AL8808EV2

AL8808EV2 EVALUATION BOARD USER GUIDE DESCRIPTION The AL8808EV2 is an evaluation board showing the application of the Diodes Incorporated AL8808 LED driver device on a board suitable for use in an MR16 LED lamp. The board has four main connections: two power inputs P1 and P2, and two LED connections Anode (A) and Cathode (K). The evaluation board is preset to drive 680mA into a single LED. The operating voltage is nominally 12VAC or DC. The 33uH 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, 680mA, is set with the 0R15 sense resistor, R1. The CTRL pin on the device is not used in this application. Warning: The LED will be hot and very bright 12-20VDC 12VAC Figure 1. AL8808EV2 evaluation board power input connection P1 and P2 Figure 2. AL8808EV2 evaluation board LED connection - Anode (A) and Cathode (K) Issue 1 – January 2013 © Diodes Incorporated, 2013 www.diodes.com AL8808EV2 AL8808 DEVICE DESCRIPTION The AL8808 is a continuous mode inductive driver in a TSOT25 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 1000mA. AL8808 DEVICE FEATURES • Drives one or more series-connected LEDs • LEDs up to 1000mA. • Internal 30V switch. • Wide input voltage: 6V to 30V. • Inherent open circuit LED protection. • Built in over temperature protection. • Brightness control using DC or PWM. DEVICE APPLICATIONS • LED flashlights. • High Power LED driving. • Low-voltage halogen replacement LEDs. • Automotive lighting. • Illuminated signs. AL8808 Device Package, Pin and Definitions TSOT25 package Pin Definition Name Pin No SW 1 GND 2 CTRL 3 SET 4 VIN 5 Description Drain of NDMOS switch. Ground (0V). Internal voltage ref. pin (5V) : • 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 AL8808EV2 DEVICE ORDER NUMBER AL8808WT-7 NOTE: Evaluation boards are subject to availability and qualified sales leads. Issue 1 – January 2013 www.diodes.com © Diodes Incorporated 2013 2 AL8808EV2 AL8808EV2 EVALUATION BOARD REFERENCE DESIGN The AL8808EV2 is configured to the reference design shown in Figure 3. For other reference designs or further applications information, please refer to the AL8808 datasheet. The operating voltage is a nominal 12V AC or DC. The nominal current is set at 680mA with a 0R15 sense resistor R1. The circuit operates in continuous mode at approximately 300kHz, (with DC voltage), with a 33uH inductor and one LED. • P1 and P2 are the power input pads, feeding into the bridge rectifier, comprising of D2, D3, D4 and D5. C5 offers 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 a single LED, only C2 is needed. C1 is not fitted. • C3 provides local decoupling for the AL8808 device U1. It is important that this is as close as possible to U1, as shown in the layout. C4 decouples the LED connections to minimise EMI and to smooth the current. • L1 smooth’s the switching at LX into a DC current for the LED string. D1 operates as the freewheeling diode, preventing large voltage spikes at LX. • L2 is not fitted, and the pads are shorted with a link on this evaluation board. Its purpose is to reduce EMI. Both L2 and C6 form a filter circuit to reduce conducted EMI if necessary. In many cases they may not be required, and where that is so, then L2 pads are shorted out and C6 is not fitted. • R1 sets the LED current. The 0R15 resistor shown gives a LED current of 680mA. • Optional extra parts are marked by an asterisk and are NOT fitted on the PCB. Figure 3. Schematic diagram of full PCB including not fitted components options on PCB Issue 1 – January 2013 www.diodes.com © Diodes Incorporated 2013 3 AL8808EV2 AL8808 Operation In normal operation, when voltage is applied at +Vin, the AL8808 internal NDMOS switch is turned on. Current starts to flow through sense resistor R1, 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 R1. The internal circuit of the AL8808 senses the voltage across R1 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, 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, which is sensed by the AL8808. A voltage proportional to the sense voltage across R1 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. Please refer to the datasheets for the threshold limits, AL8808 internal circuits, electrical characteristics and parameters. AL8808EV2 Component List PCB IDENTITY VALUE DESCRIPTION SUGGESTED SOURCE U1 D1, D2, D3, D4, D5 AL8808 DFLS240L Diodes Inc. R1 0R15 C2 C1 150uF 20v --- C3 100nF >=25v C4 1uF >=25v C5 C6 L1 L2 100nF >=25v --33uH 0R0 LED Driver IC Input bridge & freewheeling diode Resistor, 0805, +/-1%