Datasheet

12
10
8
6
4
2
0
Y Yand ( C/W)
JT JB
°
0 2
4
6 8 10
BoardCopperArea(in )
2
Y
JT
(KTW)
Y
JT
(RGW)
Y
JB
(RGW)
Y
JB
(KTW)
TPS744xx
www.ti.com SBVS066O –DECEMBER 2005–REVISED MARCH 2013
Compared with θ
JA
, the thermal metrics Ψ
JT
and Ψ
JB
are less independent of board size, but they do have
a small dependency. Figure 37 shows characteristic
performance of Ψ
JT
and Ψ
JB
versus board size.
Looking at Figure 37, the RGW package thermal
performance has negligible dependency on board
size. The KTW package, however, does have a
measurable dependency on board size. This
dependency exists because the package shape is not
point-symmetric to an IC center. In the KTW package,
for example (see Figure 36), silicon is not beneath
the measuring point of T
T
which is the center of the X
and Y dimension, so that Ψ
JT
has a dependency.
Also, because of that non-point-symmetry, device
heat distribution on the PCB is not point-symmetric,
either, so that Ψ
JB
has a dependency.
Figure 37. Ψ
JT
and Ψ
JB
vs Board Size
space
For a more detailed discussion of why TI does not
recommend using θ
JC
,Top to determine thermal
characteristics, refer to the application note Using
New Thermal Metrics (SBVA025), available for
download at www.ti.com. Also, refer to the application
note IC Package Thermal Metrics (SPRA953) (also
available on the TI web site) for further information.
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