Source Fault Protection
The MAX17061A performs a diagnostic test at startup. Open/short strings are disabled. LED fault open/short is also detected after startup. When one or more strings fails after startup, the corresponding current sources are disabled. The remaining LED strings still operate normally. When a fault is detected, bit 4 or/and bit 5, as well as bit 0 of the fault/status resister are set (see the Fault/Status Register section).
Dimming Control
The MAX17061A internally generates a DPWM signal for accurate WLED brightness dimming control. The DPWM frequency is adjustable through an external setting resistor and has 1.5% accuracy for RFSET = 464k. The duty cycle of this DPWM signal can be controlled externally through two interfaces: PWM and SMBus. The ISET pin sets the amplitude of the current sources for each LED string (Figure 3). The internal DPWM signal directly controls the duty cycle of these current sources. The resulting current is chopped and synchronized to the DPWM signal. When filtered by the slow response time of the human eye, the overall brightness is modulated in a consistent flicker-free manner.
MAX17061A
Open-Current Source Protection The MAX17061A step-up converter output voltage is regulated according to the minimum FB_ voltages on all the strings in use. If one or more strings are open, the respective FB_ pins are pulled to ground. For any FB_ lower than 175mV, the corresponding current source is disabled. The unaffected LED strings still operate normally. If all strings in use are open, the MAX17061A shuts down the step-up converter. The MAX17061A can tolerate A slight mismatch (4.4V) between LED strings. When severe mismatches (> 4.4V) or WLED shorts occur, the FB_ voltages will be uneven because mismatched voltage drops across strings. If a given FB_ voltage is higher than 5.6V (typ) after 24s blanking time when LEDs are turned on, an LED short condition is detected on the respective string. When the short continues for greater than 2ms, the string is disabled. The controller allows the unaffected LED strings to operate normally. When only one string is in operation and there are shorts on some LEDs, then the converter does not shut down. Instead, the output voltage is adjusted accordingly. The LED short-protection feature is disabled during the soft-start phase of the step-up converter.
D= tON tDPWM D = 50% DPWM tON tDPWM D = 30%
Full-Scale LED Brightness in DPWM Dimming Control The full-scale LED current in the DPWM dimming is determined by resistance from the ISET pin to ground:
ILEDMAX = 20mA x 200k RISET
The acceptable resistance range is 133k < RISET < 266k, which corresponds to full-scale LED current of 30mA > ILEDMAX > 15mA. Connect ISET to VCC for a default full-scale LED current of 25mA. The current source output is pulse-width modulated and synchronized with a DPWM signal to reduce jitter and flicker noise in the display.
DPWM DIMMING MODE D = 12.5% D = 6.25%
ILEDMAX
ILED
0A
Figure 3. LED Current Control by DPWM Signal in Dimming
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
DPWM Frequency Setting The MAX17061A uses an internal DPWM signal to perform dimming control. The DPWM frequency is specified by an external resistor connected from FSET pin to GND:
fDPWM = 109 x R[] + SMBus, it automatically remains off. In this low-power state, most of the control circuits are turned off. Even in PWM dimming mode, only the PWMI interface is used for brightness control; the MAX17061A cannot run without the SMBus interface. For sister products without the SMBus interface, contact MAXIM Integrated Products, Inc.
where: = 10.638 = 58509 The adjustable range for the FSET resistor, RFSET, is from 42k to 464k, corresponding to the DPWM frequency of 200Hz to 2kHz.
Dimming Control Register Descriptions
The MAX17061A includes four registers to monitor and control brightness, fault status, identification, and operating mode. The slave address is 0b0101100. The MAX17061A uses two multiplexers internally to direct the dimming signal processing (Figure 4). These two multiplexers are controlled by 2 bits of the device control register, PWM_SEL, and PWM_MD, respectively. The PWM_SEL bit selects either the SMBus or the PWM input to control the brightness. The PWM_MD bit selects the mode in which the PWM input is to be interpreted. Table 2 provides a complete setting of the three dimming modes (X means don't care). In PWM mode, the output LED brightness is solely controlled by the percentage duty cycle of the input signal to PWMI. In SMBus mode, the input of PWMI has no effect on the dimming control, and only the SMBus command to brightness control register adjusts the output brightness. In DPST mode, the overall brightness level is the normalized product of the SMBus command setting and PWM input duty cycle. The PWM signal starts from 100% when operating in DPST mode.
