AL8805EV2

Issue 1 – August 2010

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© Diodes Incorporated 2010

3

AL8805EV2 EVALUATION BOARD REFERENCE DESIGN

The AL8805EV2 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 680mA with a 0R15

sense resistor, R1 and the circuit operates in continuous mode at approximately 300kHz (with Dc

voltage), with a 33uH inductor and a single 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 AL8805 datasheet.

Figure 3: Schematic diagram

AL8805EV2 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 a single LED, only one capacitor of 150uF is needed.

L2 and C6 are a filter 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.

C3 provides local decoupling for the AL8805 device U1. It is important that this is as close as possible to U1, as

reflected in the layout.

R1 sets the LED current, in this case to 680mA.

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.