A8512 LED Backlight Driver for Medium/Large Displays Features and Benefits Description ▪ Six individual current sinks capable of 80 mA each ▪ Fixed frequency current mode control with integrated gate driver ▪ 300 kHz to 1 MHz adjustable switching frequency ▪ Controlled startup using options of Enable, PWM signal, or battery voltage ramp ▪ Parallel operation with one boost controller (master) and up to five slave controllers ▪ Active current sharing between LED strings for ±0.6% accuracy and matching ▪ No audible MLCC noise during PWM dimming ▪ Adjustable overvoltage protection (OVP) ▪ Open or shorted LED string protection ▪ Overtemperature, cycle-by-cycle current limit, and undervoltage protection ▪ SOIC 24-pin package for easy single-side PCB manufacturing or QFN 28-contact package with exposed thermal pad for better thermal performance The A8512 is a multi-output WLED/RGB driver for medium size display backlighting. It integrates a boost controller to drive external MOSFET, and six internal current-sinks. The boost converter operates in constant frequency (programmable) current mode control. Packages: 28-contact QFN with exposed thermal pad (ET Package) Not to scale PWM dimming allows LED currents to be controlled in 500:1 ratio. The LED sinks are capable of sinking up to 80 mA each, and can be combined together to achieve even higher currents. Multiple A8512s can be connected in parallel, with one master controller controlling the boost stage, and up to five slave controllers, which act as LED sinks. This allows up to 36 LED strings to be powered by just one boost converter. The A8512 operates from a single supply of 8 to24 V. It provides protection against overvoltage, open or shorted LED string, and overtemperature. A dual level cycle-by-cycle current limit function provides soft start and protects against overloads. The device is provided in a 24-pin SOICW package (LB), with internally fused pins for enhanced thermal dissipation, and a 28-contact 5 mm × 5 mm QFN package (ET), with an exposed thermal pad for enhanced thermal dissipation. Both packages are lead (Pb) free, with 100% matte tin leadframe plating. 24-pin SOICW with internally fused pins (LB package) Typical Application Circuit 18 LEDs per string VBAT 8 to 24 V L1 C1 Q1 C2 RSC ROVP1 P P C3 ROVP2 C4 P P VBAT DRIVER SENSE1 SENSE2 OVP VIN VBIAS R1 C5 Fault FAULT Enable A8512 (LB package) EN PWM DRIVER SENSE1 SENSE2 VIN VBIAS C7 LED1 FAULT FSET Cz1 C6 Master LED3 LED1 LED2 COMP RFSET LED4 FSET RISET ISET VREG7V Rz1 OVP PWM COMP RISET A8512 (LB package) EN LED2 PWM RFSET 18 LEDs per string VOUT D1 LED5 Rz2 LED6 Cz2 AGND AGND LGND LGND PGND LED3 LED4 ISET VREG7V C8 Slave A LED5 LED6 AGND AGND LGND LGND PGND P Control Bus P Figure 1. Typical application circuit for single IC operation, and (in dotted box) master/slave multiple IC operation. A8512L-DS To additional slaves A8512 LED Backlight Driver for Medium/Large Displays Selection Guide Part Number Packing Package A8512ELBTR-T 1000 pieces per 13-in. reel A8512EETTR-T 1500 pieces per 7-in. reel 24-pin SOICW, with internally fused pins for enhanced thermal dissipation 28-contact QFN, with exposed thermal pad for enhanced thermal dissipation Absolute Maximum Ratings Characteristic VIN Pin Input Voltage Symbol Notes Rating Unit VIN –0.3 to 34 V LED1-LED6 Pin Voltage VLEDx –0.3 to 40 V OVP Pin Input Voltage VOVP –0.3 to 50 V SENSE1 and SENSE2 Pin Input Voltage VSENx –0.3 to 1 V VBIAS, VREG7V, and DRIVER Pins –0.3 to 10 V Remaining Pins Input Voltage –0.3 to 7 V Operating Ambient Temperature TA –40 to 85 ºC Maximum Junction Temperature TJ(max) 150 ºC Tstg –55 to 150 ºC Storage Temperature Range E Thermal Characteristics may require derating at maximum conditions, see application information Characteristic Package Thermal Resistance Symbol RθJA Value Unit Package LB, on 2-layer PCB, 1-in.2 2-oz copper exposed area Test Conditions* 51 ºC/W Package LB, on 4-layer PCB, based on JEDEC standard 35 ºC/W Package ET, 4-layer PCB, based on JEDEC standard 32 ºC/W *Additional thermal information available on the Allegro website Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 2 A8512 LED Backlight Driver for Medium/Large