Dimming Control Interfaces The MAX17061A's dimming control circuit consists of two interfaces: PWM and SMBus. The block diagram of these two input interfaces is shown in Figure 4. The dimming can be performed in three modes: PWM, SMBus, or DPST. In PWM mode, the brightness is adjusted by the PWM signal applied to the PWMI pin. In SMBus mode, the brightness is adjusted by a command from uplink processor through the 2-wire SMBus. In DPST mode, the brightness is adjusted by the product of the PWM duty cycle and SMBus command value. This DPWM control provides a dimming range with 8-bit resolution down to 2.7% and supports Intel DPST to maximize battery life.
The SMBus interface can be used to adjust the dimming, as well as shut down the MAX17061A. Before the MAX17061A receives a turn-on command from the
PWMO BUFFER <=1 D 1 MUX BACKLIGHT ON/OFF DIGITAL MULTIPLIER MUX DPWM SETTING
PWMI
A
SMBus AND PWM INPUT BLOCK PWM_MD
PWM_SEL
IDENTIFICATION REGISTER 0x03
FAULT/STATUS REGISTER 0x02
DEVICE CONTROL REGISTER 0x01
BRIGHTNESS CONTROL REGISTER 0x00
SDA SCL
SMBus INTERFACE
Figure 4. MAX17061A PWM and SMBus Interface Circuit
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8-String White LED Driver with SMBus for LCD Panel Applications
Brightness control register: Address is 0x00. This register is both readable and writable for all 8 bits, BRT0-BRT7, which are used to control the LED brightness level. In SMBus dimming mode, an SMBus write byte cycle to register 0x00 sets the output brightness level. The SMBus setting of 0xFF for this register sets the backlight controller to the maximum brightness output, and 0x00 sets the minimum backlight brightness (about 2.7%). The default value for register 0x00 is 0xFF. A write byte cycle to register 0x00 has no effect when the backlight controller is in PWM mode. The SMBus read byte cycle to register 0x00 returns the current brightness level, regardless of the dimming mode.
REGISTER 0x00 BRT7 Bit 7 (R/W) BRT6 Bit 6 (R/W) BRIGHTNESS CONTROL REGISTER BRT5 Bit 5 (R/W) BRT4 Bit 4 (R/W) BRT3 Bit 3 (R/W) DEFAULT VALUE 0xFF BRT2 Bit 2 (R/W) BRT1 Bit 1 (R/W) BRT0 Bit 0
MAX17061A
Bit field definitions:
BIT FIELD Bit [7..0] DEFINITION BRT [7..0] DESCRIPTION 8-bit brightness setting, adjusting brightness levels in 256 steps, default value is 0xFF.
Device control register: Address is 0x01. This register is both readable and writable for Bit 0 to Bit 2. Bit 0, also named BL_CTL, is used as ON/OFF control for the output LEDs. Bit 1 and Bit 2, named PWM_SEL and PWM_MD, respectively, control the operating mode of the backlight controller. Bit 3 through Bit 7 are reserved bits. All reserved bits, return zero when read, and are ignored by the controller when written. A value of 1 written to BL_CTL turns on the backlight in 10ms or less after the write cycle completes. A value of zero written to BL_CTL immediately turns off the backlight.
REGISTER 0x01 RESERVE Bit 7 RESERVED Bit 6 DEVICE CONTROL REGISTER RESERVED Bit 5 RESERVED Bit 4 RESERVED Bit 3 DEFAULT VALUE 0x00 PWM_MD Bit 2 (R/W) PWM_SEL Bit 1 (R/W) BL_CTL Bit 0 (R/W)
Bit field definitions:
BIT FIELD Bit 2 Bit 1 Bit 0 DEFINITION PWM_MD PWM_SEL BL_CTL DESCRIPTION PWM mode select (1 = absolute brightness, 0 = % change), default = 0 Brightness MUX select (1 = PWM pin, 0 = SMBus value), default = 0 BL on/off control (1 = on, 0 = off), default = 0
Table 2. Operating Modes Selected by Device Control Register Bits 1 and 2
PWM_MD X 1 0 PWM_SEL 1 0 0 MODE PWM mode SMBus mode DPST mode DPWM DUTY-CYCLE SETTING PWMI input duty cycle SMBus command Product of PWMI input duty cycle and SMBus command
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
Fault/Status Register: Address is 0x02. This register has 6 status bits that allow monitoring the backlight controller's operating state. Bit 6 and Bit 7 are reserved bits, and Bit 3 is the status indicator or backlight. The other 5 bits are fault indicators. Bit 0 is a logical OR of all fault codes including LED open/short to simplify error detection. All the bits in this register are read only. The reserved bits return a zero when read.
