Al8806, Application information – Diodes AL8806 User Manual
Page 10
AL8806
Document number: DS35144 Rev. 5 - 2
10 of 15
July 2013
© Diodes Incorporated
AL8806
Application Information
(cont.)
Capacitor Selection
The small size of ceramic capacitors makes them ideal for AL8806 applications. X5R and X7R types are recommended because they retain their
capacitance over wider voltage and temperature ranges than other types such as Z5U.
A 2.2
μF input capacitor is sufficient for most intended applications of AL8806; however a 4.7μF input capacitor is suggested for input voltages
approaching 30V.
Diode Selection
For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the
maximum operating voltage and temperature. The Schottky diode also provides better efficiency than silicon PN diodes, due to a combination of
lower forward voltage and reduced recovery time.
It is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output
load current. In particular, it is recommended to have a diode voltage rating at least 15% higher than the operating voltage to ensure safe operation
during the switching and a current rating at least 10% higher than the average diode current. The power rating is verified by calculating the power
loss through the diode.
Schottky diodes, e.g. B240 or B140, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AL8806 applications.
Thermal and Layout Considerations
For continuous conduction mode of operation, the absolute maximum junction temperature must not be exceeded. The maximum power dissipation
depends on several factors: the thermal resistance of the IC package
JA
, PCB layout, airflow surrounding the IC, and difference between junction
and ambient temperature.
The maximum power dissipation can be calculated using the following formula:
where
T
J(MAX)
is the maximum operating junction temperature,
T
A
is the ambient temperature, and
JA
is the junction to ambient thermal resistance.
The recommended maximum operating junction temperature, T
J
, is 125°C and so maximum ambient temperature is determined by the AL8806’s
junction to ambient thermal resistance,
JA
. To support high LED drive at higher ambient temperatures the AL8806 has been packaged in thermally
enhanced MSOP-8EP package.
JA
, is layout dependent and the AL8806’s
JA
in MSOP-8EP on a
51 x 51mm double layer PCB with 2oz copper standing in still air is
approximately 69°C/W.
Therefore the maximum power dissipation at T
A
= +25°C is:
W
45
.
1
W
/
C
69
C
25
C
125
P
)
MAX
(
D
Figure 5, shows the power derating of the AL8806 on an FR4
51x51mm PCB with 2oz copper standing in still air.
As the ambient temperature increases and/or the PCB area reduces
the maximum allowable power dissipated by the AL8806 will
decrease.
MSOP-8EP
0
200
400
600
800
1000
1200
1400
1600
-40 -25 -10
5
20
35
50
65
80
95
110 125
Ambient temperature (°C)
P
o
w
e
r di
s
s
ipa
ti
on
(
m
W
)
Figure 5 Derating Curve