Displays Functional Block Diagram L1 D1 VIN Q1 P VIN DRIVER FSET RSC SENSE1 SENSE2 Current Mode Boost Controller OSC RFSET P ROVP P OVP Overvoltage Protection Ref OCP SS COMP CCOMP RISET 6 Control and Feedback LED1 ISET LED2 Reference Current PWM LED3 50 kΩ EN Control Logic/ UVLO LDO VREG7V 7V 0.1 μF LED Select Logic 6 Open/Short LED Detect 6 LED4 LED5 VBIAS +5 V 10 kΩ 5V 0.1 μF LED6 FAULT TSD OVP OCP ET Only LB Only PAD PGND P GND GND LGND LGND GND Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 3 A8512 LED Backlight Driver for Medium/Large Displays DRIVER VREG7V NC VIN 22 LED6 23 FAULT 24 PWM 25 DGND 26 EN 27 NC 28 PGND Pin-out Diagrams 24 PWM EN 1 23 FAULT PGND 2 1 21 LED5 DRIVER 3 22 LED6 2 20 LED4 VREG7V 4 21 LED5 3 19 LGND VIN 5 20 LED4 AGND 6 19 LGND AGND 7 18 LGND VBIAS 8 17 LED3 OVP 9 16 LED2 SENSE2 10 15 LED1 SENSE1 11 14 COMP PAD 4 18 LGND 7 15 LED1 SENSE1 SENSE2 COMP 14 OVP FSET 13 16 LED2 AGND 12 6 AGND 11 VBIAS ISET 10 17 LED3 9 5 8 NC 13 FSET ISET 12 Package ET Package LB Terminal List Table Number Name Function ET LB 26 1 EN 28 2 PGND 1 3 DRIVER Gate driver terminal to drive external MOSFET. 2 4 VREG7V Gate driver supply from internal voltage regulator. Bypass with 0.1 to 1 μF ceramic capacitor to PGND. Device Enable. Apply logic-high signal to enable, low to shut down. Power ground for external FET gate driver. Connect directly to RSC ground and to common star ground. 4 5 VIN 11, 12 6, 7 AGND Input supply voltage for the IC. Analog (signal) GND for the IC. Connect to common star ground. 6 8 VBIAS Bias supply voltage from internal regulator. Bypass with 0.1 to 1 μF ceramic capacitor to AGND 7 9 OVP Overvoltage Protection terminal. Connect this pin to output capacitor through a resistor ROVP to set the OVP threshold. 8 10 SENSE2 Connect to ground side of current sense resistor RSC. 9 11 SENSE1 Connect to high side of current sense resistor RSC. 10 12 ISET Sets 100% Current through LED strings; connect RISET from ISET to AGND. 13 13 FSET Sets switching frequency; connect RFSET from FSET to AGND. 14 14 COMP Compensation pin; connect CCOMP (1 μF typical) capacitor to AGND. 15,16,17 15,16,17 LED1-3 LED current sinks; connect unused LEDx pins to ground to disable. 18,19 18,19 LGND 20,21,22 20,21,22 LED4-6 LED current sinks; connect unused LEDx pins to ground to disable. LED current sink ground; connect to common star ground. 23 23 ¯F ¯¯A¯¯U¯¯L¯¯T ¯ This open-drain output is pulled low when fault condition occurs; connect to external pull-up resistor. 24 24 PWM Pulse width modulation LED-current control; apply logic level PWM for dimming. 25 – DGND Digital ground for input control signals (EN and PWM); connect to common star ground. 3,5,27 – NC Not connected electrically. PAD – PAD Exposed pad. Solder to GND plane for enhanced thermal dissipation. Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 4 A8512 LED Backlight Driver for Medium/Large Displays ELECTRICAL CHARACTERISTICS Valid at VIN = 12 V; TA = 25°C, RFSET = 52 kΩ, RISET = 12.4 kΩ, except indicates specifications guaranteed over the full operating temperature range with TA = TJ , unless otherwise noted Characteristics Input Voltage Range Symbol Test Conditions VIN Min. Typ.1 Max. Unit 8 – 24 V Internal Bias Voltage Range VBIAS 4.75 – 5.5 V Internal Gate Driver Voltage VDRIVER VIN ≥ 10 V 6.5 – 8 V Undervoltage Lockout Threshold for VIN VUVLO VIN falling 5.7 6.5 6.8 V Undervoltage Lockout Hysteresis for VIN VUVLOHYS Supply Current2 IVIN – 0.55 – V Switching at no load – 7 – mA Shutdown, EN = VIL, TA = 25°C – 0.1 1 μA Standby, EN = VIH, PWM = VIL, soft start completed – 2 3 mA 0.8 1 1.25 MHz Boost Controller Switching Frequency fSW Minimum Switch Off-Time toff(min) Driver output – 72 – ns Minimum Switch On-Time ton(min) Driver output – 72 – ns Logic Input Levels (EN and PWM pins) Input Voltage Level Low VIL – – 0.4 V Input Voltage Level High VIH 1.5 – – V Input Leakage Current2 IIN EN = PWM = 5 V – 100 – μA Error Amplifier COMP Pin Source Current IEA(src) VCOMP = 1.5 V – 160 – μA COMP Pin Sink Current IEA(snk) VCOMP = 1.5 V – 20 – μA ¯F ¯¯A¯¯U¯¯L¯¯T ¯= 1 – 1000 – kΩ COMP Pin Pull-Down Resistance RCOMPPD Driver Section Peak Source Current5 