Datasheet

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T
J
+ T
A
) P
D
max R
θJA
(6)
R
θJA
+
T
J
* T
A
P
D
max
(7)
P
D
max +
(
3.3 * 2.5
)
V 1A + 800mW
(8)
R
θJA
max + (125 * 55)°Cń800mW + 87.5°CńW
0
100
120
140
160
180
No Air Flow
80
60
40
20
0.1 1 10
PCB Copper Area (in
2
)
R
θ
JA
− Thermal Resistance (°C/W)
SOT223 POWER DISSIPATION
0
1
2
3
6
0 25 50 75 100 150125
T
A
= 25°C
T
A
− Ambient Temperature (°C)
4
5
4 in
2
PCB Area
0.5 in
2
PCB Area
P
D
− Maximum Power Dissipation (W)
TPS794xx
SLVS349E – NOVEMBER 2001 – REVISED DECEMBER 2005
Equation 5 simplifies into Equation 6 : 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
Rearranging Equation 6 gives Equation 7 :
directly underneath the SOT223 package enhances
the thermal performance of the package.
To illustrate, the TPS79425 in a SOT223 package
was chosen. For this example, the average input
Using Equation 6 and the computer model generated
voltage is 3.3 V, the output voltage is 2.5 V, the
curves shown in Figure 25 , a designer can quickly
average output current is 1 A, the ambient
compute the required heatsink thermal
temperature 55 ° C, no air flow is present, and the
resistance/board area for a given ambient
operating environment is the same as documented
temperature, power dissipation, and operating
below. Neglecting the quiescent current, the
environment.
maximum average power is Equation 8 :
Substituting T
J
max for T
J
into Equation 4 gives
Equation 9 :
(9)
From Figure 25 , R
θ JA
vs PCB Copper Area, the
ground plane needs to be 0.55 in
2
for the part to
dissipate 800 mW. The operating environment used
to construct Figure 25 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.
From the data in Figure 25 and rearranging equation
6, the maximum power dissipation for a different
ground plane area and a specific ambient
temperature can be computed, as shown in
Figure 26 .
Figure 25. SOT223 Thermal Resistance vs PCB
Copper Area
The SOT223 package provides an effective means
of managing power dissipation in surface-mount
Figure 26. SOT223 Maximum Power Dissipation
vs Ambient Temperature
11
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