19-5164; Rev 0; 3/10 TION KIT EVALUA BLE AVAILA Six-String WLED Driver with Integrated Step-Up Converter The MAX17127 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 converter drives the LED array, which can be configured for up to six strings in parallel and 13 LEDs per string. Each string is terminated with ballast that achieves Q2% current-regulation accuracy, ensuring even LED brightness. The MAX17127 has a wide input voltage range from 5V to 26V, and provides adjustable 10mA to 30mA full-scale LED current. The MAX17127 can implement brightness control through the PWM signal input, and LED current is directly controlled by the external dimming signal’s frequency and duty cycle. The MAX17127 has multiple features to protect the controller from fault conditions. Once an open/short string is detected, the fault string is disabled while other strings can still operate normally. The controller features cycleby-cycle current limit to provide constant operation and soft-start capability. If the MAX17127 is in current-limit condition, the step-up converter is latched off after an internal timer expires. A thermal-shutdown circuit provides another level of protection. When thermal shutdown happens, the MAX17127 is latched off. The MAX17127’s step-up controller features an internal 0.12I (typ), 48V (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 helps reduce power loss and improve efficiency. The MAX17127 features resistoradjustable switching frequency from 250kHz to 1MHz, which enables a wide variety of applications that can trade off component size for operating frequency. Features S 5V to 26V Input Supply Voltage S Up to Six Parallel Strings Multiple Series- Connected LEDs S 250kHz to 1MHz Adjustable Switching Frequency S 0.12I Internal HV Power MOSFET (48V max) S Low String Feedback Voltage: 480mV at 20mA LED Current S Full-Scale LED Current Adjustable from 10mA to 30mA S Q2% Current-Regulation Accuracy Between Strings S 400ns Minimum String On-Time S 100Hz to 25kHz PWM Input Range S Open and Short LED Protection S Output Overvoltage Protection S Thermal Shutdown S Small 20-Pin, 4mm x 4mm Thin QFN Package Ordering Information PART TEMP RANGE PIN-PACKAGE MAX17127ETP+ -40°C to +85°C 20 TQFN-EP* +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. Applications Notebook, Subnotebook, and Tablet Computer Displays Automotive Systems Handy Terminals The MAX17127 is available in a thermally enhanced, lead-free, 20-pin, 4mm x 4mm thin QFN package. Simplified Operating Circuit appears at end of data sheet. ________________________________________________________________ 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. MAX17127 General Description MAX17127 Six-String WLED Driver with Integrated Step-Up Converter ABSOLUTE MAXIMUM RATINGS VIN to AGND .........................................................-0.3V to +30V FB_, SW to PGND..................................................-0.3V to +52V PGND to AGND.....................................................-0.3V to +0.3V VDDIO, PWM, EN, FPO, I.C. to AGND......................-0.3V to +6V COMP, ISET, R_FPWM, OVP, FSLCT to AGND.................................................-0.3V to VDDIO + 0.3V SW Switch Maximum Continuous RMS Current....................1.6A Continuous Power Dissipation (TA = +70NC) TQFN (derate 16.9mW/NC above +70NC)...................1349mW Operating Temperature Range........................... -40NC to +85NC Junction Temperature......................................................+150NC Storage Temperature Range............................. -60NC to +150NC ESD HBM.................................................................................... 2kV MM....................................................................................200V Lead Temperature (soldering, 10s).................................+300NC Soldering Temperature (reflow).......................................+260NC 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, CCOMP = 0.51nF, CCOUT = 4.7µF, RCOMP = 82.5kΩ, RISET = 180kΩ, RFSLCT = 100kΩ, L = 10µH, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS VIN Input Voltage Range VIN Quiescent Current VDDIO Output Voltage MIN TYP MAX 26 V 2.7 3.2 mA 5 10 FA 5.15 5 MAX17127 is enabled, VEN = 3.3V, VIN = 26V MAX17127 is disabled, EN = AGND MAX17127 is enabled, VEN = 3.3V, 5.4V < VIN < 26V, 0A < IVDDIO < 10mA 4.85 5 MAX17127 is enabled, VEN = 3.3V, VIN = 5V, IVDDIO = 10mA, dropout condition 4.6 4.75 MAX17127 is disabled, EN = AGND, 0A < IVDDIO < 50FA 3.1 3.7 UNITS V 4.1 V VDDIO Current Limit VDDIO is forced to 4.2V 25 45 70 mA VDDIO UVLO Threshold Rising edge, typical hysteresis = 250mV 3.90 4.00 4.10 V Falling edge 4.3 4.5 4.7 Rising edge 4.55 4.75 4.95 0.12 0.25 I 1 FA VIN UVLO Threshold V BOOST CONVERTER SW On-Resistance 20mA from SW to PGND SW Leakage Current 40V on SW, TA = +25NC RFSLCT = 100kI 0.95 1.0 1.05 RFSLCT = 400kI 0.225 0.25 0.275 RFSLCT Range Operating range 90 Maximum Duty Cycle At fSW = 1MHz 91 Minimum On-Time (Note 1) SW Current Limit Duty cycle = 75% Operating Frequency 3.12 MHz 500 kI 50 80 ns 3.9 4.7 A 95 % CONTROL INPUT PWM, EN Logic-Input High Level 2.1 V PWM, EN Logic-Input Low Level EN Pulldown Resistor 120 200 2 _______________________________________________________________________________________ 