RT9199
10
DS9199-07 September 2007
www.richtek.com
The maximum power dissipation depends on the thermal
resistance of IC package, PCB layout, the rate of
surroundings airflow and temperature difference between
junction to ambient. The maximum power dissipation can
be calculated by following formula:
P
D(MAX)
= ( T
J(MAX)
T
A
) /
θ
JA
Where T
J(MAX)
is the maximum operation junction
temperature 125
°
C, T
A
is the ambient temperature and the
θ
JA
is the junction to ambient thermal resistance. The
junction to ambient thermal resistance for SOP-8 package
(Exposed Pad) is 86
°
C/W, on standard JEDEC 51-7 (4
layers, 2S2P) thermal test board. The maximum power
dissipation at T
A
= 25
°
C can be calculated by following
formula:
P
D(MAX)
= (125
°
C
25
°
C) / 86
°
C/W = 1.163W
Figure 8 shows the package sectional drawing of SOP-8
(Exposed Pad). Every package has several thermal
dissipation paths. As show in Figure 9, the thermal
resistance equivalent circuit of SOP-8 (Exposed Pad). The
path 2 is the main path due to these materials thermal
conductivity. We define the exposed pad is the case point
of the path 2.
Figure 8. SOP-8 (Exposed Pad) Package Sectional
Drawing
Ambient
Molding Compound
Gold Line
Lead Frame
Die Pad
Case (Exposed Pad)
PCB
The thermal resistance
θ
JA
of SOP-8 (Exposed Pad) is
determined by the package design and the PCB design.
However, the package design has been decided. If possible,
it
’
s useful to increase thermal performance by the PCB
design. The thermal resistance can be decreased by
adding copper under the expose pad of SOP-8 package.
Figure 10 show the relation between thermal resistance
θ
JA
and copper area on a standard JEDEC 51-7 (4 layers,
2S2P) thermal test board at T
A
= 25
°
C. We have to consider
the copper couldn
’
t stretch infinitely and avoid the tin
overflow. We use the
“
Dog-Bone
”
copper patterns on the
top layer as Figure 11.
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
Copper Area (mm
2
)
T
θ
J
Figure 10. Relation Between Thermal Resistance
θ
JA
and
Copper Area
Figure 11. Dog-Bone Layout
Exposed Pad
W
≦
2.28mm
Figure 9. Thermal Resistance Equivalent Circuit
Junction
R
DIE
R
DIE-ATTACH
R
DIE-PAD
R
GOLD-LINE
R
LEAD FRAME
Case
(Exposed Pad)
R
PCB
R
PCB
Ambient
R
MOLDING-COMPOUND
path 1
path 2
path 3