AL8811
Document number: DS36090 Rev. 2 - 2
7 of 11
www.diodes.com
June 2013
© Diodes Incorporated
L8811
Application Circuit (cont.)
Buck LED Driver
The AL8807A is a step-down DC-DC converter designed to drive LEDs with a constant current. The current through the LED is controlled by R11.
In the present Evaluation board, the current is set to around 660mA based on a resistor value of 0.15Ω. The current is set using the “CTRL” input
pin which in this new version of the IC can vary from 0 to 2.5V, controlling the current from 0mA to the maximum current at 2.5V. This control input
pin is used to lower the LED current as the TRIAC dims the LED. In this way, the energy stored in the two output capacitors of the PFC circuit will
be able to provide current throughout the AC cycle.
TRIAC Phase Detection Circuit
The phase of the TRIAC is detected by using an additional rectifier circuit that generates a voltage in proportional to the phase of the TRIAC Driver.
This is done by rectifying the input AC voltage and averaging the energy using a resistor to charge a capacitor. Two additional resistors in series,
R12 and R8, slowly discharge this circuit so it will follow the input phase change. The two resistors, R12 and R8, are used to scale the voltage so
the range is from 0 to 2.6V to the Buck LED driver control pin.
A simple transistor emitter follower circuit is used to drive a 1KΩ resistor in the emitter circuit. This low resistance is needed to drive the input
control pin of the AL8807A LED driver because the pin outputs a small current of 50uA, which limits the lowest control voltage to around 50 mV.
Setting the LED output current (AL8807A):
The LED output current is set using resister R11 and the formula:
I
LED
= V
TH
/ R11 where V
TH
is equal to 0.1V
For a current of 660mA, R11 is about 0.15Ω
.
Setting the PFC Variables (AL8811)
The choice for the size of the boost converter inductor selected in this design is based on a compromise which it is able to support a peak current to
around 1.5A since the average input voltage will be around 12-14V.
The boost converter (AL8811) includes a current limit resistor R3 which will limit the current through the inductor and thus the power delivered to the
output load. The formula for the resistor is:
I
PK(switch)
= 0.33V / R3
For a current limit of 1A, R3 is 0.33Ω.
In this evaluation design, this value was selected based on having three LEDs in series drawing about 660mA. It was found that two 68µF
capacitors mounted in parallel would just fit into the cavity of the MR16 bulb. The important design goal is to have the PFC circuit, which is used to
always draw current from the Electronic Transformer.