0.8 V 280 kI Six-String WLED Driver with Integrated Step-Up Converter (Circuit of Figure 1. VIN = 12V, CCOMP = 0.51nF, CCOUT = 4.7µF, RCOMP = 82.5kΩ, RISET = 180kΩ, RFSLCT = 100kΩ, L = 10µH, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN TYP MAX UNITS FPO OUTPUT FPO Off-Leakage Current Fault inactive, TA = +25NC 100 nA FPO On Output-Voltage Low ISINK = 1mA, fault active 0.4 V INPUT LEAKAGE PWM Leakage Current TA = +25NC, VPWM = 0V, VPWM = 5V -1 +1 FA OVP Leakage Current TA = +25NC, VOVP = 0V, VOVP = 5V -0.1 +0.1 FA RISET = 120kI 29.1 30 30.9 RISET = 180kI 19.6 20 20.4 RISET = 360kI 9.7 10 10.3 VISET < 0.7V 0.2 0.3 0.4 Operating range 100 400 Accuracy = 3% 120 360 10mA < IFB_< 30mA -2.0 +2.0 IFB_ = 30mA 400 LED CURRENT Full-Scale FB_ Output Current RISET Range Current Regulation Between Strings mA kI % 555 770 IFB_ = 20mA 460 670 IFB_ = 10mA 350 630 FB_ On-Resistance VFB_ = 50mV (includes 10I sense resistor) 17.5 28.4 I FB_ Bias Current VFB_ = 40V, TA = +25NC 0.1 1 FA 400 580 700 ns 1.23 1.25 1.27 V 7 8 9 V Minimum FB_ Regulation Voltage FB_ Minimum On-Time mV FAULT PROTECTION OVP Threshold Voltage Rising edge, typical hysteresis = 90mV FB_ Overvoltage Threshold FB_ Enable Threshold Voltage 1.2 V FB_ Open Threshold Voltage 130 280 mV FB_ Check LED Source Current 0.4 1.3 mA FB_ Check LED Time 0.7 1.3 ms Thermal-Shutdown Threshold (Note 1) Overcurrent Fault Timer Latch-off timer 1.0 +150 NC 128 Fs PWM CONTROL PWM Input On-Time 400 PWM Input Frequency Range 0.1 ns 25 kHz _______________________________________________________________________________________ 3 MAX17127 ELECTRICAL CHARACTERISTICS (continued) MAX17127 Six-String WLED Driver with Integrated Step-Up Converter ELECTRICAL CHARACTERISTICS (Circuit of Figure 1. VIN = 12V, CCOMP = 0.51nF, CCOUT = 4.7µF, RCOMP = 82.5kΩ, RISET = 180kΩ, RFSLCT = 100kΩ, L = 10µH, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS VIN Input Voltage Range VIN Quiescent Current VDDIO Output Voltage MIN TYP 5 MAX UNITS 26 V MAX17127 is enabled, VEN = 3.3V, VIN = 26V 3.2 mA MAX17127 is disabled, EN = AGND 15 μA MAX17127 is enabled, VEN = 3.3V, 5.4V < VIN < 26V, 0A < IVDDIO < 10mA 4.85 MAX17127 is enabled , VEN = 3.3V, VIN = 5V, IVDDIO = 10mA, dropout condition 4.6 5.15 V EN = AGND, 0A < IVDDIO < 50FA 3.1 VDDIO Current Limit VDDIO is forced to 4.2V 25 70 mA VDDIO UVLO Threshold Rising edge, typical hysteresis = 250mV 3.90 4.10 V Falling edge 4.3 4.7 Rising edge 4.55 4.95 VIN UVLO Threshold 4.1 V BOOST CONVERTER SW On-Resistance 20mA from SW to PGND 0.25 ω SW Leakage Current 40V on SW, TA = +25NC 1 μA Operating Frequency RFSLCT = 100kI 0.95 1.05 RFSLCT = 400kI 0.225 0.28 90 500 RFSLCT Operative Range 92 MHz kω Maximum Duty Cycle At fSW = 1MHz % Boost Output Voltage With suitable OVP network 45 V Minimum On-Time (Note 1) 80 ns CONTROL INPUT PWM, EN Logic-Input High Level 2.1 PWM, EN Logic-Input Low Level EN Pulldown Resistor 110 V 0.8 V 290 kω 0.4 V FPO OUTPUT FPO On Output-Voltage Low ISINK = 1mA, fault active LED CURRENT Full-Scale FB_ Output Current RISET Range Current Regulation Between Strings RISET = 120kI 29.1 30.9 RISET = 180kI 19.4 20.6 RISET = 360kI 9.7 10.3 VISET < 0.7V 0.2 0.4 Operating range 100 400 Accuracy = 3% 120 360 10mA < IFB_< 30mA -2.0 +2.0 IFB_ = 30mA 400 770 mA kω % Minimum FB_ Regulation Voltage IFB_= 20mA 670 IFB_= 10mA 630 FB_ On-Resistance VFB_= 50mV (includes 10I sense resistor) 28.4 ω FB_ Bias Current VFB_ = 40V, TA = +25NC 1 μA 4 _______________________________________________________________________________________ mV Six-String WLED Driver with Integrated Step-Up Converter (Circuit of Figure 1. VIN = 12V, CCOMP = 0.51nF, CCOUT = 4.7µF, RCOMP = 82.5kΩ, RISET = 180kΩ, RFSLCT = 100kΩ, L = 10µH, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN MAX UNITS 400 700 ns 1.23 1.27 V 7 9 V FB_ Open Threshold Voltage 130 280 mV FB_ Check LED Source Current 0.4 1.3 mA 0.7 1.3 ms 88 168 μs FB_ Minimum On-Time TYP FAULT PROTECTION OVP Threshold Voltage Rising edge, typical hysteresis = 90mV FB_ Overvoltage Threshold FB_ Check LED Time Overcurrent Fault Timer Latch-off timer PWM CONTROL PWM Input On-Time 400 PWM Input Frequency Range 0.1 ns 25 kHz Note 1: Specifications are guaranteed by design, not production tested. Typical Operating Characteristics (Circuit of Figure 1. VIN = 12V, TA = +25NC, unless otherwise noted.) BOOST CONVERTER EFFICIENCY vs. INPUT VOLTAGE (VS) (VOUT = 32V, IOUT = 120mA, BRIGHTNESS = 100%) 90 80 EFFICIENCY (%) EFFICIENCY (%) 88 86 84 82 MAX17127 toc02 90 MAX17127 toc01 92 BOOST CONVERTER EFFICIENCY vs. BRIGHTNESS (VS = 2V, VOUT = 32V, IOUT = 120mA AT 100%) 70 60 80 78 50 5 8 11 14 17 20 INPUT VOLTAGE (V) 23 26 0 20 40 60 80 100 BRIGHTNESS (%) _______________________________________________________________________________________ 5 MAX17127 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (continued) (Circuit of Figure 1. VIN = 12V, TA = +25NC, unless otherwise noted.) LED CURRENT (ILED = 20mA AT 100% BRIGHTNESS) vs. INPUT VOLTAGE (VS) LED CURRENT vs. BRIGHTNESS SETTING LED CURRENT (mA) 20.15 5 20.10 20.05 0 20.00 20 0 40 60 80 100 5 8 11 14 17 20 PWM DUTY CYCLE (%) INPUT VOLTAGE (V) LED CURRENT (ILED = 20mA AT 10% BRIGHTNESS) vs. INPUT VOLTAGE (VS) IN QUIESCENT CURRENT vs. IN VOLTAGE 2.02 2.00 1.98 23 26 23 26 6 