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SLVS351I – SEPTEMBER 2002 – REVISED MAY 2006
°
C/W
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, and different
air flows, etc.). Using these models, the three
thermal resistances can be combined into one
thermal resistance between junction and ambient
(R
θJA
). This R
θJA
is valid only for the specific
operating environment used in the computer model.
simplifies into
T
+
T
)
P
D
max x R
J
A
θJA
Rearranging
gives
T –T
R
+
J A
θJA
P max
D
R
θJA
max
+
(125
*
55)°C 2.5 W
+
28°C W
(9)
From
DDPAK Thermal Resistance vs
Copper Heatsink Area, the ground plane needs to be
1cm
2
for the part to dissipate 2.5W. The operating
environment used in the computer model to construct
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.
shows the side
view of the operating environment used in the
computer model.
40
No Air Flow
35
150 LFM
30
(6)
R
θ
JA − Thermal Resistance −
250 LFM
25
(7)
Using
and the computer model generated
curves shown in
and
a designer
can quickly compute the required heatsink thermal
resistance/board area for a given ambient
temperature, power dissipation, and operating
environment.
20
15
0.1
1
10
Copper Heatsink Area − cm
2
100
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 5V, the output voltage is 2.5V, the average
output current is 1A, 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 calculated as
P
D
max
+
(5
*
2.5) V x 1 A
+
2.5 W
(8)
Substituting T
J
max for T
J
into
gives
Figure 25. DDPAK Thermal Resistance vs Copper
Heatsink Area
2 oz. Copper Solder Pad
with 25 Thermal Vias
1 oz. Copper
Power Plane
1 oz. Copper
Ground Plane
Thermal Vias, 0.3 mm
Diameter, 1,5 mm Pitch
Figure 26. DDPAK Thermal Resistance
From the data in
and rearranging
the maximum power dissipation for a
different ground plane area and a specific ambient
temperature can be computed.
11
TI [ TEXAS INSTRUMENTS ]
