AN75 DIODES [Diodes Incorporated], AN75 Datasheet - Page 4

• Manufacturer Part Number: AN75
• Description: High Power Factor LED Replacement T8 Fluorescent Tube
• Manufacturer: DIODES [Diodes Incorporated]

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,
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,
The peak current is constant and set by the sense resistor R
the value of R
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,
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.
Issue 1 – January 2011
© Diodes Incorporated 2010
L
I
I
I
R
LED(min)
LED(max)
pk
BUCK
SENSE
=
pk
I
.
LED
SENSE
BUCK
=
=
=
(
=
V
nom
I
297
LED
I
pk
pk
. 0
can be easily calculated because the voltage threshold on the CS pin is 0.25V,
BUCK
depends on the desirable peak-to-peak ripple ΔI
)
-
25
-
(
mA
+
nom
0
0
Δ
0
5 .
. Inductor with tolerance rating equal or less than 10% should be chosen to
5 .
I
L
5 .
)
=
×
×
×
×
. 0
V
t
V
OFF
V
84
LED
LED
L
L
LED
L
BUCK
BUCK
Ω
BUCK
(max)
(min)
=
(
SENSE
nom
BUCK
54
×
×
LED(nom)
)
V
115
×
t
t
OFF
OFF
should be greater than I
consists of R5, R6 and R7.
×
t
OFF
13
mA
BUCK
=
=
9 .
.
=
2
2
μ
97
97
240
s
is made up of L3 and L4 as shown in Figure 1.
=
mA
mA
mA
6
4
6 .
-
-
mH
0
+
0
5 .
5 .
0
5 .
×
×
42
59
×
6
6
6 .
54
6 .
SENSE
6
V
V
mH
mH
6 .
V
×
×
pk
mH
13
13
×
. If the LD pin is tied to the VDD pin,
13
and the RMS current rating of the
9 .
9 .
L
μ
9 .
μ
in the output LED current. L
swi(max)
s
s
μ
=
s
=
253
=
234
297
below 150kHz to avoid
mA
mA
mA
pk
www.diodes.com
. As the average
AN75
BUCK
BUCK
.