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AN75

High Power Factor LED Replacement T8 Fluorescent Tube using the AL9910 High Voltage LED Controller

Diodes Incorporated. 

AN75

It is advisable to keep below the maximum switching frequency fswi(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 LBUCK.

The value of LBUCK depends on the desirable peak-to-peak ripple ΔIL in the output LED current. LBUCK

can be set with the following equation,

LBUCK

=

VLED(nom) × tOFF

ΔIL

=

54V ×13.9μs

115mA

= 6.6mH

Due to diameter limitation of the T8 tube, LBUCK is made up of L3 and L4 as shown in Figure 1.

The AL9910 constant off-time control loop regulates the peak inductor current Ipk. As the average

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

controlling Ipk.

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,

Ipk

= ILED(nom)

+

0.5 × VLED(nom)

LBUCK

× tOFF

=

240mA

+

0.5 × 54V ×13.9μs

6.6mH

=

297mA

The peak current is constant and set by the sense resistor RSENSE. If the LD pin is tied to the VDD pin,

the value of RSENSE can be easily calculated because the voltage threshold on the CS pin is 0.25V,

R SENSE

=

0.25

297mA

= 0.84Ω

In the circuit shown in Figure 1, RSENSE consists of R5, R6 and R7.

The peak current rating of the LBUCK should be greater than Ipk and the RMS current rating of the

inductor should be at least 110% of ILED(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,

ILED(min)

= Ipk

-

0.5 × VLED(max) × tOFF

LBUCK

=

297mA

-

0.5 × 59V ×13.9μs

6.6mH

=

234mA

ILED(max)

= Ipk

-

0.5 × VLED(min)

LBUCK

× tOFF

=

297mA

-

0.5 × 42V ×13.9μs

6.6mH

=

253mA

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

practical inductor LBUCK. 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.

Issue 1 – January 2011

4

© Diodes Incorporated 2010

www.diodes.com

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