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LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET KEY FEATURES * White LED Backlight Controller for Middle Size Display Panels * Up to 6 LED Strings with 0.5% Precision Current Matching * Wide Input Voltage Range from 6V to 28V * Temperature Compensated LED Current * Ultrasonic Pulse Skipping for audible noise reduction * Direct Ambient Light Sensor Interface for Brightness Control * High Efficiency over Full Dimming Range * Low Standby Current * On-chip Thermal Shut-down * Over-voltage Protection * Packaged in a Thermally Efficient 24 pin 4x4mm MLPQ Package APPLICATIONS * Notebook & Monitor Displays * Entertainment Systems * Automotive Lighting & Panels DESCRIPTION LX1996 is an integrated white LED driver for notebook size displays. It is designed to drive up to six strings of LEDs. The LX1996 consists of a boost converter and six precision current sources. It also includes a direct analog light sensor interface and can be dimmed by a PWM signal or by application of a DC control voltage. Precision current mirror circuitry sets all channels to the same current with a single programming input. The current value can be programmed up to 30mA per channel with a single resistor from the PRG pin to ground. Channel to channel matching is within 0.5 percent typical at 25C ambient of the channels mean at the rated output current. Unique design features include temperature compensated LED current, and audible noise reduction using Ultrasonic Pulse Skipping. Temperature compensated LED current is accomplished by a dedicated input pin (OT) that in conjunction with an external thermistor measures the LED's ambient temperature and thereby prevents over temperature stress to the LEDs by keeping them within their safe operating area. Additional features include low sleep mode current (6A typ.) for extending battery time, multi-mode dimming via ambient light sensor interface, analog dimming, direct PWM dimming and combination of analog and direct PWM dimming for up to a 25000:1 dimming range. The LX1996 is designed to work over a wide input voltage range from 6V to 28Vdc with adjustable over voltage protection (OVP), and short and open LED protection. The LX1996 comes in a compact thermal efficient 4x4mm 24pin MLPQ package. WWW .Microsemi .C OM IMPORTANT: For the most current data, consult MICROSEMI's website: http://www.microsemi.com Patents Pending PRODUCT HIGHLIGHT 6V to 28V N/C VIN EN 5V LX1972 Light Sensor DSEL1 VLDO DI DRV SRC PGND OVP 5V OT Large LCD Backlight LX1996 DSEL2 CS1 CS2 CS3 CS4 CS5 CS6 PRG Thermistor Multi-Mode Dimming Via Ambient Light Sensor and/or Direct PWM Dimming PWM RT PWM CT CLF GND LED Panel Temperature Compensation of LED Current LX1996 LX1996 Current Programing PACKAGE ORDER INFO THERMAL DATA TA (C) -40 to 85 LQ Plastic 4x4 mm MLPQ 24-pin RoHS Compliant / Pb-free JA = 30 C/W THERMAL RESISTANCE-JUNCTION TO AMBIENT LX1996ILQ Note: Available in Tape & Reel. Append the letters "TR" to the part number. (i.e. LX1996ILQ-TR) Junction Temperature Calculation: TJ = TA + (PD x JA). The JA numbers are guidelines for the thermal performance of the device attached to a pc-board system. All of the above assume no ambient airflow. Page 1 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET PACKAGE PIN OUT PGND ABSOLUTE MAXIMUM RATINGS Supply Input Voltage (VIN), EN ........................................................................ -0.3V to 30V CS# pins .............................................................................................................. -0.3V to 28V DSEL1, DSEL2, DI and PWM pins .................................................................. -0.3V to 6.0V OT, DRV, SRC, OVP, PRG, CLF, CT, RT Pins .................................. -0.3V to VLDO + 0.3V CS# Current ................................................................................................................... 35mA Operating Temperature Range ............................................................................... -40 to 85C Maximum Operating Junction Temperature ...................................................................150C Storage Temperature Range............................................................................. -65C to 150C Package Peak Temp. for Solder Reflow (40 seconds maximum exposure) ....... 