REGISTER 0x02 RESERVED Bit 7 (R) RESERVED Bit 6 (R) FAULT STATUS REGISTER 2_CH_SD Bit 5 (R) 1_CH_SD Bit 4 (R) BL_STAT Bit 3 (R) OV_CURR Bit 2 (R) DEFAULT VALUE 0x00 THRM_SHDN Bit 1 (R) FAULT Bit 0 (R)
Bit field definitions:
BIT FIELD Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 DEFINITION 2_CH_SD 1_CH_SD BL_STAT OV_CURR THRM_SHD FAULT DESCRIPTION Two or more LED output channels are faulted (1 = faulted, 0 = OK) At least one LED output channel is faulted (1 = faulted, 0 = OK) Backlight status (1 = BL on, 0 = BL off) Input overcurrent (1 = overcurrent condition, 0 = current OK) Thermal shutdown (1 = thermal fault, 0 = thermal OK) Any fault, logic OR of all fault conditions (1 = fault condition, 0 = no fault)
Identification Register: Address is 0x03. The ID register contains two bit fields to denote the manufacturer and the silicon revision of the controller IC. The bit field widths were chosen to allow up to 32 vendors with up to eight silicon revisions each. This register is read only. The value for the MAX17061A is 0x81.
REGISTER 0x03 LED PANEL Bit 7 = 1 MFG3 Bit 6 (R) MFG2 Bit 5 (R) ID REGISTER MFG1 Bit 4 (R) MFG0 Bit 3 (R) DEFAULT VALUE 0x81 REV2 Bit 2 (R) REV1 Bit 1 (R) REV0 Bit 0 (R)
Bit field definitions:
BIT FIELD Bit 7 Bit [6..3] Bit [2..0] DEFINITION LED panel MFG[3..0] REV[2..0] DESCRIPTION Display panel using LED backlight, bit 7 = 1 Manufacturer ID; see Table 3, default = 0 Silicon rev (revs 0-7 allowed for silicon spins), default = 1
The list of ID values for vendors is shown in Table 3.
Table 3. Vendor IDs
ID 0 1 2 3 4 5 6 7-14 15 VENDOR Maxim Micro Semi MPS O2 Micro TI ST Analog Devices Reserved Vendor ID register not implemented
Thermal Shutdown
The MAX17061A includes a thermal-protection circuit. When the local IC temperature exceeds +160C (typ), the controller and current sources shut down and do not restart until the die temperature drops by 15C. When thermal shutdown occurs, Bit 0 and Bit 1 of fault/status register is set to 1.
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8-String White LED Driver with SMBus for LCD Panel Applications
Design Procedure
All MAX17061A designs should be prototyped and tested prior to production. Table 4 provides a list of power components for the typical applications circuit. Table 5 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. where 24.7mV is a scale factor from the slope compensation, the LCCM(MIN) is the minimum inductor value for stable operation in CCM, and RS =12m (typ) is the equivalent sensing scale factor from the controller's internal current-sense circuit. The controller can also operate in discontinuous conduction mode (DCM). In this mode, the inductor value can be lower, but the peak inductor current is higher than in CCM. In DCM, the maximum inductor value is calculated with the following equation: VIN(MIN) LDCM(MAX) = 1 - VOUT(MAX) + VDIODE x VIN(MIN)2 x 2 x fOSC(MAX) x VOUT(MAX) x IOUT(MAX)
MAX17061A
Step-Up Converter Current Calculation
To ensure the stable operation, the MAX17061A includes slope compensation, which sets the minimum inductor value. In continuous-conduction mode (CCM), the minimum inductor value is calculated with the following equation: L CCM(MIN) =
(VOUT(MAX) + VDIODE - 2 x VIN(MIN) ) x RS
2 x 24.7mV x fOSC(MIN)
where the LDCM(MAX) is the maximum inductor value for DCM, is the nominal regulator efficiency (85%), and IOUT(MAX) is the maximum output current.
