New prod uc t al8808, Application information, Al8808 – Diodes AL8808 User Manual
Page 15
AL8808
Document number: DS35648 Rev. 2 - 2
15 of 20
January 2013
© Diodes Incorporated
NEW PROD
UC
T
AL8808
Application Information
(cont.)
Thermal 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 (θ
JC
), PCB layout, airflow surrounding the IC, and difference
between junction and ambient temperature.
The maximum power dissipation can be calculated using the following formula:
JA
A
)
MAX
(
J
)
MAX
(
D
T
T
P
θ
−
=
where
T
J(MAX)
is the maximum operating junction temperature; for the AL8808 this is +125°C.
T
A
is the ambient temperature, and
θ
JA
is the junction to ambient thermal resistance.
The major thermal path for the TSOT25 package is pin 2 (GND pin) and it is important for minimizing the θ
JA
that a suitable area and thermal
mass is associated with pin 2. The thermal impedance from the AL8808 junction to pin 2 is approximately 57°C/W.
The AL8808’s θ
JA
on a 25 x 25mm double sided FR4 PCB with minimum recommended pad layout on top layer and thermal vias to maximum
area on bottom layer with 1oz copper standing in still air is approximately 209°C/W. Yielding a maximum power dissipation at 25°C of 0.47W
The AL8808’s θ
JA
on a 25 x 25mm double sided FR4 PCB with maximum area top and bottom with vias is approximately 151°C/W; which gives a
maximum power dissipation at 25°C of 0. 66W.
Figure 42 shows the power derating of the AL8808 on different area PCB with maximum area on bottom of PCB with 1 and 2oz copper standing in
still air.
Figure 42. Derating Curve for Different PCB
(50mm) with 2oz Cu max area top
2
(50mm) with 1oz Cu max area top
2
(25mm) with 2oz Cu max area top
2
(25mm) with 1oz Cu min area top
2
(25mm) with 2oz Cu min area top
2
(25mm) with 1oz Cu min area top
2
(50mm) with 1oz Cu min area top
2
-10
5
-40
-25
50
65
20
35
110
125
80
95
AMBIENT TEMPERATURE (C)
P
O
WE
R
DI
SSI
P
A
T
IO
N (
W
)
1.0
0.9
0.1
0
0.8
0.7
0.3
0.2
0.6
0.5
0.4