260C (+0 -5) Notes: Exceeding these ratings could cause damage to the device. All voltages are with respect to Ground. Currents are positive into, negative out of specified terminal. Solder reflow to follow: IPC/JEDEC J-STD-020B 7/02 Pb-Sn Small Body Profile WWW .Microsemi .C OM DRV SRC 24 23 22 21 20 EN VLDO DSEL1 DSEL2 DI OT OVP 19 18 17 N/C VIN 1 2 3 4 5 6 7 8 9 10 11 12 CS6 CS5 CS4 CS3 CS2 CS1 MSC LX1996 XXXX Connect Bottom Pad to GND 16 15 14 13 RT CLF CT PWM LQ PACKAGE (Top View) RoHS / Pb-free 100% Matte Tin pin Finish XXXX = Date code / Lot code identifier FUNCTIONAL PIN DESCRIPTION Name EN VLDO DSEL1 DSEL2 DI OT Pin # 1 2 3 4 5 6 Description Enable Input - Logic input. Pull low to put the system into sleep mode; If not used must be tied to VIN. Internal 5V Regulator Output - Power Pin. Provides a regulated 5V for internal 5V circuits. It can also power an external light sensor and/or SM Buss chip (limited to 1mA). Must be decoupled closely to the GND with a ceramic capacitor typical value of 4.7F. Dimming Mode Select Input 1. Three level input. Refer to Table 1 in the applications section for functionality Dimming Mode Select Input 2. - Two Level Logic Input. Refer to Table 1 in the applications section for functionality Dimming Input - Signal Pin. Connect a DC dimming voltage or the light sensor output to this pin. LED Ambient Over Temperature Protection - Signal Pin - Connect to an external NTC thermistor to monitor the LED light bar temperature. If NTC is not used, then tie this pin to the IC's VLDO pin. The NTC needs to be 100k at 25C and B constant of 4000. Please see the applications section for more details regarding the OT option. Pulse Width Modulated Dimming Signal - Signal Input. PWM input used for multi-dimming mode. With DSEL1 pin tied low, a PWM signal input in the range of 100Hz to 25kHz provides direct digital dimming for the LED outputs. With DSEL1 pin open and DSEL2 pin tied high, this pin is used as a sync input for synchronizing the LX1996's internal PWM dimming clock with an external clock signal of 400ns or greater width. Current source setting resistor. Must be 100K between this pin and ground when Analog to Digital Dimming or Ultrasonic Pulse Skipping modes are used. If neither mode is used, this pin may be left open or tied to VLDO. Internal Digital Dimming frequency setting capacitor. A capacitor between this pin and ground sets the internal Digital Dimming frequency. The relationship between the CT pin capacitor and internal frequency is; F(Hz) = 5e-6/CT(Farads) This pin should be left open or grounded when the internal oscillator is not used. PLL Loop filter connection pin. A loop filter capacitor should be connected between this pin and ground when the internal dimming is used with synchronization mode engaged. Typical capacitor values are; 0.1uF for 200Hz; 1nF for 20Khz. This pin should be left open or grounded when the synchronization option is not used. LED current programming. Connect a resistor between this pin and ground to set the LED current. Refer to applications section for functionality Signal ground. Should be tied at a single common point to Power Ground (PGND pin 20). Current Sink pins - controls individual LED string current. CS pins can be wired in parallel for higher currents. The lowest CS voltage is kept at approx. 0.8V at 20mA. PWM 7 RT 8 GND PRG LX1996 LX1996 CT 9 CLF PRG GND CS1-6 10 11 12 13 to18 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 2 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET FUNCTIONAL PIN DESCRIPTION Name OVP PGND SRC DRV N/C VIN Pin # 19 20 21 22 23 24 Description Over Voltage Protection - Signal input. An external voltage divider sets the maximum boost voltage limit in case of open LED strings. Refer to applications section for functionality. Power Ground. Should be tied at a single common point to Signal Ground (GND pin 12). Boost Converter Current Sense Pin. This connects to the junction of the external N-FET source and the current sense resistor. It is used to set the peak inductor current. External N-FET Gate Drive. Connect to the Gate of the external N-channel MOSFET No Connection. Power Supply Input. Must be closely decoupled to PGND with a 4.7uF ceramic capacitor. WWW .Microsemi .C OM LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 3 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET FUNCTIONAL BLOCK DIAGRAM WWW .Microsemi .C OM SRC VIN 5V REGULATOR VLDO COIL CURRENT SENSE 4-bit DAC BOOST CONVERTER CONTROL LOGIC + OFF TIMER Toff=f(VCC) + ULTRASONIC TIMER DRV NGATE DRIVER FB OVP 2V EN DSEL1 DSEL2 MODE CONTROL LED CURRENT OUTPUT, LED OPEN/SHORT DETECTION, BOOST CONVERTER REGULATION REFERENCE CURRENT SOURCE OUTPUT #6 GND CS6 OT LED CURRENT TEMPRATURE PROFILER LED CURRENT REFERENCE CURRENT SOURCE OUTPUT #5 CURRENT SOURCE OUTPUT #4 CURRENT SOURCE OUTPUT #3 FBref CURRENT SOURCE OUTPUT #2 FB BOOST FB + + + + - CS5 CS4 PRG LED CURRENT REFERENCE FBref Gen. CS3 CS2 DI ANALOG DIMMING 0.2V/2V LIMIT MU X PWM DIGITAL DIMMING CLOCK CS1 FBref CS OVER VOLTAGE LIMIT LED SHORT DETECTION 25V RT CT TRIANGLE WAVE GENERATOR ANALOG TO DIGITAL DIMMING CS OVER VOLTAGE 15V PGND 0.2V LED OPEN DETECTION OUTPUT CONTROL LOGIC CS OPEN CLF PLL OUTPUT ENABLE CURRENT SOURCE OUTPUT #1 LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 4 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET ELECTRICAL CHARACTERISTICS Unless otherwise specified, the following specifications apply over the operating ambient temperature of -400C < Temp <850C, and the following test conditions: VIN=12V, VOT = 3V, EN = VIN , RPRG= 4.99 k, L=6.8H, RSRC= 0.075, COUTPUT = 4.7uF Parameter Symbol Test Conditions / Comment LX1996 Min 6 EN 2.4V, output not switching, VCS = 0.8V EN 0.4V 6 Typ 12 Max 28 2.5 15 Units WWW .Microsemi .C OM Power Supply Input Voltage