MAX17127 toc06 MAX17127 toc05 2.04 5 QUIESCENT CURRENT (mA) LED CURRENT (mA) 10 MAX17127 toc04 fPWM = 200Hz 15 LED CURRENT (mA) 20.20 MAX17127 toc03 20 4 100% BRIGHTNESS 3 2 200Hz/1% BRIGHTNESS 1 1.96 0 10 8 11 14 17 20 23 26 5 7 11 14 17 20 INPUT VOLTAGE (V) IN VOLTAGE (V) IN SHUTDOWN CURRENT vs. IN VOLTAGE SWITCHING WAVEFORMS (VS = 5V, BRIGHTNESS = 100%) MAX17127 toc08 MAX17127 toc07 5 SHUTDOWN CURRENT (µA) MAX17127 Six-String WLED Driver with Integrated Step-Up Converter EN = LOW 8 VOUT = 32V, IOUT = 120mA VLX 20V/div 0V 6 4 INDUCTOR CURRENT 500mA/div 2 0mA 0 5 8 11 14 17 20 23 26 1µs/div IN VOLTAGE (V) 6 _______________________________________________________________________________________ Six-String WLED Driver with Integrated Step-Up Converter SWITCHING WAVEFORMS (VS = 26V, BRIGHTNESS = 100%) STARTUP WAVEFORMS (BRIGHTNESS = 100%) MAX17127 toc09 MAX17127 toc10 VOUT = 32V, IOUT = 120mA VLX 20V/div 0V INDUCTOR CURRENT 200mA/div VLX 20V/div 0mA VOUT 20V/div 0V 0V 12V 1µs/div 1ms/div STARTUP WAVEFORMS (BRIGHTNESS = 20%) LED CURRENT WAVEFORMS (BRIGHTNESS = 50%) MAX17127 toc11 MAX17127 toc12 VEN 5V/div 0V INDUCTOR CURRENT 500mA/div 0A VFB1 10V/div 0V ILED 20mA/div 0mA VLX 20V/div 0V INDUCTOR CURRENT 500mA/div 0mA VOUT 20V/div 0V 12V VEN 5V/div 0V INDUCTOR CURRENT 500mA/div 0A 2ms/div 1ms/div LED CURRENT WAVEFORMS (BRIGHTNESS = 1%) LED-OPEN FAULT PROTECTION (BRIGHTNESS = 100%, LED OPEN ON FB1) MAX17127 toc13 MAX17127 toc14 VFB1 10V/div 0V VFB1 1V/div 0V IFB1 20mA/div 0mA VFB2 10V/div 0V INDUCTOR CURRENT 500mA/div 0mA 1ms/div 0mA 20mA 200ms/div 32V VOUT 10V/div 10V IFB2 10mA/div _______________________________________________________________________________________ 7 MAX17127 Typical Operating Characteristics (continued) (Circuit of Figure 1. VIN = 12V, TA = +25NC, unless otherwise noted.) Typical Operating Characteristics (continued) (Circuit of Figure 1. VIN = 12V, TA = +25NC, unless otherwise noted.) LINE-TRANSIENT RESPONSE (VS = 9V 21V, BRIGHTNESS = 100%) LED-SHORT FAULT PROTECTION (BRIGHTNESS = 100%, 3 LEDs SHORT ON FB1) MAX17127 toc15 MAX17127 toc16 VFB1 10V/div 0V 9V VOUT (AC-COUPLED) 2V/div 0V VS 10V/div 21V 0V IFB1 50mA/div 0A INDUCTOR CURRENT 1A/div IFB1 10mA/div 0mA 0mA 20mA 10µs/div 200µs/div LINE-TRANSIENT RESPONSE (VS = 21V 9V, BRIGHTNESS = 100%) MAXIMUM UNBALANCE RATE BETWEEN STRING vs. BRIGHTNESS (VS = 12V, ILED = 20mA) MAX17127 toc17 MAXIMUM UNBALANCE RATE (%) 21V 0A INDUCTOR CURRENT 1A/div IFB1 10mA/div 20mA MAX17127 toc18 1.0 0V VOUT (AC-COUPLED) 1V/div VS 10V/div 0V 9V 0.8 0.6 0.4 0.2 MAXIMUM = IFB_ − IFB(AVG) MAX % UNBALANCE RATE (%) IFB(AVG) 0 0mA 10 200µs/div 20 30 40 50 60 70 80 90 100 BRIGHTNESS (%) MAXIMUM UNBALANCE RATE BETWEEN STRINGS (ILED = 20mA) vs. INPUT VOLTAGE (VS) MAX17127 toc19 0.8 MAXIMUM UNBALANCE RATE (%) MAX17127 Six-String WLED Driver with Integrated Step-Up Converter 0.7 0.6 0.5 0.4 0.3 MAXIMUM = IFB_ − IFB(AVG) MAX % UNBALANCE RATE (%) IFB(AVG) 0.2 5 8 11 14 17 20 23 26 INPUT VOLTAGE (V) 8 _______________________________________________________________________________________ Six-String WLED Driver with Integrated Step-Up Converter PGND OVP R_FPWM FB1 FB2 TOP VIEW 15 14 13 12 11 SW 16 10 FB3 I.C. 17 9 AGND 8 FB4 7 FB5 6 FB6 MAX17127 COMP 18 VIN 19 2 3 4 5 ISET FPO VDDIO 1 FSLCT + EN PWM 20 EP THIN QFN (4mm × 4mm) Pin Description PIN NAME FUNCTION 1 VDDIO 5V Linear Regulator Output. VDDIO provides power to the MAX17127. Bypass VDDIO to AGND with a ceramic capacitor of 1FF or greater. 2 EN Enable Pin. EN = high enables the MAX17127. An internal 200kI (typ) pulldown resistor keeps the MAX17127 in disabled mode if the EN pin is high impedance. 3 FSLCT Oscillator Frequency-Adjustment Pin. The resistance from FSLCT to AGND sets the step-up converter’s oscillator frequency: fSW = 1MHz O 100kI/RFSLCT The acceptable resistance range is 100kI < RFSLCT < 400kI, which corresponds to the switching frequency of 1MHz > fSW > 250kHz. Full-Scale LED Current-Adjustment Pin. The resistance from ISET to AGND controls the full-scale current in each LED string: ILEDMAX = 20mA O 180kI/RISET 4 ISET The acceptable resistance range is 120kI < RISET < 360kI, which corresponds to a full-scale LED current of 30mA > ILEDMAX > 10mA. Connecting ISET to AGND sets the test mode for 0.3mA (typ) full-scale LED current. 5 FPO Fault-Diagnostic Output. Open drain, active low. The FPO output is asserted low when the following faults occur: overcurrent fault, thermal fault, output-voltage short condition, or output overvoltage. 6 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 30mA. If unused, connect FB6 to AGND. _______________________________________________________________________________________ 9 MAX17127 Pin Configuration MAX17127 Six-String WLED Driver with Integrated Step-Up Converter Pin Description (continued) PIN NAME FUNCTION 7 FB5 LED String 5 Cathode Connection. FB5 is the open-drain output of an internal regulator, which controls current through FB5. FB5 can sink up to 30mA. If unused, connect FB5 to AGND. 8 FB4 LED String 4 Cathode Connection. FB4 is the open-drain output of an internal regulator, which controls current through FB4. FB4 can sink up to 30mA. If unused, connect FB4 to AGND. 9 AGND 10 FB3 LED String 3 Cathode Connection. FB3 is the open-drain output of an internal regulator, which controls current through FB3. FB3 can sink up to 30mA. If unused, connect FB3 to AGND. 11 FB2 LED String 2 Cathode Connection. FB2 is the open-drain output of an internal regulator, which controls current through FB2. FB2 can sink up to 30mA. If unused, connect FB2 to AGND. 12 FB1 LED String 1 Cathode Connection. FB1 is the open-drain output of an internal regulator, which controls current through FB1. FB1 can sink up to 30mA. If unused, connect FB1 to AGND. 13 R_FPWM 14 OVP 15 PGND 16 SW Boost Regulator Power Switch Node 17 I.C. Internal Connection. Not connected externally. 