DF06S datasheet(4/12 Pages) DIODES

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AN75

Issue 1 – January 2011

www.diodes.com

It is advisable to keep below the maximum switching frequency f

swi(max) below 150kHz to avoid

excessive switching loss.

Inductor selection and setting the LED current

The fixed off-time architecture of the AL9910 regulates the average current through the inductor L

BUCK.

The value of L

BUCK depends on the desirable peak-to-peak ripple ΔIL in the output LED current. LBUCK

can be set with the following equation,

115

OFF

)

nom

(

LED

BUCK

Due to diameter limitation of the T8 tube, L

BUCK is made up of L3 and L4 as shown in Figure 1.

The AL9910 constant off-time control loop regulates the peak inductor current I

pk. As the average

inductor current equals the average LED current, the average LED current can be regulated by

controlling I

pk.

Given a fixed inductor value, the change in the inductor current over time is proportional to the voltage

applied across the inductor. During the off-time, the voltage seen by the inductor is the LED stack

voltage. So, the peak inductor current should be regulated to,

297

240

BUCK

OFF

)

nom

(

LED

)

nom

(

LED

The peak current is constant and set by the sense resistor R

SENSE. If the LD pin is tied to the VDD pin,

the value of R

SENSE can be easily calculated because the voltage threshold on the CS pin is 0.25V,

297

RSENSE

In the circuit shown in Figure 1, R

SENSE consists of R5, R6 and R7.

The peak current rating of the L

BUCK should be greater than Ipk and the RMS current rating of the

inductor should be at least 110% of I

LED(nom).

Although the described solution, working in fixed off-time and Continuous Conduction Mode (CCM),

works as a constant current source, a limitation to the output LED current accuracy is its dependency

on the number of LEDs and overall LED chain voltage. The best result can be achieved using a fixed

number of LEDs. A variable number of LEDs results in reduced current precision.

The two extremes of the output LED current can be approximated as,

234

BUCK

OFF

(max)

LED

LED(min)

253

BUCK

OFF

(min)

LED

LED(max)

The above equation shows that the precision of the LED current also depends on the tolerance of

practical inductor L

BUCK. Inductor with tolerance rating equal or less than 10% should be chosen to

ensure good LED current precision at mass production.

Power MOSFET calculation

The power MOSFET is chosen based on maximum voltage stress, peak MOSFET current, total power

losses, maximum allowable working temperature and the gate driver capability of the AL9910.

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