Quiescent current Sleep Current VIN IIN ON IIN sleep VDSEL1-H V DSEL1-L I DSEL1_H I DSEL1_L DSEL1 = 2.4V DSEL1 = 0.8V -10 1.2 2.4 0.8 600 2.4 0.8 IEN_H IEN_L VPWM-H VPWM-L 600 VDI =0.2 to 2.0V, Analog dimming VTH_DI_H VTH_DI_L FPWM FPWM RT = 100k FINTPWM CT = 200pF, RT = 100k CT =.047F, RT = 100k Sync. freq. Capture Range Minimum PWM Input Pulse Width Percent of Internal PWM Frequency For synchronization -25 400 2 25 100 +25 VDI =2.0V, Analog dimming VDI =0.2V, Analog dimming 100 1.83 2.0 0.2 25000 4 12V EN 2.4V EN < 0.4V 2.4 0.8 4.0 0.4 10 1 4 -4 1.6 2.0 V mA A Logic DSEL1 Input High Voltage DSEL1 Input Low Voltage DSEL1 input current High DSEL1 input current Low DSEL1 Open Voltage DSEL2 Input High Voltage DSEL2 Input Low Voltage DSEL2 Pull-down Resistor EN Input High Voltage EN Input Low Voltage EN input current EN input current PWM Input High Voltage PWM Input Low Voltage PWM Pull-down Resistor 2.4 0.8 10 V V A A V V V k V V A A V V k DIMMING DI Input Bias Current Maximum PRG voltage Minimum PRG voltage Direct Digital Dimming Frequency Direct Digital Dimming Min. PWM Pulse Width (Note 2) RT Reference Voltage Internal PWM Frequency 1 2.17 A V V Hz us V kHz Hz % ns LX1996 LX1996 Device Protection OVP Threshold Voltage Copyright (c) 2007 Rev. 1.0, 2007-12-14 VTH_OVP 1.9 2.0 2.1 V Page 5 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET ELECTRICAL CHARACTERISTICS Unless otherwise specified, the following specifications apply over the operating ambient temperature of -400C < Temp <850C, and the following test conditions: VIN=12V, VOT = 3V, EN = VIN , RPRG= 4.99 k, L=6.8H, RSRC= 0.075, COUTPUT = 4.7uF WWW .Microsemi .C OM LED Short Protection Threshold CS pin Over-voltage Limit Vcs VCS Voltage at CS pin that latches off it's respective string. Voltage at CS pin that clamps Boost Output 15 23 25 27 V V LED Current Sink Outputs LED DC Current Programming Accuracy (including string to string matching) Analog Dimming Accuracy Direct Digital Dimming Accuracy ICS VCS = 0.8V; DI 2.0V; Direct Analog Dimming Mode RPRG = 6.65k (LED current set for15mA);, VCS = 0.8V; DI 2.0V; Direct Analog Dimming Mode DI=0.5V; Direct Analog Dimming Mode Direct Digital Dimming Mode; VCS = 0.8V; PWM input at 25kHz , 25% duty cycle; Average (DC) LED current. VIN = 6V to 28V; -40C to 85C; VCS = 0.8V to 5V; DI 2.0V; Direct Analog Dimming Mode At the lowest CS pin, ICS = 20mA TCS_Rise TCS_Fall ICS/IPRG VPRG VOT VOT 19.2 14.4 20 15 5 5 20.8 15.6 mA mA mA mA ICS ICSAVG Current Matching between strings (Note 3). Regulated CS Pin Voltage LED current On Rise Time LED current Off Fall Time Csx current to PRG current Ratio PRG Reference Voltage ICS-MATCH -1.3 0.5 0.75 200 400 50 2.0 1.3 % V ns ns V LED Current Temperature Profiler LED Over Temperature Shutdown Voltage Threshold OT Recovery Voltage Threshold OT Input Bias Current VOT Input vs. PRG pin Reference Voltage (Note 1, 2) 1.47 1.8 1 VPRG VPRG VPRG VOT = 2.48V VOT = 1.8V VOT = 1.47V 2.0 1.2 0.8 V V A V V V VLDO Regulator VLDO Output Voltage VLDO Source Current UVLO UVLO Hysteresis VLDO IVLDO VUVLO Vin 6.5V; 0mA IVLDO 6.0mA Vin < 6.5V; 0mA IVLDO 6.0mA External available current VLDO rising 4.0 4.25 0.25 4.75 4.5 5.0 5.25 5.25 1 4.5 V V LX1996 LX1996 mA V V Boost Converter DRV Rise Time DRV Fall Time TRISE TFALL CL = 1000pF; 10% to 90% of VLDO CL = 1000pF; 90% to 10% of VLDO 25 14 nsec ns Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 6 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET ELECTRICAL CHARACTERISTICS Unless otherwise specified, the following specifications apply over the operating ambient temperature of -400C < Temp <850C, and the following test conditions: VIN=12V, VOT = 3V, EN = VIN , RPRG= 4.99 k, L=6.8H, RSRC= 0.075, COUTPUT = 4.7uF WWW .Microsemi .C OM Ultrasonic Pulse Skipping (USPS) Maximum Switching Interval RT=100K, Ultrasonic mode 23 40 50 s Thermal Protection Over Temperature Shutdown 140 C (Note 2) Thermal Shutdown Recovery 100 C (Note 2) Notes 1. With a 100K NTC thermistor, Bconstant = 4250, and Rbias = 22.1K, VOT threshold values equate to the following temperature points: a. OT shut off temperature = 85C b. OT derating threshold = 60C c. OT recovery temperature = 75C 2. This parameter is bench characterized but not ATE tested. 3. Based on % variation from (ICS Max+ICS Min)/2. LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 7 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET SYSTEM EFFICIENCY Typical Characteristics @ 25C (Refer to Figure 1) VS LOAD SLEEP CURRENT VS INPUT VOLTAGE WWW .Microsemi .C OM 100% 90% 80% Efficiency 70% 60% System EfficiencyOutput load Load Efficiency Vs Vs Output Analog Dimming Mode Analog dim m ing m ode Sleep current Vs input voltage 11 10 9 8 7 6 5 4 3 6 8 10 12 14 16 18 20 22 24 26 28 Vin (V) 6V-Eff 50% 40% 0 20 40 60 80 Total LED current (m A) 100 120 12V-Ef f 18V-Ef f SUPPLY CURRENT VS INPUT VOLTAGE ( NO LOAD) I_sleep (uA) STRING CURRENT MATCHING Vs Input Voltage Quiescent current Vs input voltage LED String Current Matching Vs. Input Voltage ISET = 20.04mA Percent Variation from Range Midpoint 0.5% 0.4% 0.3% 0.2% 0.1% 0.0% -0.1% -0.2% -0.3% -0.4% -0.5% 3 6 9 12 Input Voltage (V) 15 18 21 Variation ILED1 Variation ILED2 Variation ILED3 Variation ILED4 Variation ILED5 Variation ILED6 2 1.8 I_Q (mA) 1.6 1.4 1.2 1 6 8 10 12 14 16 18 20 22 24 26 28 Vin (V) ANALOG INPUT VS INTERNAL DIGITAL DIMMING PWM output Vs Analog input 100 90 ANALOG INPUT VS ICS(X) LED current Vs Analog input 2.36 2.2 1.97 20 20.1 24 LX1996 LX1996 PWM output (% duty cycle) 80 70 60 50 40 30 20 10 0 0.25 LED current (mA) 1.78 1.59 . 