Report number: DIAPR0165 Issue: 1 Project number: AL8806 User guide Engineer: Silvestro Russo Applications Report Date: 26 January 2011 Customer: internal Title: AL8806 preliminary Evaluation Board for MR16 user guide AL8806 DESCRIPTION AL8806EV2 is an evaluation board showing the application of the Diodes Incorporated AL8806 LED driver device on a board suitable for use in an MR16 lamp. The board has four main connections: two power inputs P1 and P2, and two LED connections Anode (A) and Cathode (C). The evaluation board is preset to drive 1300mA into two LEDs. The operating voltage is nominally 12VAC or DC. The 10uH inductor used in the circuit is based on this nominal supply, which should be connected across the P1 and P2 pins. Note: The input bridge rectifier provides the board with reverse battery protection. The nominal current, 1300mA, is set with the 0R075 sense resistors, R1 and R2 in parallel. The CTRL Terminal is not used in this application. Warning: At 12V nominal operation with 1300mA output, the LED will be hot and very bright CONFIDENTIAL 1 of 11 Figure 1: Top Layer Figure 2: Bottom Layer AL8806 DEVICE DESCRIPTION The AL8806 is a continuous mode inductive driver in a MSOP8 EP package, for driving one or more series-connected LEDs efficiently from a voltage source higher than the LED voltage. The device includes the output switch and a current sense circuit, which requires an external sense resistor to set the nominal current up to 1500mA. AL8806 DEVICE FEATURES DEVICE APPLICATIONS • • • • • • • • • Drives one or more series-connected LEDs LEDs up to 1500mA. Internal 30V switch. Wide input voltage: 6V to 30V. Inherent open circuit LED protection. Brightness control using DC or PWM. Low-voltage halogen replacement LEDs High Power LED driving. Illuminated signs. CONFIDENTIAL 2 of 11 AL8806 Device Packages, Pin and Definitions SET GND GND CTRL 1 2 3 4 AL8806 8 7 6 5 VIN N/C SW SW MSOP8 EP AL8806 Device Pin Definition Name Pin No Description SW 1 Drain of NDMOS switch. GND 2 Ground (0V). CTRL 3 Internal voltage ref. pin (2.5V) : • • • SET 4 VIN 5 Leave floating for normal operation. Connect to GND to turn off output current. Drive with DC voltage (0.4V to 2.5V) or with PWM signal to Adjust output current Connect a sense resistor, Rs, from the SET pin to VIN to sense the nominal output current. Nominal Iout = 0.1/ R1 Input voltage: 6V to 30V. Decouple to ground with a 4.7uF or higher ceramic capacitor. ORDERING INFORMATION EVALBOARD ORDER NUMBER DEVICE ORDER NUMBER AL8806E5TA AL8806EV2 Please note: Evaluation boards are subject to availability and qualified sales leads. CONFIDENTIAL 3 of 11 AL8806EV2 EVALUATION BOARD REFERENCE DESIGN The AL8806EV2 is configured to the reference design in Figure 3. The operating voltage is a nominal 12V AC or DC. The nominal current is set at 1300mA with two 0R15 sense resistor, R1 and R2, in parallel and the circuit operates in continuous mode at approximately 600kHz (with Dc voltage), with a 10uH inductor and a two LED. An accurate way of determining the current is to measure the voltage on the sense resistor. A 10K resistor and a 1uF capacitor can be used to form a low pass filter, the voltage across the capacitor representing a more stable DC reading of current. 100mV represents 1 Amp when using a 0R1 sense resistor. For other reference designs or further applications information, please refer to the AL8806 datasheet. D1 DFLS240 SW R2 0R15 VIN Anode Anode C5 100nF C3 100nF GND Figure 3: Schematic diagram CONFIDENTIAL 4 of 11 GND 150uF 150uF SW CTRL NC D5 SBR2A30 D4 SBR2A30 150uF 150uF L1 AL8806 4 GND C7 3 C6 2 C2 TAB C1 AC2 9 P2 7 P1 C4 1uF U1 SET AC1 8 D2 SBR2A30 Vin D3 SBR2A30 1 R1 0R15 SW Cathode 5 6 AL8806 10uH Cathode AL8806EV2 Circuit description P1 and P2 are the power input pads, feeding into the bridge rectifier, comprising of D2, D3, D4 and D5. C5 offers an optional EMI filtering at the input pads. C1 and C2 form the bulk reservoir capacitance, used to sustain operation of the device during the low part of the AC wave. In the case of a system driving two LED, four capacitors of 150uF is needed. C3 provides local decoupling for the AL8806 device U1. It is important that this is as