18 COMP 19 VIN 20 PWM — EP Analog Ground Connect R_FPWM to AGND Overvoltage Sense. Connect OVP to the boost converter output through a resistor: VOVP = 1.25V O (1 + R1/R2 ) Boost Regulator Power Ground Step-Up Converter Compensation Pin. Connect a ceramic capacitor in series with a resistor from COMP to AGND. Supply Input. VIN biases the internal 5V linear regulator that powers the device. Bypass VIN to AGND directly at the pin with a 0.1FF or greater ceramic capacitor. PWM Signal Input. This signal is used for brightness control. The brightness is proportional to the PWM duty cycle, and the PWM signal directly controls the LED turning on/off. 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. 10 ������������������������������������������������������������������������������������� Six-String WLED Driver with Integrated Step-Up Converter The MAX17127 typical operating circuit is shown as Figure 1. Table 1 lists some recommended components, L1 10µH VS 5V TO 26V and Table 2 lists the contact information for component suppliers. D1 COUT 4.7µF CIN 4.7µF R1 2.21MI VIN VIN 0.1µF SW VDDIO PGND 1µF OVP RISET 180kI ISET COMP FSLCT AGND RFSLCT 100kI RCOMP 82.5kI CCOMP 510pF R2 71.5kI MAX17127 I.C. EN FB1 PWM FB2 R_FPWM FB3 3.3V FB4 FB5 10kI FPO FAULT INDICATOR FB6 EP Figure 1. Typical Operating Circuit Table 1. Component List DESIGNATION DESCRIPTION DESIGNATION CIN 4.7FF Q10%, 25V X5R ceramic capacitor (1206) Murata GRM319R61E475KA12D L1 10FH, 1.2A power inductor Sumida CR6D09HPNP-100MC TDK VLP6810T-100M1R2 C1, C2 2.2FF Q20%, 50V X7R ceramic capacitors (1206) Murata GRM31CR71H225K White LED 3.2V (typ), 3.5V (max) at 20mA Nichia NSSW008C D1 DESCRIPTION 2A, 40V Schottky diode (M-flat) Toshiba CMS11 ______________________________________________________________________________________ 11 MAX17127 Typical Operating Circuit MAX17127 Six-String WLED Driver with Integrated Step-Up Converter Table 2. Component Suppliers SUPPLIER PHONE WEBSITE Murata Electronics North America, Inc. 770-436-1300 www.murata.com Nichia Corp. 248-352-6575 www.nichia.com Sumida Corp. 847-545-6700 www.sumida.com Toshiba America Electronic Components, Inc. 949-455-2000 www.toshiba.com/taec Vishay 203-268-6261 www.vishay.com VIN 1.25V FAULT CONTROL EN 5V LINEAR REGULATOR OVP ERROR COMPARATOR VDDIO SW CONTROL AND DRIVER LOGIC N VDDIO FSLCT SLOPE COMPENSATION OSCILLATOR CURRENT SENSE Σ PGND 8V TO FAULT CONTROL OVERVOLTAGE COMPARATOR ERROR AMPLIFIER COMP OVP Gm HVC S&H ERROR AMPLIFIER 1.25V CLAMP LVC Gm FB6 FB5 FB4 FB3 FB2 VSAT FB1 ISET ISET EN EN PWM N MAX17127 PWM CONTROL AGND R_FPWM I.C. VDDIO FPO FAULT CONTROL CURRENT SOURCE FB2 CURRENT SOURCE FB3 CURRENT SOURCE FB4 CURRENT SOURCE FB5 CURRENT SOURCE FB6 Figure 2. Functional Diagram 12 ������������������������������������������������������������������������������������� Six-String WLED Driver with Integrated Step-Up Converter The MAX17127 is a high-efficiency driver for arrays of white LEDs. It contains a fixed-frequency current-mode PWM step-up controller, a 5V linear regulator, a dimming control circuit, an internal power MOSFET, and six regulated current sources. Figure 2 shows the MAX17127 functional diagram. 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 MAX17127 features resistor-adjustable switching frequency (250kHz to 1MHz), which allows trade-offs between external component size and operating efficiency. The MAX17127 can implement brightness control through the PWM signal input. The LED current is directly controlled by the external dimming signal's frequency and duty cycle. The MAX17127 has multiple features to protect the controller from fault conditions. Separate feedback loops limit the output voltage in all circumstances. The MAX17127 checks each FB_ voltage during operation. If one or more strings are open, the corresponding FB_ voltages are pulled below 180mV (max), and an opencircuit fault is detected. As a result, the respective current sources are disabled. When one or more LEDs are shorted and the related FB_ voltage exceeds 8V, short fault is detected and the respective current source is disabled if at least one FB_ voltage is lower than the minimum FB_ regulation voltage +460mV (typ). When in LED open or short conditions, the fault string is disabled while other strings can still operate normally. The MAX17127 also includes other kinds of fault protections, which are overcurrent, thermal shutdown, and output overvoltage. The MAX17127 features cycle-bycycle current limit to provide consistent operation and soft-start protection. In an overcurrent condition, the IC latches off if the fault still exists after a 128Fs overcurrent fault timer expires. The output overvoltage is a nonlatched operation, and