1.2 0.99 0.795 0.598 0.4 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 16 12 10.4 15.6 1.379 8 4 2 4.9 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 Analog input (V) Analog input (V) Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 8 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET SWITCHING WAVEFORMS STARTUP WAVEFORMS Analog dimming. Full load Vled Boost FET Drain EN VCS 6X LED Current +5V 6x LED Current Inductor Current Analog dimming. 50% Load WWW .Microsemi .C OM LED CURRENT WAVEFORMS D = 50% at 17.5kHz, Digital dimming LED CURRENT WAVEFORMS D = 5% at 17.5kHz, Digital dimming VCS VCS 6X LED Current Inductor Current PWM Input 6X LED Current Inductor Current PWM Input LED CURRENT WAVEFORMS D = 1% at 200Hz, Digital dimming Inductor Current Boost Output VCS Inductor Current Boost Output 6X LED Current LED Current LED CURRENT WAVEFORMS USPS Operation; LED Current = 0.5mA LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 9 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET LED CURRENT WAVEFORMS Line Transient Response; VIN 20V to 10V Step Boost Output LED CURRENT WAVEFORMS Line Transient Response; VIN 10V to 20V Step WWW .Microsemi .C OM Boost Output LED Current LED Current VIN VIN LED PROTECTION WAVEFORMS Shorted LED Response - 1 to 5 Shorted LEDS in a Single String 5 LED Short - VCS incremented by shorting one LED at a time up to VCS short threshold of 15V LED PROTECTION WAVEFORMS LED Open Response OVP THRESHOLD Boost Current Boost Voltage VOVP VBOOST VCS CS Current VCS LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 10 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET TYPICAL APPLICATION WWW .Microsemi .C OM 6.8uH 6V to 28V 3 AMP B160 Si3458DV 4.7uF/50V X5R, 1210 0.075 1M Large LCD Backlight 4.7uF/50V X5R 1210 23 24 1 3 N/C VIN EN DSEL1 VLDO DI DSEL2 RT PWM CT CLF GND DRV SRC PGND OVP 22 21 20 19 5V LX1800 SMBus SCL SCD GND ADR VDD REF Aout Ain 5V 2 5 4 8 OT 6 22.1k NTC 100K@25C B=4250 78.7K LX1972 24.9K DPST PWM 4.7uF 7 9 10 12 4.7uF/6.3V,X7R CS1 CS2 CS3 CS4 CS5 CS6 PRG 13 14 15 16 17 18 11 4.99k Figure 1 - Typical Application - This circuit demonstrates the LX1996 and LX1972 interfacing to the LX1800, which is an SMBus ADC/DAC. This application also uses the Display Power Saving Technology (DPST) signal. Key Components - Bill of Materials LX1996ILQ = Multi-string LED Back-light Controller, Microsemi LX1800ILD = SMBus to Analog Interface, Microsemi LX1972IBC = Ambient Light Detector, Microsemi GRM188R60J475KE19 = 4.7uF/6.3V, X7R, 0603, Murata GRM32ER71H475KA88 = 4.7uF/50V, X5R, 1210, Murata SLF6028T-6R8M1R5-PF, 6.8H, 1.5A, TDK SI3458DV = N channel MOSFET, 60V/1.2A, TSOP-6,Vishay NCP18WB104J03RA = NTC, 100K at 25C, B = 4250, Murata B160 = Schottky Diode 1A/60V, SMA, Vishay LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 11 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET OPERATION THEORY BOOST CONVERTER The LX1996 uses a variable peak current variable off time hysteretic PFM converter topology where inductor peak current is digitally controlled. It is a current mode step up converter in PFM burst mode. The LX1996 controls the inductor peak and the off time to minimize the switching frequency variation over a wide input power supply range. It regulates the boost output voltage based on the lowest voltage among the CS pins. This converter stays in burst PFM mode for the entire load range and has extremely fast load response. It automatically increases or decreases the inductor peak current depending on the output load to minimize the output voltage ripple while maintaining the output in regulation. Maximum coil peak current can be up to 1.55A with a 75m sense resistor. For a higher power system, a user can use a smaller resistance current sense resistor. Conversely, a lower power system can use a larger resistance current sense resistor. Typical switching frequency is over 1MHz. A 6.8uH inductor is recommended for systems that require direct digital dimming. A larger inductor may be used for a higher efficiency if the system runs only in analog dimming mode. VLDO REGULATOR LED current can be set with an external resistor using the following formula: I LED = VPRG x 50 R PRG WWW .Microsemi .C OM VPRG = 2V for either Direct Digital Dimming or Direct Analog Dimming. In combined Analog and Digital Dimming, VPRG = 2V with DI 2V. When DI < 2V, then VPRG = DI. DIMMING CONTROL The LX1996 offers 6 methods of dimming using only 2 pins (See Table 1). DIRECT DIGITAL DIMMING (PWM) The LED current source outputs are directly turned on and off by the PWM