Table 4. Component List
SWITCHING FREQUENCY White LED Number of WLEDs Input Voltage Inductor Input Capacitors Output Capacitor COUT Diode Rectifier 1MHz Nichia NSSW008C 3.2V (typ), 3.5V (max) at 20mA 10 pcs x 4 strings, 25mA (max) 7V to 21V 10H, 1.2A power inductor TDK VLP6810T-100M1R2; Sumida CR6D09HPNP-100MC 4.7F 10%, 25V X5R ceramic capacitor (1206) Murata GRM319R61E475KA12D 0.33F 10%, 50V X7R ceramic capacitor (1206) (6x) Murata GRM319R71H334K TDK C3216JB1H334K 0.7A, 60V Schottky diode (US-flat) Toshiba CUS04 1MHz Nichia NSSW008C 3.2V (typ), 3.5V (max) at 20mA 10 pcs x 8 strings, 25mA (max) 7V to 21V 10H, 2.5A power inductor TDK SLF10145T-100M2R5-PF 10F 10%, 25V X5R ceramic capacitor (1206) Murata GRM31CR61E106KA 1F 10%, 50V X7R ceramic capacitor (1206) (4x) Murata GRM31MR71H105KA TDK C3216X7R1H105K 3A, 60V Schottky diode Nihon EC31QS06
Table 5. Component Suppliers
SUPPLIER Murata Electronics North America, Inc. Nichia Corp. Sumida Corp. Toshiba America Electronic Components, Inc. Vishay PHONE 770-436-1300 248-352-6575 847-545-6700 949-455-2000 203-268-6261 WEBSITE www.murata-northamerica.com www.nichia.com www.sumida.com www.toshiba.com/taec www.vishay.com
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
The output current capability of the step-up converter is a function of current limit, input voltage, operating frequency, and inductor value. Because the slope compensation is used to stabilize the feedback loop, the inductor current limit depends on the duty cycle, and is determined with the following equation: ILIM = 1.9A + 24.7mV x (0.75 - D) RS Finding the best inductor involves the compromises among circuit efficiency, inductor size, and cost. In choosing an inductor, the first step is to determine the operating mode: continuous conduction mode (CCM) or discontinuous conduction mode (DCM). The MAX17061A has a fixed internal slope compensation that requires 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 works in 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 ripples 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 for CCM can be described as follows:
where 24.7mV is the scale factor from the slope compensation, 1.9A is a typical current limit at 75% duty cycle, and D is the duty cycle. The output current capability depends on the currentlimit value and operating mode. The maximum output current in CCM is governed by the following equation: 0.5 x D x VIN IOUT _ CCM(MAX) = ILIM - fOSC x L where ILIM is the current limit calculated above, is the nominal regulator efficiency (85%), and D is the duty cycle. The corresponding duty cycle for this current is: D= VOUT - VIN + VDIODE VOUT - ILIM x RON + VDIODE
where VDIODE is the forward voltage of the rectifier diode and RON is the internal MOSFET's on-resistance (0.15 typ). The maximum output current in DCM is governed by the following equation: IOUT _ DCM(MAX) = L x ILIM2 x fOSC x 2 x (VOUT + VDIODE - VIN )
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. Low inductor values decrease the physical size, but increase the current ripple and peak current.
Inductor Selection in CCM Operation
1) Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current (I OUT(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
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8-String White LED Driver with SMBus for LCD Panel Applications
The MAX17061A has a minimum inductor value limitation for a stable operation in CCM mode at low input voltage because of the internal fixed-slope compensation. The minimum inductor value for stability is calculated with the following equation: L CCM(MIN) =
(VOUT(MAX) + VDIODE - 2 x VIN(MIN) ) x RS
2 x 24.7mV x fOSC(MIN)
Inductor Selection Design Examples: Considering the Typical Operating Circuit with four 10-LED strings and 25mA LED full-scale current, the maximum load current (IOUT(MAX)) is 100mA with a 35.9V output and a minimal input voltage of 7V.
Choosing a CCM operating mode with LIR = 1 at 1MHz and estimating efficiency of 85% at this operating point: 7V 35.9V - 7V 0.85 L= = 9.44H 35.9V 100mA x 1MHz 1 In CCM, the inductor has to be higher than LCCM(MIN): L CCM(MIN) =
2
MAX17061A
where 24.7mV is a scale factor from slope compensation, and the RS is the equivalent current-sensing scale factor (12m typ): 1) 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
(35.9V + 0.4V - 2 x 7V) x 12m = 6.0H
2 x 24.7mV x 0.9MHz
2) 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
(
)
A10H inductor is chosen, which is higher than the minimum L that guarantees stability in CCM. The peak-inductor current at minimum input voltage is calculated as follows: 100mA x 35.9V IPEAK = 7V x 0.85 7V x (35.9V - 7V ) = 0.92A + 2 x 10H x 35.9V x 0.9MHz Alternatively, choosing a DCM operating mode at 750kHz and estimating efficiency of 85% at this operating point: 7V LDCM(MAX) = 1 - 35.9V + 0.4V x (7V)2 x 0.85 = 5.6H 2 x 0.825MHz x 35.9V x 100mA
Inductor Selection in DCM Operation When DCM operating mode is chosen to minimize the inductor value, the calculations are different from those above in CCM mode. The maximum inductor value for DCM mode is calculated with the following equation:
The peak-inductor current in DCM is calculated with following equation: VIN(MIN) LDCM(MAX) = 1 - VOUT(MAX) + VDIODE x VIN(MIN)2 x 2 x fOSC(MAX) x VOUT(MAX) x IOUT(MAX)
A 4.7H inductor is chosen. The peak inductor current at minimum input voltage is calculated as follows:
IPEAK = 4.7uH x 0.675MHz x 0.85 x (35.9V + 0.4V ) 100mA x 2 x 35.9V x (35.9V + 0.4V - 7V ) = 1.47A
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) VRIPPLE(C) and: IOUT(MAX) VOUT(MAX) - VIN(MIN) COUT VOUT(MAX)fOSC
I IPEAK = IIN(DCMAX) + RIPPLE , 2 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.
