Al8806, Application information – Diodes AL8806 User Manual
Page 12
AL8806
Document number: DS35144 Rev. 5 - 2
12 of 15
July 2013
© Diodes Incorporated
AL8806
Application Information
(cont.)
Application Example
(cont.)
The output of the 12V transformer is fed into the bridge rectifier, comprising of D2, D3, D4 and D5. C10 offers an optional EMI filtering at the input.
(see figure 9)
Capacitor, C
BULK
, (capacitors C1 and C2 on the PCB and BOM are multiple components to reduce size) forms the bulk reservoir capacitance -
used to sustain operation of the device during the low part of the rectified AC wave. In the case of a system driving two LEDs at 1.1A, one 470µF
plus 47µF capacitors are required.
Capacitors C3 and C9 provide local decoupling for the AL8806 and for the schottky freewheeling diode, D1. It is important that C3 is as close as
possible to the AL8806’s V
IN
pin and that C9 is as close to the V
IN
and shottky diode as possible, as reflected in the layout shown below in figure 8.
The LED current is determined by the total resistance between the V
IN
and SET pins (R1 and R2 in figures 7 (schematic) and 8 (top PCB) and
BOM).
C4 decouples the LED connections, again to minimize EMI, as well as smoothing the current.
D1 operates as the freewheeling diode, providing a current path for the LED current when the power switch at SW pin is off.
Capacitors C5 to C9, 1.5nF have been added to improve the overall EMI performance. C5 is connected across the Supply pins to reduce radiation
via the wiring. C6 and C7 also assist the reduction of EMI via the wiring. C8 snubs high frequency radiation from the harmonics in the switching
regulator.
A large number of vias have been used to improve the grounding effect throughout the PCB to improve the evaluation board’s EMI performance.
Figure 8 AL8806EV6 evaluation board (Top)
Figure 9 AL8806EV6 evaluation board (Bottom)