signal. A high on PWM input pin turns on the output. The current pulse magnitude is independent of PWM duty cycle and is set by the PRG resistor with a fixed 2V reference at the PRG pin. COMBINED ANALOG AND DIGITAL DIMMING (PWM) The LED current source outputs are directly turned on and off by the PWM signal. A high on PWM input pin turns on the output. The current pulse magnitude is dependent on the voltage input at the DI pin. Current is set by the PRG resistor, with a reference voltage at the PRG pin that varies with the voltage input at the DI pin in a 1:1 ratio. The minimum and maximum reference voltage at the PRG pin is clamped at 0.2 and 2 Volts, respectively. Regulated 5V is generated from VIN. It provides power to the internal 5V circuits, an external NFET, and an optional external light sensor (LX1972) or SMBus chip (LX1800). The total maximum output current is limited to 1mA from the VLDO pin. A capacitor must be closely connected across the VLDO pin and GND for noise decoupling and phase compensation. A value of 4.7F is required. The VLDO Regulator shuts off while the chip is in sleep mode (EN=low) to minimize the sleep ANALOG TO DIGITAL DIMMING mode current. The VLDO output voltage is monitored by the Analog to digital dimming mode converts a DC voltage on the under voltage lockout circuit. DI input into a PWM output. DI input will be in the range of LED CURRENT SINKS 0.4 to 2.4V. An internal PWM signal is generated to control the CS1 through CS6 are the LED current sinks. Voltage LED output, just as in direct digital dimming. Two external information on these pins is used to regulate the boost converter components are required to make this mode work; CT and RT. output. Boost converter output regulation is based on the lowest RT is a reference current setting resistor for the internal CS pin voltage. This regulation scheme insures that the LED dimming oscillator, and the ultrasonic pulse skipping timer. RT string with the largest total VF has ample headroom for must be a fixed 100k. The internal oscillator frequency is set regulation, while maximizing the total efficiency for all other by the value of the CT capacitor, and is found by the following strings. CS pin voltage (at the lowest CS pin) is linearly equation: proportional to CS current. For example, typical VCS will be 800mV at ICS = 20mA, and 80mV at ICS = 2mA. The LX1996 uses active current sources for the best string current matching between strings. The high impedance outputs have excellent voltage compliances, therefore, VF matching of the LEDs is less critical. The current sources can be paralleled for higher current rating. F = 5e-6/CT, CT in Farad, F in Hertz. LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 12 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET OPERATION THEORY The table below summarizes the dimming modes. ANALOG TO DIGITAL DIMMING WITH EXTERNAL SYNC The internal dimming oscillator can be synchronized to an external clock on the PWM input. A PLL loop filter capacitor must be connected between CLF and GND pins for synchronization. Use 0.1uF for 200Hz, 0.001uF for 20KHz. The RT, CT and CLF pins can be left open if both the internal dimming and ultrasonic pulse skipping modes are not used. WWW .Microsemi .C OM DSEL1 Low Low Hi Hi Open Open DIRECT ANALOG DIMMING In this mode, a DC voltage on the DI pin is used to vary the reference voltage at the PRG pin. The PRG pin reference voltage, in conjunction with the resistor connected between the PRG pin and ground, sets the LED current. The ratio of DI input voltage to PRG pin voltage is 1:1, up to 2V input. The DI input voltage range is 0V to 5V, however, the voltage at PRG pin is limited to 0.2V to 2V with an internal voltage clamp. Thus the dimming ratio is limited to 10:1. A higher dimming ratio is obtained in the Combined Analog and Digital Dimming mode. Table 1 - Dimming Modes DSEL2 Dimming Method Low Direct Digital Dimming Hi Combined Analog and Digital Dimming Modes Low Direct Analog Dimming Hi Direct Analog Dimming with Ultrasonic Pulse Skipping (USPS) Low Analog to Digital dimming with internal clock Hi Analog to Digital dimming with external clock synchronization. LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 13 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET OPERATION THEORY DIGITAL DIMMING MODES OF OPERATION Direct Digital Dimming Freq. Range * PWM 100Hz to 25kHz * * Controller Performance LED current will be turned On/Off directly by PWM. The maximum LED current is set by a programmable resistor. Dimming ratio is 10:1 at internal frequency = 25kHz. It will be proportionally higher at lower frequency. The minimum pulse width is 4sec Controller Performance Signal into DI pin will be either from an Ambient Light Sensor (ALS) or a variable voltage source, VDI The LED current is linearly controlled by a programmable resistor and VDI Valid VDI range is 0.2V to 2.0V LED current will be turned On/Off directly by PWM. Dimming ratio is 100:1 at PWM = 25kHz. It will be proportionally higher at lower frequency. The minimum pulse width is 4sec