AN75

Issue 1 – January 2011

4

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© Diodes Incorporated 2010

It is advisable to keep below the maximum switching frequency f

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

The value of L

can be set with the following equation,

mH

6

.

6

mA

115

s

μ

9

.

13

V

54

I

t

V

L

L

OFF

)

nom

(

LED

BUCK

=

×

=

Δ

×

=

Due to diameter limitation of the T8 tube, L

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

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

controlling I

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,

mA

297

mH

6

.

6

s

μ

9

.

13

V

54

5

.

0

mA

240

L

t

V

5

.

0

I

I

BUCK

OFF

)

nom

(

LED

)

nom

(

LED

pk

=

×

×

+

=

×

×

+

=

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

the value of R

Ω

=

=

84

.

0

mA

297

25

.

0

In the circuit shown in Figure 1, R

The peak current rating of the L

inductor should be at least 110% of I

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,

mA

234

mH

6

.

6

s

μ

9

.

13

V

59

5

.

0

-

mA

97

2

L

t

V

5

.

0

-

I

I

BUCK

OFF

(max)

LED

pk

LED(min)

=

×

×

=

×

×

=

mA

253

mH

6

.

6

s

μ

9

.

13

V

42

5

.

0

-

mA

97

2

L

t

V

5

.

0

-

I

I

BUCK

OFF

(min)

LED

pk

LED(max)

=

×

×

=

×

×

=

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

practical inductor L

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.