AN75
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
ΔI
L
in the output LED current. L
BUCK
can be set with the following equation,
L
BUCK
=
V
LED(nom )
×
t
OFF
Δ
I
L
=
54 V
×
13.9
μ
s
=
6.6mH
115mA
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,
I
pk
=
I
LED(nom)
+
0.5
×
V
LED(nom )
×
t
OFF
L
BUCK
=
240mA
+
0.5
×
54 V
×
13.9
μ
s
=
297mA
6.6mH
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,
R
SENSE
=
0.25
=
0.84
Ω
297mA
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 I
pk
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,
I
LED(min)
=
I
pk
-
0.5
×
V
LED(max)
×
t
OFF
L
BUCK
=
297mA -
0.5
×
59 V
×
13.9
μ
s
=
234mA
6.6mH
I
LED(max)
=
I
pk
-
0.5
×
V
LED(min)
×
t
OFF
L
BUCK
=
297mA -
0.5
×
42V
×
13.9
μ
s
=
253mA
6.6mH
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.
Issue 1 – January 2011
© Diodes Incorporated 2010
4
www.diodes.com
DIODES [ DIODES INCORPORATED ]