Freq. Range Analog to Digital dimming WWW .Microsemi .C OM * * * * Controller Performance PWM duty cycle controlled by the signal into the DI pin Signal into DI pin will be either from an Ambient Light Sensor (ALS) or a variable voltage source, VDI VDI will be in the range of 0.4V to 2.4V PWM signal is generated by an internal oscillator. Dimming frequency is set by a capacitor connected to the CT pin The maximum LED current is set by a programmable resistor connected to the RPRG pin. LED current will be turned On/Off directly by the internal PWM. Dimming ratio is 10:1 at internal frequency = 25kHz. It will be proportionally higher at lower frequency. The minimum pulse width is 4sec Internal frequency 100Hz - 25kHz Combined Analog and Digital Dimming Freq. Range * * * * * * * * PWM 100Hz to 25kHz Analog to Digital dimming with external synchronization clock Freq. Range * * * * Controller Performance PWM duty cycle controlled by the signal into the DI pin Signal into DI pin will be either from an Ambient Light Sensor (ALS) or a variable voltage source, VDI VDI will be in the range of 0.4V to 2.4V PWM signal is generated by an internal oscillator. Dimming frequency is set by a capacitor connected to the CT pin The maximum LED current is set by a programmable resistor connected to the RPRG pin. LED current will be turned On/Off directly by the internal PWM The internal PWM synchronizes to a clock on the PWM input pin Dimming ratio is 10:1 at internal frequency = 25kHz. It will be proportionally higher at lower frequency. The minimum pulse width is 4sec ANALOG DIMMING MODES OF OPERATION Direct Analog Dimming Freq. Range * N/A * * * Controller Performance Signal into DI pin will be either from Ambient Light Sensor (ALS) or the adjustable voltage source, VDI The LED current is linearly controlled by a programmable resistor and VDI Valid VDI range is 0.2V to 2.0V The Dimming ratio can go up to 10:1. Internal frequency 100Hz - 25kHz * * * * LX1996 LX1996 Direct Analog Dimming with Ultra Sonic Pulse Skipping (USPS) Freq. Range N/A * Controller Performance Enables USPS in analog dimming mode to reduce boost inductor noise by moving the switching frequency out of the audible range. Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 14 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET OPERATION THEORY Ultra Sonic Pulse Skipping (USPS) At low LED current level, the boost frequency may go to audible range. With an USPS option (analog dimming mode only), the audible noise is completely eliminated. When this mode is enabled, maximum switching interval is limited to 40S, thus keeping the switching frequency beyond audible range. USPS is an option for analog dimming only. PROTECTION MODES THERMAL SHUTDOWN The LX1996 includes a thermal protection circuit. When the IC senses temperature over +140C (typ), the LX1996 turns off the LED output current and other circuits. The output recovers when the die temperature drops down to 100C. The thermal protection does not latch off the IC. OVER VOLTAGE PROTECTION A simple resistor divider between the boost output and ground is used to program the LX1996's OVP limit. The over-voltage threshold is set to 2V at the OVP pin. When OVP limit threshold is met, the LX1996 will regulate the boost output to the OVP 2V threshold. OVP limit should be set to a voltage approximately 2 Volts above the maximum total VF of the individual diode strings. LED OPEN/SHORT PROTECTION In case of an open string, the open string CS voltage will drop to zero and the boost voltage will rise up to its programmed OVP limit or the CS pin maximum voltage limit. When the boost output reaches the OVP limit or the voltage of any CS pin reaches 25V, the open string is latched off from the regulation loop. At the moment the open string latches off, The boost converter will stop switching until the lowest CS pin voltage gets down to 0.8V. The change in boost output voltage should have no impact to the remaining active LED string currents since they are provided by high impedance current sinks. Once the open condition is corrected, cycling power or the EN input will reset the latched off CS string. In case of the LED short condition, LX1996 output works normally until the CS voltage rises to 15V (up to four shorted LEDs, typical). When the CS pin voltage reaches 15V, the shorted string is latched off and is disconnected from the boost converter's regulation loop. If the latched off string CS pin voltage reaches 25V, the circuit will hold the CS pin voltage at this level. LED CURRENT TEMPERATURE PROFILER For LED Bar temperature sensing, the LED current profiler circuit receives a voltage on the OT pin from the output of a voltage divider network. This network consists of a thermistor and a bias resistor connected between +5V and GND. The LED current profiler circuit will reduce the LED current setting by reducing the PRG pin voltage. With our recommended external components, derating begins at 60C (recommended by most LED manufacturers). The LED current will reduce proportional to the temperature, and shut off