the step-up converter stops switching during the fault. A thermal-shutdown circuit provides another level of protection. The MAX17127 is latched off once thermal shutdown occurs. The MAX17127 includes a 5V linear regulator that provides the internal bias and gate driver for the step-up controller. Fixed-Frequency Step-Up Controller The MAX17127’s fixed-frequency, current-mode, stepup controller automatically chooses the lowest active FB_ voltage to regulate the feedback voltage. Specifically, the difference between the lowest FB_ voltage and the current source control signal plus an offset is integrated at the COMP output. The resulting error signal is compared to the internal switch current plus slope compensation to determine the switch on-time. As the load changes, the error amplifier sources or sinks current to the COMP output to deliver the required peak inductor current. The slope-compensation signal is added to the current-sense signal in order to improve stability at high duty cycles. Internal 5V Linear Regulator and UVLO The MAX17127 includes an internal low-dropout linear regulator (VDDIO). When VIN is higher than 5.0V, this linear regulator generates a 5V supply to power the internal PWM controller, control logic, and MOSFET driver. The VDDIO voltage drops to 3.3V in shutdown. If 5V < VIN < 5.5V, VDDIO and VIN can be connected together and powered from an external 5V supply. There is a body diode from VDDIO to VIN, so VIN must be greater than VDDIO (see Figure 2). The MAX17127 is disabled until VDDIO exceeds the UVLO threshold. The hysteresis on UVLO is approximately 250mV. In standby mode, the internal LDO is in low-power mode with 10FA (max) input current and approximately regulated at 3.3V (typ). When EN = high, the internal LDO is enabled and regulated accurately at 5V (typ). The VDDIO pin should be bypassed to AGND with a minimum 1FF ceramic capacitor. Startup At startup, the MAX17127 performs a diagnostic test of the LED array. In the test phase, all FB_ pins are pulled up by a given current source (0.4mA min) during 1ms (typ). If some FB_ voltage is lower than 1.2V (max), the string is considered to be unused. Therefore, when a string is not in use, it should be connected to AGND. All other strings with FB_ higher than 1.2V (max) are detected as in use. After the LED string diagnostic phases are finished, the boost converter starts. An additional 1ms after boost soft-start end is used as minimum FB_ control. The total startup time is less than 10ms, including 2ms (typ) soft-start. Figure 3 shows the sequence. ______________________________________________________________________________________ 13 MAX17127 Detailed Description MAX17127 Six-String WLED Driver with Integrated Step-Up Converter Shutdown The MAX17127 can be placed into shutdown by pulling the EN pin low. When a critical failure is detected, the IC also enters shutdown mode. In shutdown mode, all functions of the IC are turned off, including the 5V linear regulator. Only a crude linear regulator remains on, providing a 3.3V (typ) output voltage to VDDIO with 1FA current-sourcing capability. Frequency Selection The boost converter switching frequency can be adjusted by the external resistor on the FSLCT pin. The switching frequency adjustable range is 250kHz to 1MHz. High-frequency (1MHz) operation optimizes the regulator for the smallest component size at the expense of efficiency due to increased switching losses. Lowfrequency (250kHz) operation offers the best overall efficiency, but requires larger components and PCB area. VIN 0V VOUT ILED 0V VEN 0V VDDIO 0V CHECK LED STEP-UP REGULATOR SOFT-START MIN FB_ CONTROL (1ms) Figure 3. Startup Sequence 14 ������������������������������������������������������������������������������������� Six-String WLED Driver with Integrated Step-Up Converter LED Current Sources Maintaining uniform LED brightness and dimming capability is critical for backlight applications. The MAX17127 is equipped with a bank of six matched current sources. These specialized current sources are accurate within P 3% and match each other within 2%. They can be switched on and off at PWM frequencies of up to 25kHz. LED full-scale current is set through the ISET pin (10mA < ILED < 30mA). The minimum voltage drop across each current source is 480mV (typ) when the LED current is 20mA. The lowvoltage 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 connecting the respective FB_ pin to AGND at startup. When the IC is enabled, the controller scans settings for all FB_ pins. If an FB_ pin is not connected to AGND, an internal circuit pulls this pin high, and the controller enables the corresponding current source to regulate the string current. If the FB_ pin is connected to AGND, the controller disables the corresponding current regulator. The current regulator cannot be disabled by connecting the respective FB_ pin to AGND after the IC is enabled. Current-Source Fault Protection LED fault open/short is detected after startup. When one or more strings fail after startup, the corresponding current source is disabled. The remaining LED strings are still operated normally. The LED open/short detection is not executed when LED on-time is less than 2Fs. The MAX17127 can tolerate a slight mismatch between LED strings. When