AL8807AEV3 EVALUATION BOARD USER GUIDE DESCRIPTION The AL8807AEV3, Figure 1, is a double sided evaluation board for the AL8807A step-down, or ‘buck’, LED driver with internal switch. The evaluation board is preset to drive 1A into a single LED, or multiple LEDs, the maximum number of which depends on their total forward voltage drop and the supply voltage. The operating voltage is between 6 and 36 volts, but it must be greater than the voltage drop across the LED chain. The 68uH inductor used in the circuit is based on this nominal supply. The evaluation board should be connected as in Figure 1 below. Note: The evaluation board does not have reverse supply protection. The nominal output current, 1A, is set with the 0R1 sense resistor, R1. Terminal CTRL provides a connection point for DC dimming. Warning: At 36V nominal operation with 1A output, the LED will be hot and very bright + V 1-8 LEDs 6-36VDC Figure 1: AL8807AEV3 evaluation board and connection diagram Issue 2 – October 2012 © Diodes Incorporated, 2012 www.diodes.com AL8807AEV3 AL8807AEV3 Connection Point Definition Name Description Vin Positive supply voltage. 6 to 36V GND Supply Ground (0V). CTRL LED A LED K Internal voltage ref. pin (2.5V). This pin can be used for dimming. Leave floating for full brightness. LED A connects to the external LED anode LED K connects to the external LED cathode AL8807A DEVICE DESCRIPTION The AL8807A is a continuous mode inductive driver in a SOT25 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 1A. AL8807A DEVICE FEATURES • Drives one or more series-connected LEDs • LEDs up to 1A. • Internal 40V switch. • Wide input voltage: 6V to 36V. • Inherent open circuit LED protection. • Brightness control using a DC voltage. DEVICE APPLICATIONS • LED flashlights. • High Power LED driving. • Low-voltage halogen replacement LEDs. • Automotive lighting. • Illuminated signs. AL8807A Device Packages, Pin and Definitions SET 1 8 GND 2 7 6 5 GND 3 CTRL 4 VIN N.C. SW SW Top View SOT25 package MSOP8EP package AL8807A 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) : • Leave floating for normal operation. • Drive with DC voltage (0.25V to 2.5V) to adjust output current SET 4 Connect a sense resistor, R1, from the SET pin to VIN to sense the nominal output current. Nominal Iout = 0.1/ R1 VIN 5 Input voltage: 6V to 36V. Decouple to ground with a 2.2uF or higher ceramic capacitor. Issue 2 – October 2012 www.diodes.com © Diodes Incorporated 2012 2 AL8807AEV3 ORDERING INFORMATION EVALBOARD ORDER NUMBER AL8807AEV3 DEVICE ORDER NUMBER AL8807AMP-13 Please note: Evaluation boards are subject to availability and qualified sales leads. AL8807AEV3 EVALUATION BOARD REFERENCE DESIGN The AL8807AEV3 is configured to the reference design in Figure 2. The operating voltage is up to 36V. The nominal current is set at 1A with a 0R1 sense resistor R1. The circuit operates in continuous mode between approximately 40kHz and 130kHz, depending on the input voltage, with a 68uH inductor and one LED. For DC dimming, the CTRL pin may be driven between 0.25V and 2.5V adjusting the output current from approximately 10% to 100% of ILED. Shorting R3 will connect the test pin CTRL directly to device pin CTRL, if required. For other reference designs or further applications information, please refer to the AL8807A datasheet. Figure 2: Schematic diagram Issue 2 – October 2012 www.diodes.com © Diodes Incorporated 2012 3 AL8807AEV3 AL8807A Operation In normal operation, when a voltage is applied at +Vin, the AL8807A internal NDMOS switch is turned on. Current starts to flow through sense resistor R1, inductor L1, and the LED. The current ramps up linearly, the ramp rate being determined by the input voltage +Vin and the inductor L1. This rising current produces a voltage ramp across R1. The internal circuit of the AL8807A 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 AL8807A. 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, AL8807A internal circuits, electrical characteristics and parameters. AL8807AEV3 Component list Ref Value Package Part Number U1 AL8807A MSOP AL8807AMP-13 D1 60V, 2A R1 R2 0R1 R3 C1, C5 1k0 4.7uF 50V 0805 1210 C2, C3 100nF, 100V 0805 DFLS260 1206 NOT FITTED C1210X475K5RAC NMC0805X7R104K100 PF GRM21BR71H104KA01L C4 Manufactu rer Diodes Zetex Diodes Zetex Generic 1uF 100V 1206 NMC1206X7R104K100 C6, C7, C8, C9 C12 0805 Generic Generic KEMET Generic NIC Comps MURATA Generic NIC Comps Generic NOT FITTED C10, C11 1n5, 100V L1 68uH 0805 Generic MSS1038-683ML NPIS24H680MTRF NPIS104F680MTRF Notes DC-DC converter Schottky diode 1% 250mW Fit with R1 to adjust LED current if required 1% 125mW X7R X7R X7R Fit to improve EMC performance if required C0G Coilcraft NIC Comps 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. Issue 2 – October 2012 www.diodes.com © Diodes Incorporated 2012 4 AL8807AEV3 AL8807AEV3 Basic operation at full voltage 1. Connect Vin and GND Warning: The board does not feature reverse battery/supply protection. 2. Set the PSU to 36V 3. Turn on the PSU. The LED will illuminate and the current should be approximately 1A. Warning: Do not stare at the LED directly. Minimising the output current Shorting the CTRL pin to GND will cause the LED current to be reduced. On some boards, however, the LED may not be completely turned off. Soft start Adding capacitor C3 will create a soft-start power-up sequence (0.1ms/nF). Changing the LED current 1. Remove R1. 2. Calculate and replace 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. The device calculator at the address below can be used to speed up the redesign phase: http://www.diodes.com/destools/calculators.html PERFORMANCE The system efficiency depends on the sense resistor, supply voltage, switching frequency and the number of LEDs. With a 12V supply and two LEDs, the switching frequency is typically 102kHz, and the efficiency level is 88%. For further advice, please contact your local Diodes Field Applications Engineer, or one of our sales offices listed on the back page of this document. Issue 2 – October 2012 www.diodes.com © Diodes Incorporated 2012 5 AL8807AEV3 INTENTIONALLY BLANK Issue 2 – October 2012 www.diodes.com © Diodes Incorporated 2012 6 AL8807AEV3 INTENTIONALLY BLANK Issue 2 – October 2012 www.diodes.com © Diodes Incorporated 2012 7 AL8807AEV3 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). Diodes Incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or other changes without further notice to this document and any product described herein. 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