when the sensing temperature reaches 85C. The LED current will recover when the sensing temperature drops to 75C. See example chart below (recovery not shown). LED C urrent Vs. Tem perature 25m A WWW .Microsemi .C OM 20m A 15m A 10m A B = 4500 B = 4000 B = 3000 5m A 0C 20C 40C 60C 80C 100C LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 15 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET APPLICATION INFORMATION INDUCTOR CURRENT SENSE RESISTOR SELECTION (RSRC) Peak inductor current is determined by the resistor connected between SRC pin and GND, RSRC. It's value is determined by the following equation: IL PK 112.5mV VCC x 20ns = + mA R SRC L BOOST THERMAL PROFILER COMPONENT SELECTION The following circuit is a typical application using the OT pin for temperature profiling: 5V R1 WWW .Microsemi .C OM Typically, Peak current is determined by the size of the FET desired. Peak current will also determine the amount of energy stored in the inductor during a single switching cycle. The VCS detect signal is fed into a 4 bit up/down counter to determine the peak current level for each boost condition (VIN, VOUT, and IOUT). There are 15 total current steps available. The DC maximum threshold, VSCR, is equal to 112.5mV, and the constant voltage increment is equal to 112.5mV/15 = 7.5mV. If we select RSRC = 75m, then step current: ISTEP.= 7.5mV/75m = 100mA Maximum inductor peak current: IPEAK (MAX) = 100mA x 15 +50mA = 1.55A Note: 50mA term is due to propagation delay and is based on 4.7uH inductor and VIN = 12V. If higher output power is needed, the value of RSENSE must be lowered accordingly. For example: If 50m is used: Each current step will be: ISTEP = 7.5mV/50m = 150mA Maximum inductor peak current: IPEAK (MAX) = 150mA x 15 + 50mA = 2.3A NTC OT Use NTC thermistor with resistance of 100k at 25C. B constant can be from 3000 to 4500. 1. Find out the NTC resistance at 85C (R85) by manufacturing chart or formula: 2. Calculate R1 value to set the shut off temperature at 85C by: R1 = 2.4 x R85. This will set VOT = 1.47V when NTC is at 85C. Thermal profile is set by the internal circuit with the following equation: VPRG=(VOT-0.8)*1.2, VOT=(RTHERM/(RTHERM + R1))*5; Where RTHERM is the thermistor resistor At PRG pin, 2V is the 100% value and 0.8V is the shut off The current sense signal is very sensitive; therefore, the LX1996 point (40%). SRC pin must be connected directly to the junction of the sense resistor, RSRC, and the NFET Source, as close to RSRC as practical. The return side of the sense resistor must tie directly to the PGND pin, and then tie the PGND pin to the ground plane. LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 16 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET APPLICATION INFORMATION BOOST CONVERTER INDUCTOR SELECTION (LBOOST) The LX1996 boost converter operates always in PFM mode, and in conjunction with the current sink circuit to support digital dimming mode with a minimum pulse width of 4s. Therefore, a boost inductor must be selected to satisfied digital dimming mode minimum pulse width requirements, with enough energy to supply the output during this period. The LX1996 controls the off time and peak current for hysteretic PFM converter topology. The off time (TOFF) will vary proportionally with input voltage per the following formula: TOFF = 150 + (22 x VIN ) (nsec) (1) And PFM frequency: FPFM = VIN/(VOUT x TOFF) (Hz) (2) Substitute values into equations 1, 2, and 3: WWW .Microsemi .C OM TOFF FPWM LMIN = = = 282nsec 654.7kHz 4.96uH From equation 4, the maximum value of the boost inductor is: LMAX = 7.7uH The inductor value will be in the range of 4.96uH to 7.7uH. Lower inductance provides faster transient response; higher inductance reduces ripple current and improves efficiency. When selecting the inductor, peak current rating, series resistance, and physical size should be all considered. The right candidate for this part is 6.8uH, 1.6A (6x6mm2) from TDK or Coil craft. For any instant, the stored energy must be large enough to supply the output load and output capacitor. Therefore: OVP SETTING The worst case for white LED forward voltage VF = 3.5V. From equation 1, 2, and 3, the minimum boost inductor value Therefore, the boost voltage that is set for 10 LEDs will be: can be calculated. VLED = 10xVF +2V = 37V With a specified minimum pulse-width of 4sec, the maximum boost inductor value must allow the inductor to reach the To minimize power loss through the OVP resistor network, maximum peak current in less than 4sec. Otherwise, the LED select a top resistor R The bottom resistor is TOP = 1M. current can not get to the maximum. selected: LMAX = VIN dt/di (4) RBOTTOM = VOVP x RTOP/( VLED - VOVP); RBOTTOM = (2 x 1M)/(37-2) = 57.1k 1/2 LMIN*IFPWM VOUT x IOUT (3) To calculate the maximum inductor value, VIN will be at minimum, di will be at maximum, and dt is set at 2us which is Select the closest resistor value: R BOTTOM = 56.2k. 1/2 of the minimum pulse width. For example: With 10 white LED in series for 6 strings with I-LED = 20mA, total IOUT = 120mA. Minimum