RT9009
8
DS9009-01 April 2011www.richtek.com
Applications Information
Thermal Considerations
For continuous operation, do not exceed absolute
maximum operation junction temperature. 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, T
A
is the ambient temperature and the θ
JA
is
the junction to ambient thermal resistance.
For recommended operating conditions specification of
RT9009, the maximum operating junction temperature is
125°C. The junction to ambient thermal resistance θ
JA
is
layout dependent. As shown in Figure 2, RT9009
TO-263S-5 with 15mm x 15mm PCB copper area on the
standard JEDEC 51-7 four layers thermal test board
thermal resistance θ
JA
is about 29°C/W. The maximum
power dissipation at T
A
= 25°C can be calculated by
following formula :
P
D(MAX)
= (125°C − 25°C) / (29°C/W) = 3.448W for
TO-263S-5 packages
Figure 2. Thermal Resistance θ
JA
vs. Copper Area of
TO-263S-5 Package
The maximum power dissipation depends on operating
ambient temperature for fixed T
J(MAX)
and thermal
resistance θ
JA
. For the RT9009, the Figure 3 of de-rating
curve allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation allowed.
Figure 3. Derating Curve for RT9009 Package
Thermal Resistance vs. Copper Area
0
10
20
30
40
50
60
70
0 50 100 150 200 250 300 350 400
PCB Copper Area (mm
2
)
Thermal Resistance (°C/W)
Single Layout PCB
Four Layout PCB
TO-263S-5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0255075100125
Ambient Temperature (°C)
Maximum Power Dissipation (W)
TO-263S-5
Four Layers PCB