Ipk(src) Measured at VDRIVER = 0 V – 2 – A Peak Sink Current5 Ipk(snk) Measured at VDRIVER = VREG7V – 2 – A High Side Gate Drive On Resistance RDS(on)H Measured at VDRIVER = VREG7V / 2 – 4 – Ω Low Side Gate Drive On Resistance RDS(on)L Measured at VDRIVER = VREG7V / 2 – 3 – Ω VSEN VSENSE1 – VSENSE2 80 95 110 mV LEDx Pin Regulation Voltage VLEDx ILED = 80 mA – 1.4 – V ISET to ILEDx Current Gain AISET ISET = 100 μA – 640 – A/A ISET Pin Voltage VISET – 1.235 – V ISET 41 – 125 μA Sense Overcurrent Threshold Voltage LED Current Sinks ISET Allowable Current Range2 Continued on the next page… Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 5 A8512 LED Backlight Driver for Medium/Large Displays ELECTRICAL CHARACTERISTICS (continued) Valid at VIN = 12 V; TA = 25°C, RFSET = 52 kΩ, RISET = 12.4 kΩ, except indicates specifications guaranteed over the full operating temperature range with TA = TJ , unless otherwise noted Characteristics ErrILEDX LEDx Matching4 ΔILEDX LEDx Switch Leakage Min. Typ.1 Max. Unit LED1 through LED6 = 1.5 V, at 100% Current –3 ±0.6 3 % LED1 through LED6 = 1.5 V, ISET = 100 μA –3 ±0.6 3 % Symbol LEDx Accuracy3 Current2 Test Conditions VLEDx = 12 V, EN = 0 – 48 – μA RLEDX ISL PWM = Low, VLEDx = 10 V – 250 – kΩ Soft Start Sense Threshold Voltage VSENS Sense voltage for boost switch current sensing – 28.5 – mV Soft Start LEDx Current Limit Relative to LED 100% Current ILED(SS) Current through enabled LEDx pins during soft start – 8 – % TTSD TJ rising – 165 – °C Short Circuit Detect Voltage VSC Measured on any LEDx pin Output Overvoltage Threshold VOVP ROVP = 0 IOVPLK VOVP = 22 V, EN = VIL, or PWM=VIL LEDx Bleeder Resistor to GND Soft Start Protection Features Thermal Shutdown Threshold OVP Pin Leakage Current2 Overvoltage Protection Sense Current2 IOVPH – 25 – V 18.0 19.5 21.0 V – 0.1 – μA 183 200 217 μA ¯F ¯¯A¯¯U¯¯L¯¯T ¯ Pin Output Leakage2 IFLT V=5V – – 1 μA ¯F ¯¯A¯¯U¯¯L¯¯T ¯ Pin Output Voltage VOL I = 500 μA – – 0.4 V 1Typical specifications are at TA = 25ºC. input and output current specifications, negative current is defined as coming out of the node or pin (sourcing), positive current is defined as going into the node or pin (sinking). 3LED accuracy is defined as (I SET × 640 – ILED(av)) / (ISET × 640), ILED(av) measured as the average of ILED1 through ILED6. Refer to characterization chart for variation over temperature range. 4LED current matching is defined as (I LEDx – ILED(av)) / ILED(av), with ILED(av) as defined in footnote 3. Refer to characterization chart for variation over temperature range. 5Guaranteed by design and characterization. 2For Variation of Total LED Current versus Ambient Temperature Normalized Total ILED(av) (%) 100% Current = 64 mA per channel at 25°C 101.0 100.5 100.0 99.5 99.0 98.5 98.0 -60 -40 -20 0 20 40 60 Temperature, TA (°C) 80 100 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 6 A8512 LED Backlight Driver for Medium/Large Displays Efficiency versus Battery Voltage for Various LED Configurations Efficiency versus Battery Voltage for Various LED Configurations FET = IRFR120N, VIN = 12 V, fSW = 500 kHz L = 22 μH, Load = OVS3W LEDs at 112 mA per string FET = FQB17N08L, VIN = 12 V, fSW = 500 kHz L = 22 μH, Load = OVS3W LEDs at 112 mA per string 95 95 94 94 93 93 92 92 91 91 90 90 Efficiency (%) Efficiency (%) Characteristic Performance 89 88 87 86 3 strings 18 LEDs per string VOUT ≈ 60 V, POUT ≈ 20.2 W 85 83 82 87 86 3 strings 18 LEDs per string VOUT ≈ 60 V, POUT ≈ 20.2 W 2 strings 18 LEDs per string VOUT ≈ 60 V, POUT ≈ 13.4 W 3 strings 14 LEDs per string VOUT ≈ 47 V, POUT ≈ 15.8 W 84 83 82 2 strings 14 LEDs per string VOUT ≈ 47 V, POUT ≈ 10.5 W 81 88 85 2 strings 18 LEDs per string VOUT ≈ 60 V, POUT ≈ 13.4 W 3 strings 14 LEDs per string VOUT ≈ 47 V, POUT ≈ 15.8 W 84 89 2 strings 14 LEDs per string VOUT ≈ 47 V, POUT ≈ 10.5 W 81 80 80 10 11 12 13 14 15 16 17 18 19 20 21 22 23 10 11 12 13 14 VBAT (V) 15 16 17 18 19 20 21 22 23 VBAT (V) Efficiency of the boost converter stage is affected by the selection of power MOSFET, switching frequency, input/output voltages, and output power. The external MOSFET used for the above chart is the IRFR120N, which has a