input voltage is 6V. Per Nichia VF (AVG) = 3.15V, and 3 = 0.15V, then the worst case for the peak LED voltage is 31.7V. Adding 0.8V for the output diode forward drop, the boost voltage will be: VOUT = 31.7V + 0.8V = 32.5V The maximum peak inductor current is 1.55A with 75m current sense resistor. LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 17 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET APPLICATION INFORMATION OUTPUT CAPACITOR SELECTION The output capacitor is preferred to be ceramic for low ESR, small size and relatively lower cost. The capacitor value needs to be large enough to minimize the ripple voltage, which is sensed at CS pins for the feedback current sink regulation. For up to 42 LEDs, we recommend a minimum of 4.7uF; for larger number of LEDs, use larger capacitor such as 6.8uF or 10uF. OUTPUT DIODE SELECTION A high switching frequency demands a high-speed rectifier. A Schottky diode is recommended with a rating of 1A/60V. 1A rating is more than adequate to meet the peak and average current required. 60V rating is to cover up to OVP limit when using 10LEDs per string. SWITCHING MOSFET SELECTION The switching MOSFET should be an N-channel MOSFET with a low RDSON rated at VGS = 4.5V, and a low gate charge. Selection of a FET with the best combined low RDSON and gate charge (Qg) will generally improve efficiency. The MOSFET breakdown rating must be higher than the OVP setting. Good candidates are Si3458DS or AM2358N. WWW .Microsemi .C OM LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 18 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET PCB LAYOUT GUIDE PCB LAYOUT RECOMMENDATIONS (SEE FIGURE 2 SCHEMATIC) It is recommended that decoupling capacitors C7 and C4 be placed as close as possible to the LX1996. The Inductor current sense resistor R1 must tie directly to PGND pin, and then tie to the power ground plane at that pin. For a good practical layout of switching boost circuit, keep loops as short as possible for the boost switching components ( Q1, L1, D1, & C3). Connect a separate signal ground plane to GND pin, a separate power ground plane to PGND pin, and then tie them together at GND pin. It is important that the LX1996 have a good thermal path to ambient to dissipate heat. The simplest way to do this is to heat sink the LX1996 bottom pad directly to the ground plane by placing 4 or more vias in the ground pad directly under the LX1996 footprint. Place the LDO filter capacitor (C5), R5 (RT resistor), C2 (CT capacitor), C1 (CLF capacitor), and R4 (RPRG resistor) as close to the IC as practical. PCB LAYOUT GUIDE - TOP LAYER WWW .Microsemi .C OM LX1996 LX1996 Actual PCB size (H x W): 10mm x 40mm Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 19 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET PCB LAYOUT GUIDE PCB LAYOUT GUIDE - BOTTOM LAYER WWW .Microsemi .C OM PCB LAYOUT GUIDE - TOP SILKSCREEN LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 20 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET PCB LAYOUT GUIDE WWW .Microsemi .C OM L1 J1 1 2 3 4 5 6 7 F1 3.15 A D1 1N5819 4.7uF/50V 1210 GND R8 100k .1uF/50V Vin (6 - 28V) EN 5V DSEL1 DI PWM SLF6028T-4R7M1R6-PF 5V C7 C4 6 x 6 mm Q1 Si2308DS Powermite R2 1M C3 SOT23 4.7uF/50V SM07B-SRSS-TB 4.95 x 8mm R7 N/U 5V 23 24 1 3 4 5 2 N/C Vcc EN DSEL1 DSEL2 DI +5V U1 LX1996 DRV SRC 22 21 5V 1206 R3 R1 0.075 20 R6 22.1k Thermistor 68.1k PGND GND C5 4.7uF 12 AGND OVP 19 1 2 3 4 5 6 7 8 9 10 J2 TO LEDs ASSEMBLY R5 R4 4.99k C1 0.1uF C2 2.2nF 100k 8 9 10 11 7 RT CT CLF PRG PWM OT CS6 CS5 CS4 CS3 CS2 CS1 6 18 17 16 15 14 13 FH12F-10S-0.5SH 6.4 x 9.1mm Unless otherwise specified: - PCB dimension = 10mm x 40mm - All resistors are in ohm, and 0603 size - All capacitors size are 0603 Figure 2 - LX1996 Example Schematic LX1996 LX1996 Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 21 LX1996 TM (R) Multi-string LED Back-light Controller PRODUCTION DATASHEET PACKAGE DIMENSIONS WWW .Microsemi .C OM Dim MILLIMETERS MIN MAX INCHES MIN MAX D b L E D2 E2 e A1 A3 A A A1 A3 b D E e D2 E2 L Notes: 0.80 1.00 0 0.05 0.20 REF 0.18 0.30 4.00 BSC 4.00 BSC 0.50 BSC 2.30 2.55 2.30 2.55 0.30 0.50 0.031 0.039 0 0.002 0.008 REF 0.007 0.011 0.157 BSC 0.157 BSC 0.019 BSC 0.090 0.100 0.090 0.100 0.012 0.020 1. Controlled dimensions are in mm, inches are for reference only. 2. Dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm (.006") on any side. Lead dimension shall not include solder coverage. LX1996 LX1996 PRODUCTION DATA - INFORMATION CONTAINED IN THIS DOCUMENT IS PROPRIETARY TO MICROSEMI AND IS CURRENT AS OF PUBLICATION DATE. THIS DOCUMENT MAY NOT BE MODIFIED IN ANY WAY WITHOUT THE EXPRESS WRITTEN CONSENT OF MICROSEMI. PRODUCT PROCESSING DOES NOT NECESSARILY INCLUDE TESTING OF ALL PARAMETERS. MICROSEMI RESERVES THE RIGHT TO CHANGE THE CONFIGURATION AND PERFORMANCE OF THE PRODUCT AND TO DISCONTINUE PRODUCT AT ANY TIME. Copyright (c) 2007 Rev. 1.0, 2007-12-14 Microsemi Analog Mixed Signal Group 11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570 Page 22 |
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