Datasheet
SLVS293D − NOVEMBER 2000 − REVISED MAY 2002
14
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
THERMAL INFORMATION
Equation 2 summarizes the computation:
T
J
+ T
A
) P
D
max x
ǒ
R
θJC
) R
θCS
) R
θSA
Ǔ
(2)
The R
θJC
is specific to each regulator as determined by its package, lead frame, and die size provided in the
regulator’s datasheet. The R
θSA
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 21(a), can have R
θCS
values ranging
from 5°C/W for very large heatsinks to 50°C/W for very small heatsinks. The R
θCS
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
θCS
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’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.
Equation 2 simplifies into equation 3:
T
J
+ T
A
) P
D
max x R
θJA
(3)
Rearranging equation 3 gives equation 4:
R
θJA
+
T
J
–T
A
P
D
max
(4)
Using equation 3 and the computer model generated curves shown in Figures 22 and 25, a designer can quickly
compute the required heatsink thermal resistance/board area for a given ambient temperature, power
dissipation, and operating environment.
PowerPAD is a trademark of Texas Instruments.