Datasheet

T
J
+ T
A
) P
D
max x R
θJA
R
θJA
+
T
J
–T
A
P
D
max
P
D
max +
(
5 * 2.5
)
V x 1 A + 2.5 W
R
θJA
max + (125 * 55)°Cń2.5 W + 28°CńW
TPS72501
TPS72515, TPS72516
TPS72518, TPS72525
SLVS341E –MAY 2002–REVISED JUNE 2010 www.ti.com
Even if no external black body radiator type heatsink is attached to the package, the board on which the
regulator is mounted provides some heatsinking through the pin solder connections. Some packages, like the
DDPAK and SOT223 packages, use a copper plane underneath the package or the circuit board's ground plane
for additional heatsinking to improve their thermal performance. Computer-aided thermal modeling can be used
to compute very accurate approximations of an integrated circuit's thermal performance in different operating
environments (e.g., different types of circuit boards, different types and sizes of heatsinks, different air flows,
etc.). Using these models, the three thermal resistances can be combined into one thermal resistance between
junction and ambient (R
qJA
). This R
qJA
is valid only for the specific operating environment used in the computer
model.
Equation 4 simplifies into Equation 5:
(5)
Rearranging Equation 5 gives Equation 6:
(6)
Using Equation 5 and the computer model generated curves shown in Figure 21 and Figure 24, a designer can
quickly compute the required heatsink thermal resistance/board area for a given ambient temperature, power
dissipation, and operating environment.
DDPAK Power Dissipation
The DDPAK package provides an effective means of managing power dissipation in surface mount applications.
The DDPAK 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 DDPAK package enhances the thermal performance of the
package.
To illustrate, the TPS72525 in a DDPAK package was chosen. For this example, the average input voltage is 5
V, the output voltage is 2.5 V, the average output current is 1 A, the ambient temperature 55°C, the air flow is
150 LFM, and the operating environment is the same as documented below. Neglecting the quiescent current,
the maximum average power is:
(7)
Substituting T
J
max for T
J
into Equation 6 gives Equation 8:
(8)
From Figure 21, DDPAK Thermal Resistance vs Copper Heatsink Area, the ground plane needs to be 1 cm
2
for
the part to dissipate 2.5 W. The operating environment used in the computer model to construct Figure 21
consisted of a standard JEDEC High-K board (2S2P) with a 1 oz. internal copper plane and ground plane. The
package is soldered to a 2 oz. copper pad. The pad is tied through thermal vias to the 1 oz. ground plane.
Figure 22 shows the side view of the operating environment used in the computer model.
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