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SLVS351I – SEPTEMBER 2002 – REVISED MAY 2006
5
T
A
= 55°C
180
°
C/W
160
140
120
100
80
60
40
20
No Air Flow
150 LFM
3
No Air Flow
2
Rθ JA − Thermal Resistance −
4
250 LFM
P
D
Maximum (W)
1
0.1
1
10
Copper Heatsink Area − cm
2
100
0
0.1
1
PCB Copper Area − in
2
10
Figure 27. Maximum Power Dissipation vs
Copper Heatsink Area
Figure 28. SOT223 Thermal Resistance vs PCB
Area
From the data in
and rearranging
the maximum power dissipation for a
different ground plane area and a specific ambient
temperature can be computed (see
6
T
A
= 25°C
5
SOT223 Power Dissipation
The SOT223 package provides an effective means
of managing power dissipation in surface mount
applications. The SOT223 package dimensions are
provided in the
Mechanical Data
section at the end
of the data sheet. The addition of a copper plane
directly underneath the SOT223 package enhances
the thermal performance of the package.
To illustrate, the TPS72525 in a SOT223 package
was chosen. For this example, the average input
voltage is 3.3V, the output voltage is 2.5V, the
average output current is 1A, the ambient
temperature 55°C, no air flow is present, and the
operating environment is the same as documented
below. Neglecting the quiescent current, the
maximum average power is calculated as
P
D
max
+
(3.3
*
2.5) V x 1 A
+
800 mW
(10)
Substituting T
J
max for T
J
into
gives
R
max + (125 * 55)°C 800 mW + 87.5°C W
θJA
(11)
4
P
D
Maximum (W)
4 in
2
PCB Area
3
0.5 in
2
PCB Area
2
1
0
0
25
50
75
T
A
(°C)
100
125
150
From
R
θJA
vs PCB Copper Area, the
ground plane needs to be 0.55in
2
for the part to
dissipate 800mW. The operating environment used
to construct
consisted of a board with 1-oz.
copper planes. The package is soldered to a 1-oz.
copper pad on the top of the board. The pad is tied
through thermal vias to the 1-oz. ground plane.
Figure 29. SOT223 Power Dissipation
12
TI [ TEXAS INSTRUMENTS ]