severe mismatches or WLED shorts occur, the FB_ voltages are uneven because of mismatched voltage drops across strings. At each LED turn-on, the FB_ voltage is brought down to the regulation voltage quickly. When FB_ voltage is higher than 8V (typ) after LED turn-on, an LED short is detected if at least one FB_ voltage is lower than the minimum FB_ regulation voltage +460mV (typ). The remaining LED strings can still operate normally. The LED short protection is disabled during the soft-start phase of the step-up regulator. Open Current-Source Protection The MAX17127 step-up regulator 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 180mV, the corresponding current source is disabled. The remaining LED strings can still operate normally. If all strings in use are open, the MAX17127 shuts the step-up regulator down. FPO Function The fault conditions trigger FPO function and pull the FPO pin low. Table 3 shows the state of the FPO pin with different fault conditions. Dimming Control The MAX17127 performs brightness control with a PWM input signal. Dimming duty cycle and frequency of current sources follow the signal at the PWM pin directly. All FB_ pins in use are combined to extract a lowest FB_ voltage (LVC) (see Figure 2). LVC is fed into the step-up regulator’s error amplifier and is used to set the output voltage. Table 3. FPO Function Table FAULT CONDITION THERMAL FAULT OUTPUT OVERVOLTAGE INPUT OVERCURRENT LATCHED Yes No (stop switching) Yes (after time expires) FPO PIN STATE Low Low Low ______________________________________________________________________________________ 15 MAX17127 Overvoltage Protection To protect the step-up regulator when the load is open, or if the output voltage becomes excessive for any reason, the MAX17127 features a dedicated overvoltagefeedback input (OVP). The OVP pin is connected to the center tap of a resistive voltage-divider from the high-voltage output. When the OVP pin voltage, VOVP, exceeds 1.25V (typ), a comparator turns off the internal power MOSFET. This step-up regulator switch is reenabled after the VOVP drops 90mV (typ hysteresis) below the protection threshold. This overvoltage-protection feature ensures the step-up regulator fail-safe operation when the LED strings are disconnected from the output. MAX17127 Six-String WLED Driver with Integrated Step-Up Converter Full-Scale and Low-Level LED Current The full-scale LED current is set by: ILED_MAX = 20mA × 180kΩ RISET The acceptable resistance range for ISET is 120kI < RISET < 360kI, which corresponds to full-scale LED current of 30mA > ILED_MAX > 10mA. Thermal Shutdown The MAX17127 includes a thermal-protection circuit. When the local IC temperature exceeds +150NC (typ), the controller and current sources shut down. When the thermal shutdown happens, the FPO output pin is asserted low. The controller and current sources do not restart until the next enable signal is sent or input supply is recycled. 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) L DCM(MAX) = 1 − VOUT(MAX) + VDIODE × VIN(MIN) 2 × η 2 × fSW(MAX) × VOUT(MAX) × IOUT(MAX) where the LDCM(MAX) is the maximum inductor value for DCM, E is the nominal regulator efficiency (85%), and IOUT(MAX) is the maximum output current. All MAX17127 designs should be prototyped and tested prior to production. The output current capability of the step-up regulator 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: 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 the inductor is known, choose the diode and capacitors. SF ILIM = R × 0.97, when D < 30% S SF I = × (1.27-D), when D > 30% LIM RS Design Procedure Step-Up Converter Current Calculation To ensure the stable operation, the MAX17127 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 × VIN(MIN) ) × R S 2 × SF × fSW(MIN) where: SF = 72mV, when VIN < 12.5V 72mV , when VIN > 12.5V SF = − 12.5V V 1 + IN 10.6V SF is a scale factor from the slope compensation depending on input voltage (this allows a higher current capability), the LCCM(MIN) is the minimum inductor value for stable operation in CCM, and RS = 15mI (typ) is the equivalent sensing scale factor from the controller’s internal current-sense circuit. where SF is the scale factor from the slope compensation, 2.5A is the current limit specified 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 × D × VIN VIN I OUT_CCM(MAX) = ILIM − ×η × f × L V SW OUT where ILIM is the current limit calculated above, E 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 × R ON + VDIODE where VDIODE is the forward voltage of the rectifier diode and RON is the internal MOSFET’s on-resistance (0.2I typ). 