close as possible to U1, as reflected in the layout. R1 and R2 set the LED current, in this case to 1300mA. C4 decouples the LED connections, again to minimise EMI, as well as smoothing the current. L1 smoothes the switching at LX into a DC current for the LED string. D1 operates as the freewheeling diode, preventing large voltage spikes at LX. AL8806 Operation In normal operation, when voltage is applied at +Vin, the AL8806 internal NDMOS switch is turned on. Current starts to flow through sense resistor, inductor L1, and the LED. The current ramps up linearly, and the ramp rate is determined by the input voltage +Vin and the inductor L1. This rising current produces a voltage ramp across the sense resistor. The internal circuit of the AL8806 senses the voltage across R1||R2 and applies a proportional voltage to the input of the internal comparator. When this voltage reaches an internally set upper threshold, the NDMOS switch is turned off. The inductor current continues to flow through R1||R2, L1, the LED and the schottky diode D1, and back to the supply rail, but it decays, with the rate of decay determined by the forward voltage drop of the LEDs and the schottky diode. This decaying current produces a falling voltage at R1||R2, which is sensed by the AL8806. A voltage proportional to the sense voltage across R1||R2 is applied at the input of the internal comparator. When this voltage falls to the internally set lower threshold, the NDMOS switch is turned on again. This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor R1||R2. Please refer to the datasheets for the threshold limits, AL8806 internal circuits, electrical characteristics and parameters. CONFIDENTIAL 5 of 11 AL8806EV2 Component list QUANTITY PCB IDENT VALUE DESCRIPTION 1 U1 AL8806 Diodes Zetex LED Driver IC 5 D1, D2, D3, D4, D5 DFLS230HL Diodes Zetex freewheeling diode and bridge 2 R1, R2 0R15 Resistor, 0805, +/-1% Generic SR732ATTDR150F 4 C1, C2, C6, C7 150uF 20v SMD tantalum Kemet T491X157K020AT <+/-300ppm KOA D case, 1 C3 100nF >=25v X7R 0805 Generic Kemet C0805C104K5RAC (50v) NIC NMC0805X7R104K50TRPF (50v) 1 C4 100nF >=25v X7R 1206 Generic 1 C5 100nF >=25v X7R 0603 Generic 0 C6 1 L1 NOT FITTED 10uH MSS7341– 103ML Note: The component part numbers are correct at the time of publication. Diodes Inc reserves the right to substitute other parts where necessary, without further notification. AL8806EV2 Basic operation at full voltage 1. Connect P1 and P2 to the power supply 2. Set the PSU to 12VAC or 12VDC CONFIDENTIAL 6 of 11 3. Turn on the PSU and the LED will illuminate and the current should be approximately 1300mA. Warning: Do not stare at the LED directly. Changing the LED current 1. Remove R1||R2. 2. Calculate and fit a new sense resistor, R1, the value of which is based on the required LED current without dimming. R1 can be calculated using following equation : R1 = 0.1V/IOUT where IOUT = the LED current. R1 = the sense resistor value in ohms. 0.1V is the nominal sense voltage with ‘CTRL’ open circuit or set to 2.5V. An on-line Calculator is available to speed up the design process at: http://www.diodes.com/destools/calculators.html PERFORMANCE The system efficiency depends on the sense resistor, supply voltage, switching inductor and the number of LEDs. With a 12VDC supply, the switching frequency is typically 600kHz, and the efficiency level is >80% . For further advice, please contact our local FAE or contact our local sales offices as listed below. CONFIDENTIAL 7 of 11 Performances VIN Power IN VLED ILED Temp Temp Inductor T Bridge Efficiency PFC AL8806 12 DC 9.84W 6.69 1.27A 82.3 77.8 86C 86.3% NA 12 Ac 10.2W 6.7V 1.26A 89.5C 78C 91.5C 82% 0.614 CONFIDENTIAL 8 of 11 Figure 4: Switching waveform with VIN=12V DC Figure 5: AL8806 surface temperature with VIN=12VDC CONFIDENTIAL 9 of 11 Figure 6: AL8806 surface temperature with VIN=12VAc Figure 7: Bridge rectifier and freewheeling diode surface temperature with VIN=12VDAC CONFIDENTIAL 10 of 11 IMPORTANT NOTICE DIODES INCORPORATED MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARD TO THIS DOCUMENT, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION). 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