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)
TPS749xx
www.ti.com SBVS082G –JUNE 2007–REVISED NOVEMBER 2010
Compared with q
JA
, the new thermal metrics Ψ
JT
and
Ψ
JB
are less independent of board size, but they do
have a small dependency. Figure 31 shows
characteristic performance of Ψ
JT
and Ψ
JB
versus
board size.
Looking at Figure 31, 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 30), 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,
Figure 31. Ψ
JT
and Ψ
JB
vs Board Size
either, so that Ψ
JB
has a dependency.
space
For a more detailed discussion of why TI does not
recommend using q
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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