16 ������������������������������������������������������������������������������������� Six-String WLED Driver with Integrated Step-Up Converter I OUT_DCM(MAX) = L × ILIM 2 × fSW × η × (VOUT + VDIODE ) 2 × VOUT × (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 increase physical size and I2R copper losses. Low inductor values decrease the physical size but increase the current ripple and peak current. Finding the best inductor involves 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 MAX17127 has a fixed internal slope compensation, which requires a minimum inductor value. When CCM mode is chosen, the ripple current and the peak current of the inductor can be minimized. If a small-size inductor is required, DCM mode can be chosen. In DCM mode, the inductor value and size can be minimized, but the inductor ripple current and peak current are higher than those in CCM. The controller can be stable, independent of the internal slopecompensation 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 detailed design procedure for CCM can be described as follows. Calculate the approximate inductor value using the typical input voltage (VIN), the maximum output current (IOUT(MAX)), the expected efficiency (ETYP) taken from an appropriate curve in the Typical Operating Characteristics, and an estimate of LIR based on the above discussion: VIN(MIN) L = VOUT 2 VOUT − VIN(MIN) η TYP I OUT(MAX) × fSW LIR The MAX17127 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 with the following equation: L CCM(MIN) = (VOUT(MAX) + VDIODE − 2 × VIN(MIN) ) × R S 2 × SF × fSW(MIN) where SF is a scale factor from slope compensation, and RS is the equivalent current-sensing scale factor (15mI typ). 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 (EMIN) taken from an appropriate curve in the Typical Operating Characteristics: IIN(DC,MAX) = I OUT(MAX) × VOUT VIN(MIN) × ηMIN ______________________________________________________________________________________ 17 MAX17127 The maximum output current in DCM is governed by the following equation: MAX17127 Six-String WLED Driver with Integrated Step-Up Converter Calculate the ripple current at that operating point and the peak current required for the inductor: ( VIN(MIN) × VOUT(MAX) − VIN(MIN) IRIPPLE = L × VOUT(MAX) × fSW The peak inductor current at minimum input voltage is calculated as follows: ) IPEAK = I IPEAK = IIN(DC,MAX) + RIPPLE 2 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: VIN(MIN) L DCM(MAX) = 1 − VOUT(MAX) + VDIODE × A 10FH inductor is chosen, which is higher than the minimum L that guarantees stability in CCM. 7V × (32V − 7V) 120mA × 32V + = 0.95A 7V × 0.85 2 × 10µH × 32V × 0.9MHz Alternatively, choose a DCM operating mode by using lower inductance and estimating efficiency of 85% at this operating point. Since DCM has higher peak inductor current at lower input, it causes current limit when the parameters are not chosen properly. Considering the case with six 10-LED strings and 20mA LED full-scale current to prevent excessive switch current from causing current limit: 7V L DCM(MAX) = 1 − 32V + 0.4V VIN(MIN) 2 × η 2 × fSW(MAX) × VOUT(MAX) × IOUT(MAX) The peak inductor current in DCM is calculated with the following equation: I OUT(MAX) × 2 × VOUT(MAX) IPEAK = ( × VOUT(MAX) + VDIODE − VIN(MIN) ( ) L × fSW(MIN) × η × VOUT(MAX) + VDIODE ) × (7V) 2 × 0.85 = 3.9µH 2 × 1.1MHz × 32V × 120mA A 3.3FH inductor is chosen. The peak inductor current at minimum input voltage is calculated as follows: IPEAK = 120mA × 2 × 32V × (32V + 0.4V − 7V) 3.3µH × 1.1MHz × 0.85 × (32V + 0.4V) = 1.40A Output Capacitor Selection The inductor’s saturation current rating should exceed IPEAK, and the inductor’s DC current rating should exceed IIN(DC,MAX). For good efficiency, choose an inductor with less than 0.1I series resistance. 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): Considering the circuit with six 10-LED strings and 20mA LED full-scale current, the maximum load current (IOUT(MAX)) is 120mA with a 32V output and a minimal input voltage of 7V. VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR) Choosing a CCM operating mode with LIR = 0.7 at 1MHz and estimating efficiency of 85% at this operating point: 2 7V 32V − 7V 0.85 L= = 12.1µH 32V 120mA × 1MHz 0.7 In CCM, the inductor has to be higher than LCCM(MIN): L CCM(MIN) = (32V + 0.4V − 2 × 7V) × 13.7mΩ = 5.5µH 2 × 25.5mV × 0.9MHz VRIPPLE(C) ≈ I OUT(MAX) VOUT(MAX) − VIN(MIN) C OUT VOUT(MAX) × fSW and: VRIPPLE(ESR) ≈ IPEAKR ESR(COUT) where IPEAK 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). 18 ������������������������������������������������������������������������������������� Six-String WLED Driver with Integrated Step-Up Converter Rectifier Diode Selection The MAX17127’s high switching frequency demands a high-speed rectifier. Schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. The diode should be rated to handle the output voltage and the peak switch current. Make sure that the diode’s peak current rating is at least IPEAK calculated in the Inductor Selection section and that its breakdown voltage exceeds the output voltage. Overvoltage-Protection Determination The overvoltage-protection circuit ensures 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 OVP pin is connected to the center tap of a resistive voltage-divider (R1 and R2 in Figure 1) from the highvoltage output. When the controller detects the OVP pin voltage reaching the threshold VOVP_TH, typically 1.25V, overvoltage protection is activated. Hence, the step-up converter output overvoltage-protection point is: R1 VOUT(OVP) = VOVP_TH × (1 + ) R2 VOUT(OVP) depends on how many LEDs are used for each string and VOUT(OVP) = 1.25V x VOUT, generally and where VOUT is the LED’s operating voltage for each string. In Figure 1, the output OVP voltage is set to: VOUT(OVP) = 1.25V × (1 + 2.21MΩ ) = 39.71V 71.5kΩ Input Capacitor Selection The input capacitor (CIN) filters the current peaks drawn