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
51 3 7 9
Y
JT
Y
JB
DRC
RGW
Y Y
JT J T JT D
:T =T + P·
Y Y
JB J B JB D
:T =T + P·
TPS74801
www.ti.com SBVS074J –JANUARY 2007– REVISED JANUARY 2012
Figure 29 shows the variation of θ
JA
as a function of By looking at Figure 30, the new thermal metrics (Ψ
JT
ground plane copper area in the board. It is intended and Ψ
JB
) have very little dependency on board size.
only as a guideline to demonstrate the effects of heat That is, using Ψ
JT
or Ψ
JB
with Equation 6 is a good
spreading in the ground plane and should not be way to estimate T
J
by simply measuring T
T
or T
B
,
used to estimate actual thermal performance in real regardless of the application board size.
application environments.
NOTE: When the device is mounted on an
application PCB, it is strongly recommended to use
Ψ
JT
and Ψ
JB
, as explained in the Estimating Junction
Temperature section.
ESTIMATING JUNCTION TEMPERATURE
Using the thermal metrics Ψ
JT
and Ψ
JB
, as shown in
the Thermal Information table, the junction
temperature can be estimated with corresponding
formulas (given in Equation 6). For backwards
compatibility, an older θ
JC
,Top parameter is listed as
well.
(6)
Figure 30. Ψ
JT
and Ψ
JB
vs Board Size
Where P
D
is the power dissipation shown by
Equation 4, T
T
is the temperature at the center-top of
For a more detailed discussion of why TI does not
the IC package, and T
B
is the PCB temperature
recommend using θ
JC(top)
to determine thermal
measured 1mm away from the IC package on the
characteristics, refer to application report SBVA025,
PCB surface (see Figure 31).
Using New Thermal Metrics, available for download
at www.ti.com. For further information, refer to
NOTE: Both T
T
and T
B
can be measured on actual
application report SPRA953, IC Package Thermal
application boards using a thermo-gun (an infrared
Metrics, also available on the TI website.
thermometer).
For more information about measuring T
T
and T
B
, see
the application note SBVA025, Using New Thermal
Metrics, available for download at www.ti.com.
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