relatively high RDS(on) = 0.21 Ω. This causes higher conduction loss, especially at lower input voltage. The power MOSFET is replaced with FQB17N08L, which has a lower RDS(on) = 0.115 Ω. This results in less conduction loss at lower input voltage, however, the switching loss becomes more significant at higher input voltage. LED Current versus ISET Resistor Value Switching Frequency versus FSET Resistor Value VREF = 1.24 V, Gain = 640 fSW (MHz) = 52 / RFSET (kΩ) 80 1100 1000 70 900 fSW (kHz) ILED (mA) 60 50 40 800 700 600 500 30 400 20 300 10 12 14 16 18 20 22 RISET (kΩ) 24 26 28 30 50 60 70 80 90 100 110 RFSET (kΩ) 120 130 140 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 150 7 A8512 LED Backlight Driver for Medium/Large Displays Normal Startup Power Sequences VBAT = 12 V, Load = 6 strings, 16 LEDs each string, 56 mA per string, Output capacitors = 2 × 2.2 μF ceramic The A8512 can startup with any combination of input and power sequences, as shown in waveforms below: VOUT C3,C4 C2 ILED Symbol Parameter Units/Division C1 C2 C3 C4 t VEN VPWM VOUT Total ILED time 5V 5V 10 V 100 mA 2 ms Normal startup with EN = Low-toHigh transition (PWM = High) VPWM VEN C1 t VOUT C3,C4 Symbol Parameter Units/Division C1 C2 C3 C4 t VEN VPWM VOUT Total ILED time 5V 5V 10 V 100 mA 2 ms Normal startup with PWM = Low-toHigh transition (EN = High) ILED VPWM C2 VEN C1 t VOUT C3,C4 C2 Symbol Parameter Units/Division C1 C2 C3 C4 t VEN VPWM VOUT Total ILED time 5V 5V 10 V 100 mA 2 ms Normal startup with PWM signal toggling at 500 Hz, 50% duty cycle ILED VPWM VEN C1 t ILED C2 VPWM C1 VEN Parameter Units/Division C1 C2 C3 C4 t VEN VPWM VOUT Total ILED time 5V 5V 10 V 100 mA 2 ms Normal startup with battery voltage ramping up from 2 to 12 V (EN= PWM = High) VOUT C3,C4 Symbol t Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 8 A8512 LED Backlight Driver for Medium/Large Displays Typical PWM Operation Waveforms VBAT = 12 V, Load = 6 strings, 16 LEDs each string, 56 mA per string, Output capacitors = 2 × 2.2 μF ceramic VOUT Symbol Parameter Units/Division C1 C2 –* C4 t VEN VPWM VOUT Total ILED time 5V 5V 1V 100 mA 2 ms *Offset = 46 V C2 PWM dimming at 200 Hz 10% duty cycle; Output voltage ripple approximately 0.8 V (out of 50 V) VPWM VEN C1 t VOUT C4 C2 Symbol Parameter Units/Division C1 C2 –* C4 t VEN VPWM VOUT ILED time 5V 5V 1V 100 mA 50 μs *Offset = 46 V ILED PWM dimming at 5 kHz 10% duty cycle VPWM VEN C1 t Ratio of LED Current versus PWM Duty Cycle Ratio of LED Current versus PWM Frequency PWM frequency = 200 Hz PWM duty cycle = 10% 100 100 Ratio of LED Current (%) Ratio of LED Current (%) 90 10 1 80 70 60 50 Compensated pulse width 40 Uncompensated pulse width 30 20 10 0.1 0 0.1 1 PWM Duty Cycle (%) 10 100 0.1 1 PWM Frequency (kHz) 10 To improve the accuracy of PWM dimming at very high frequency and/or very low duty cycle, it is necessary to compensate the PWM pulse width, as described in Application Information section. Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 9 A8512 LED Backlight Driver for Medium/Large Displays Normal Operation and Fault Conditions VBAT = 12 V, Load = 6 strings, 18 LEDs each string, 56 mA per string, Output capacitors = 2 × 2.2 μF ceramic, ROVP = 249 kΩ (OVP at 69 V) VOUT C4 C2,C3 Symbol Parameter Units/Division C1 C2 C3 C4 t VEN VPWM VOUT Total ILED time 5V 5V 10 V 100 mA 2 ms Normal startup with VBAT = 12 V, (VOUT ≈ 58 V when cold) ILED VPWM VEN C1 t OVP tripped C2,C3 C1 Parameter Units/Division C1 C2 C3 C4 t VEN VPWM VOUT Total ILED time 5V 5V 10 V 100 mA 2 ms Startup with one LED string open; (OVP tripped at ≈69 V. Open string removed from regulation. Remaining strings operate normally.) VOUT C4 Symbol ILED VPWM VEN t OVP tripped VOUT ILED C1 Parameter Units/Division C1 C2 C3 C4 t VEN VPWM VOUT Total ILED time 5V 5V 10 V 100 mA 2 ms Startup with all LED strings connected, then one LED string becomes open; (OVP tripped at ≈69 V. Open string removed from regulation. Remaining strings operate normally) C4 C2,C3 Symbol VPWM VEN t VOUT C4 C2,C3 LED shortdetect tripped ILED VPWM VFAULT Symbol Parameter Units/Division C1 C2 C3 C4 t VFAULT VPWM VOUT Total ILED time 5V 5V 10 V 100 mA 2 ms OVP setpoint too