from the input supply and reduces noise injection into the IC. A 4.7FF 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 directly from the output of another regulated supply. In some applications, CIN can be reduced below the values used in the typical operating circuit. Ensure a low-noise supply at IN by using adequate CIN. Alternatively, greater voltage varia- tion 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 on regulator performance. LED characteristics vary significantly from manufacturer to manufacturer. Consult the respective LED data sheets to determine the range of output voltages for a given brightness and LED current. In general, brightness increases as a function of bias current. This suggests that the number of LEDs could be decreased if higher bias current is chosen; however, high current increases LED temperature and reduces operating life. Improvements in LED technology are resulting in devices with lower forward voltage while increasing the bias current and light output. LED manufacturers specify LED color at a given LED current. With lower LED current, the color of the emitted light tends to shift toward the blue range of the spectrum. A blue bias is often acceptable for business applications, but not for high-image-quality applications such as DVD players. Direct-DPWM dimming is a viable solution for reducing power dissipation while maintaining LED color integrity. Careful attention should be paid to switching noise to avoid other display-quality problems. Using fewer LEDs in a string improves step-up converter efficiency, and lowers breakdown voltage requirements of the external MOSFET and diode. The minimum number of LEDs in series should always be greater than 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. Applications Information LED VFB_Variation The forward voltage of each white LED may vary up to 25% from part to part and 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 8V (typ). Otherwise, the internal LED short-protection circuit disables the high FB_ voltage string. ______________________________________________________________________________________ 19 MAX17127 The voltage rating and temperature characteristics of the output capacitor must also be considered. MAX17127 Six-String WLED Driver with Integrated Step-Up Converter FB Pin Maximum Voltage The current through each FB_ pin is controlled only during the step-up converter’s on-time. During the converter off-time, the current sources are turned off. The output voltage does not discharge and stays high. The MAX17127 disables the FB_ current source, 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. 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, and 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, and to the positive terminal of the output capacitors, reconnecting between the output capacitor and input capacitor ground terminals. Avoid using vias in the high-current paths. If vias are unavoidable, use multiple vias in parallel to reduce resistance and inductance. 3) Create a ground island (PGND) consisting of the input and output capacitor ground. Connect all these together with short, wide traces or a small ground plane. Maximizing the width of the power ground traces improves efficiency and reduces output voltage ripple and noise spikes. Create an analog ground island (AGND) consisting of the overvoltage detection divider (R1 and R2) ground connection; the ISET, FSLCT, COMP resistor connections; and the device’s exposed backside pad. Connect the AGND and PGND islands by connecting the AGND pins directly to the exposed backside pad. Make no other connections between these separate ground planes. 4) Place the overvoltage-detection divider resistors as close to the OVP pin as possible. The divider’s center trace should be kept short. Placing the resistors far away causes the sensing trace to become antennae that can pick up switching noise. Avoid running the sensing traces near SW. 5) Place the VIN pin and VDDIO pin bypass capacitors as close to the device as possible. The ground connection of the bypass capacitors should be connected directly to AGND pins with a wide trace. 6) Minimize the size of the SW node while keeping it wide and short. Keep the SW node away from the feedback node and ground. If possible, avoid running the SW node from one side of the PCB to the other. Use DC traces as a shield if necessary. Refer to the MAX17127 Evaluation Kit data sheet for an example of proper board layout. 20 ������������������������������������������������������������������������������������� Six-String WLED Driver with Integrated Step-Up Converter VIN VDDIO SW PGND ISET OVP COMP FSLCT AGND MAX17127 I.C. EN FB1 PWM FB2 R_FPWM FB3 3.3V FB4 FB5 FPO FB6 EP Chip Information PROCESS: BiCMOS Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE DOCUMENT NO. 20 TQFN T2044+3 21-0139 ______________________________________________________________________________________ 21 MAX17127 Simplified Operating Circuit (Direct-PWM Mode) MAX17127 Six-String WLED Driver with Integrated Step-Up Converter Revision History REVISION NUMBER REVISION DATE 0 3/10 DESCRIPTION Initial release PAGES CHANGED — 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. 22 © 2010 Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.