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
VRIPPLE(ESR) IPEAKRESR(COUT) where I PEAK is the peak inductor current (see the Inductor Selection section). The output voltage ripple 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 actual capacitance of a ceramic capacitor is reduced by DC voltage biasing. Ensure the selected capacitor has enough capacitance at actual DC biasing. directly from the output of another regulated supply. In some applications, CIN can be reduced below the values used in the Typical Operating Circuit (Figure 1). 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.
LED Selection and Bias
The series/parallel configuration of the LED load and the full-scale bias current have a significant effect or 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 and while increasing the bias current and light output. LED manufacturers specify LED color at a given LED current. With lower LED current, the color of the emitted light tends to shift toward the blue range of the spectrum. A blue bias is often acceptable for business applications but not for high-image-quality applications such as DVD players. Direct DPWM dimming is a viable solution for reducing power dissipation while maintaining LED color integrity. Careful attention should be paid to switching noise to avoid other display quality problems. Using fewer LEDs in a string improves step-up converter efficiency, and lowers breakdown voltage requirements of the external MOSFET and diode. The minimum number of LEDs in series should always be greater than maximum input voltage. If the diode voltage drop is lower than 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 are ideal for input voltages up to 20V.
Rectifier Diode Selection
The MAX17061A'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 I PEAK calculated in the Inductor Selection section and that its breakdown voltage exceeds the output voltage.
Overvoltage Protection Determination
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 tied to the center tap of a resistive voltage-divider (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: R1 VOUT(OVP) = VOV _ TH x 1 + R2 In Figure 1, the output OVP voltage is set to: 2.21M VOUT(OVP) = 1.236V x (1 + ) 45V 61.9k
Applications Information
LED VFB_ Variation
The MAX17061A has accurate (1.5%) matching for each current source. However, the forward voltage of each white LED can vary up to 25% 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 4.8V (typ). Otherwise, the internal LED shortprotection circuit disables the high FB string.
Input Capacitor Selection
The input capacitor (C IN ) 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 impedance since the step-up regulator often runs
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8-String White LED Driver with SMBus for LCD Panel Applications
FB Pin Maximum Voltage
The current through each FB_ pin is controlled only during the step-up converter's on-time. During the converter's off-time, the current sources are turned off. The output voltage does not discharge and stays high. The MAX17061A disables the FB current source to which the string is shorted. In this case, the step-up converter's output voltage is always applied to the disabled FB pin. The FB_ pin can withstand 45V. 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 powerground 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 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 as possible to the OV pin. 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 IN pin bypass capacitor as close as possible to the device. 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 shield if necessary. Refer to the MAX17061A evaluation kit for an example of proper board layout.
MAX17061A
PCB Layout Guidelines
Careful PCB layout is important for proper operation. Use the following guidelines for good PCB layout: 1) Minimize the area of high current switching loop of rectifier diode, internal MOSFET, and output capacitor to avoid excessive switching noise. 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 internal MOSFET, then 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.
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8-String White LED Driver with SMBus for LCD Panel Applications MAX17061A
Pin Configuration
PGND1 N.C. SCL LX1 LX2
Chip Information
TRANSISTOR COUNT: 21,800 PROCESS: BiCMOS
TOP VIEW
21 VDD 22 VCC 23 CCV 24 OV 25 ISET 26 FB1 27 FB2 28 1 FB3 IN
20
19
PGND2
18
17
16
15 14 13 12 SDA FSET PWMO PWMI OSC FB8 FB7
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE 28 TQFN PACKAGE CODE T-2844-1 DOCUMENT NO. 21-0139
MAX17061AETI+
11 10 9 8
2 FB4
3 GND
4 N.C.
5 FB5
6 N.C.
7 FB6
THIN QFN 4mm x 4mm
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 24
(c) 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.

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