high for the application. Startup with one string open; voltage at LEDx pin exceeded Short-Detect threshold (25 V) before OVP could be tripped. IC shuts down. C1 t Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 10 A8512 LED Backlight Driver for Medium/Large Displays Functional Description Overview The A8512 is a multi-output WLED/RGB controller for backlighting medium-size displays. It has an integrated gate driver for driving an external N-channel boost MOSFET. The gate driver voltage is regulated at 7 V, which allows a wide selection of power MOSFET (in contrast to being limited to logic-level MOSFETs when using a 5 V gate driver). The boost controller operates in fixed-frequency current-mode control. The switching frequency can be set in the range from 300 kHz to 1 MHz, by an external resistor, RFSET , connected between FSET and ground. The external MOSFET switch is protected by pulse-by-pulse current limiting. The current limit is independent of duty cycle, and is set using an external sense resistor, RSC. The A8512 has six well-matched current sinks that provide regulated current through the LEDs for uniform display brightness. The boost converter is controlled by monitoring all LEDx pins simultaneously and continuously. Multiple A8512 can be connected in parallel, for applications that require more than six LED strings. One master controller is in charge of the boost converter stage, while other slave controllers act as LED current sinks only. The converter output voltage will be boosted to a level just sufficient for all LED currents to be within regulation. Up to six A8512s (1 master + 5 slaves) can be connected in parallel, which allows up to 36 LED strings to be powered by just one boost converter. The maximum number of LEDs within each string is limited only by the voltage ratings of the external power components (MOSFET, diode, and capacitors). LED Current Setting The maximum LED current can be set, at up to 80 mA/channel, through the ISET pin. Connect a resistor, RISET , between this pin and ground to set the reference current level, ISET . The value of ISET (mA) is determined by: ISET = 1.235 / RISET (kΩ) . (1) The resulting current is multiplied internally with a gain of 640 and mirrored on all enabled LEDx pins. This sets the maximum current through each LEDx, referred as the 100% Current. The LEDx current can be reduced from the 100% Current value by applying an external PWM signal on the PWM pin. Conversely, we can calculate RISET according to the LED current required: RISET = ( 1.235 / ILED ) × 640 . (2) Boost Switching Frequency Setting Connect an external resistor between the FSET pin and GND, to set boost switching frequency, fSW . The value of fSW (MHz) is determined by: fSW = 52 / RFSET , (3) where fSW is in MHz and RFSET is in kΩ. The typical range of RFSET is approximately 51 to 174 kΩ, which corresponds to 1 MHz to 300 kHz. Enable The IC turns on when a high signal is applied on the EN pin, and turns off when this pin is pulled low. The LED current sinks are turned on when both the EN and the PWM inputs are high. Channel Selection The A8512 can be used to drive 1 to 6 LED channels. During startup, the IC detects LED sink pins which are shorted to ground, and disables the corresponding LED channel. Therefore, any unused LED pins must be connected to ground, otherwise the IC will go into overvoltage protection fault during startup. LED pins can be paralleled together for higher current. For example for a 3 parallel string configuration, connect LED1-2, LED3-4, and LED5-6 together to deliver up to 160 mA per LED. PWM Dimming The A8512 has a very wide range for PWM signal input. It can accept a PWM signal from 100 Hz to 5 kHz. When a PWM high signal is applied, the LEDx pins sink 100% Current. When the PWM signal is low, the LED sinks turn off. Referring to figure 13, there is a ramp-up delay between when the PWM signal is applied and when the current reaches the 90% level. To improve current dimming linearity for PWM pulse widths less than 100 μs, increase the applied PWM pulse-width by 3 to 5 μs to compensate for this delay. PWM tD ILED Figure 13. Propagation delay from the PWM signal