Datasheet
www.ti.com
A
B
C
A
B
C
T
J
A
R
θ
JC
T
C
B
R
θ
CS
T
A
C
R
θ
SA
(a) (b)
TO−263 Package
TO−220 Package
T
J
+ T
A
)P
D
max @
ǒ
R
QJC
)R
QCS
)R
QSA
Ǔ
(4)
T
J
+ T
A
)P
D
max @ R
QJA
(5)
R
QJA
+
T
J
*T
A
P
D
max
(6)
TPS75801 , , TPS758A01
TPS75815 , TPS75818
TPS75825 , TPS75833
SLVS330F – JUNE 2001 – REVISED APRIL 2007
THERMAL INFORMATION (continued)
Figure 22. Thermal Resistances
Equation 4 summarizes the computation:
The R
Θ JC
is specific to each regulator as determined by its package, lead frame, and die size provided in the
regulator's data sheet. The R
Θ SA
is a function of the type and size of heatsink. For example, black body radiator
type heatsinks, like the one attached to the TO-220 package in Figure 22 (a), can have R
Θ CS
values ranging from
5 ° C/W for very large heatsinks to 50 ° C/W for very small heatsinks. The R
Θ CS
is a function of how the package is
attached to the heatsink. For example, if a thermal compound is used to attach a heatsink to a TO-220 package,
R
Θ CS
of 1 ° C/W is reasonable.
Even if no external black body radiator type heatsink is attached to the package, the board on which the
regulator is mounted will provide some heatsinking through the pin solder connections. Some packages, like the
TO-263 and TI's TSSOP PowerPAD™ packages, use a copper plane underneath the package or the circuit
board ground plane for additional heatsinking to improve their thermal performance. Computer-aided thermal
modeling can be used to compute very accurate approximations of integrated circuit thermal performance in
different operating environments (for example, 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
Θ JA
). This R
Θ JA
is valid only for the specific operating
environment used in the computer model.
Equation 4 simplifies into Equation 5 :
Rearranging Equation 5 results in Equation 6 :
Using Equation 5 and the computer model generated curves shown in Figure 23 and Figure 26 , a designer can
quickly compute the required heatsink thermal resistance/board area for a given ambient temperature, power
dissipation, and operating environment.
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