rising edge to ILEDx reaching the 90% level Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 11 A8512 LED Backlight Driver for Medium/Large Displays Startup Sequence When EN is pulled high, the IC enters soft start. The IC first tries to determine which LEDx pins are being used, by raising the LEDx pin voltage with a small current. After a duration of 512 switching cycles, the LEDx pin voltage is checked. Any LEDx channel with a drain voltage smaller then 100 mV is removed from the control loop. This is the reason why unused LEDx pins should be connected to GND, After the first PWM positive trigger, the boost current is limited to 30% of normal value and all active LEDx pins sink 1/12 of the set current until output voltage reaches sufficient regulation level. When the device comes out of soft start, boost current and the LEDx pin currents are set to normal operating level. Within a few cycles, the output capacitor charges to the voltage required to supply full LEDx current. After output voltage, VOUT , reaches the required level, LEDx current toggles between 0% and 100% with each PWM command signal. In case of a heavy overload on VOUT at startup, the device will stay in soft start mode indefinitely, as the output voltage cannot rise to the LED regulation level. LED Short Detect Any LEDx pins that have a voltage exceeding the Short Circuit Detect Voltage, VSC , cause the device to shut down and this condition is latched. This faults occurs when multiple LEDs short. In case only a few LEDs short, the IC will continue to work as long as power dissipation in the IC is limited. Overvoltage Protection The A8512 has an adjustable overvoltage protection feature to protect the power components (external MOSFET, output diode and capacitors) against output overvoltage. The overvoltage level can be set, from 19.5 V to a higher voltage, with an external resistor, ROVP . When the current though the OVP pin exceeds 200 μA, internal OVP comparator goes high and the device shuts down. The OVP fault disables all LEDx strings that are below regulation, thus preventing them from controlling the boost output voltage. Calculate the value for ROVP (Ω) as follows: ROVP = (VOVP – 19.5) / 200 μA , (4) where VOVP is the required OVP level in V. For single-IC operation, select ROVP such that its OVP setpoint is approximately 10 V above the LED operating voltage at cold. For example, given the pin regulation voltage, VLEDx of 1.4 V (typ.), if LED VF = 3.4 V (max.) and there are 15 LEDs in series, then the operating voltage is approximately: VOUT = 3.4 V × 15 + 1.4 V = 52.4 V . In this case, select OVP at about 60 V, which gives ROVP = 200 kΩ. Open LED Protection During normal operation, if any enabled LED string opens, voltage on the corresponding LEDx pin goes to zero. The boost loop operates in open loop till the OVP level is reached. The A8512 identifies the open LED string when overvoltage is detected. Open strings are then removed from the regulation loop. Afterwards, the boost controller operates in normal manner, and the output voltage is regulated to drive the remaining strings. If the open LED string is reconnected, it will sink current up to the programmed current level. Note: Open strings are removed from boost regulation, but not disabled. This keeps the string in operation if LEDs open for only a short length of time, or reach OVP level on a transient event. The disconnected string can be restored to normal mode by reenabling the IC. It can also be restored to normal operation if the fault is removed from the corresponding LEDx pin, but an OVP event occurs on any other LEDx pin. Overcurrent Protection The IC provides pulse-by-pulse current limiting for the boost MOSFET. The current limit level, ISC (A), can be set by selecting the external resistor, RSC (Ω): (5) RSC = 0.095 / ISC . If the boost output voltage is unable to reach the regulation target even when the switch is operating at maximum current limit, the boost control loop will force the compensating capacitor, CCOMP , to rise in voltage until it reaches the overcurrent fault level (3.4 V approximately). The overcurrent fault forces the device into soft start. Thermal Shutdown (TSD) The IC shuts down when junction temperature exceeds 165°C. It will recover automatically when the junction temperature falls below 125°C approximately. VIN Undervoltage Lockout (UVLO) The device is shut down when input voltage, VIN , falls below VUVLO. Any existing latched fault is cleared. VIN Operating Range Considerations When VIN is above VUVLO and below 10 V, the IC will operate correctly, but its gate driver voltage may not reach the regulation target of 7 V. This may cause excessive switching and conduction loss if the external MOSFET is not fully enhanced. During normal operation, the IC draws approximately 10 to 15 mA from the VIN pin, depending on switching frequency and the external MOSFET. At VIN = 12 V, this translates into 120 to 180 mW of power consumption, most of it dissipated in internal linear regulators. This power increases proportionally Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 12 A8512 LED Backlight Driver for Medium/Large Displays with input voltage. Therefore it is highly recommended to keep VIN between 10 and 24 V during normal operation. If the input battery voltage must be higher than 24 V, a better solution is to power the VIN pin separately using a 12 V supply. Doing this reduces the heat dissipation of the IC, and improves the overall system efficiency. Fault Mode in Single-Controller Operation Fault State AutoRestart Description Yes Fault occurs when output voltage exceeds the OVP setpoint voltage. Used to prevent the output voltage from damaging the power components. Yes Fault occurs when the current through the external MOSFET increases such that the voltage across the SENSE1 and SENSE2 pins exceeds 95 mV typical. The MOSFET switch is turned off on a cycle-per-cycle basis. Overcurrent Protection Yes Multiple pulse-by-pulse current limits will cause the COMP pin voltage to rise. After a time period determined by the COMP pin current and the COMP capacitor, the COMP voltage will exceed the overcurrent detect threshold, forcing a fault. System may hiccup if the total current requirement is too high. Overtemperature Protection Yes Fault occurs when the die temperature exceeds the over-temperature threshold, 165°C typical. LED Short Protection No Fault occurs when the LED pin voltage exceeds VSC , 25 V typical. VIN UVLO No Fault occurs when VIN drops below VUVLO, 6.5 V typical. This fault resets all latched faults. Overvoltage Protection Pulseby-Pulse Current Limit Parallel Operation The A8512 is designed to operate with up to six A8512 devices connected in parallel, in order to drive a greater number of LED strings. In this case, the A8512 which controls the boost converter is designated the master, while the other devices are slaves which serve as current sinks for their own LED strings. Slaves communicate with the master through the shared COMP signal. PWM dimming and protection mechanisms work consistently across all devices. Select ROVP1 for the master controller such that its OVP setpoint is approximately 10 V above the LED operating voltage at cold. Select ROVP2 for each slave controller at approximately 15 to 25 kΩ lower than that for the master. This ensures that, in the case in which an open-LED fault occurs, the slave controllers will enable OVP before the master does. Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 13 A8512 LED Backlight Driver for Medium/Large Displays Application Information PCB Layout Guidelines As with any switching power supply, care should be taken in laying out the board. A switching power supply has sources of high dv/dt and high di/dt which can cause malfunction. All general norms should be followed for board layout. Refer to figure 14 for a typical application schematic. The A8512 evaluation board provides a useful model for designing application circuit layouts. The following guidelines should be observed: • Place bypass capacitors physically close to their respective pins (VIN, VBIAS, and VREG7V). • Place the resistors RFSET and RISET, and the compensation components (Rz and Cz) close to the FSET, ISET, and COMP pins, respectively. Connect the other ends to the common star ground. • A8512 has 50 kΩ internal pull-down resistors on the EN and PWM pins to keep these pins low while driving through tri-state state (for example, shutdown). Add external resistors R2 and R3 between the EN and PWM pins and ground, for added noise immunity. Connect these resistors close to the pins and return to the common star ground. • Sense voltage across RSC with smaller length traces. Place the SENSE1 and SENSE2 traces as close to each other as possible to minimize noise pickup. Connect the SENSE2 trace to the negative end of the resistor and do not connect it to power ground plane. • Route analog ground, digital signal ground, LED ground (LGND pin), and power ground (PGND pin) separately. Connect all these grounds at the common ground plane under the A8512, serving as a star ground. • Place the input capacitors (C1, C2), inductor (L1), boost diode (D1), MOSFET (Q1), and output capacitors (C3, C4) so that they form the smallest loop practical. Avoid long traces for these paths. • Provide a substantial copper plane near MOSFET Q1 and the IC, to provide good thermal conduction. VOUT VBAT 8 to 24 V L1 C1 Q1 C2 RSC ROVP P P 18 LEDs per string D1 C3 C4 P DRIVER SENSE1 SENSE2 OVP VIN VBIAS R1 C5 FAULT EN A8512 (LB package) LED1 LED2 PWM R2 COMP RFSET FSET RISET R3 Cz1 LED4 ISET VREG7V Rz1 LED3 C6 LED5 LED6 AGND AGND LGND LGND PGND P R2, R3 optional (A8512 has internal pull-down resistors) Figure 14. Typical application circuit with single controller; VIN pin tied to VBAT . Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 14 A8512 LED Backlight Driver for Medium/Large Displays Package ET 28-Contact QFN 0.30 5.00 ±0.15 1.15 28 1 2 0.50 28 1 A 5.00 ±0.15 3.15 4.80 3.15 29X D SEATING PLANE 0.08 C C 4.80 C +0.05 0.25 –0.07 PCB Layout Reference View 0.90 ±0.10 0.50 For Reference Only; not for tooling use (reference JEDEC MO-220VHHD-1) Dimensions in millimeters Exact case and lead configuration at supplier discretion within limits shown 0.73 MAX A Terminal #1 mark area B 3.15 2 1 28 3.15 B Exposed thermal pad (reference only, terminal #1 identifier appearance at supplier discretion) C Reference land pattern layout (reference IPC7351 QFN50P500X500X100-29V1M); All pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary to meet application process requirements and PCB layout tolerances; when mounting on a multilayer PCB, thermal vias at the exposed thermal pad land can improve thermal dissipation (reference EIA/JEDEC Standard JESD51-5) D Coplanarity includes exposed thermal pad and terminals Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 15 A8512 LED Backlight Driver for Medium/Large Displays Package LB 24-Pin SOICW with Internally Fused Pins 15.40 ±0.20 8° 0° 24 24 0.33 0.20 7.50 ±0.10 2.20 10.30 ±0.33 9.60 A 1.40 REF 1 2 1.27 0.40 Branded Face 24X SEATING PLANE 0.10 C 0.51 0.31 1.27 BSC 1 2 0.65 1.27 0.25 BSC SEATING PLANE GAUGE PLANE C B PCB Layout Reference View 2.65 MAX 0.30 0.10 Pins 6, 7, 18, and 19 internally fused for enhanced thermal dissipation For Reference Only; not for tooling use (reference MS-013AD) Dimensions in millimeters Dimensions exclusive of mold flash, gate burrs, and dambar protrusions Exact case and lead configuration at supplier discretion within limits shown A Terminal #1 mark area B Reference pad layout (reference IPC SOIC127P1030X265-24M) All pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary to meet application process requirements and PCB layout tolerances Copyright ©2010, Allegro MicroSystems, Inc. The products described here are manufactured under one or more U.S. patents or U.S. patents pending. Allegro MicroSystems, Inc. reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the information being relied upon is current. Allegro’s products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the failure of that life support device or system, or to affect the safety or effectiveness of that device or system. The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use; nor for any infringement of patents or other rights of third parties which may result from its use. For the latest version of this document, visit our website: www.allegromicro.com Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com 16