Datasheet
140
120
100
80
60
40
20
0
q
JA
( C/W)
°
0 1 2 3
4 5
6
7
8 9 10
Board Copper Area ( )in
2
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
Y Y
JT J T JT D
:T =T + P·
Y Y
JB J B JB D
:T =T + P·
1mm
T on PCB
B
T on top
of IC surface
T
1mm
T
B
T
T
TPS74701
www.ti.com
SBVS099F –NOVEMBER 2007–REVISED NOVEMBER 2010
Where P
D
is the power dissipation shown by
Equation 5, T
T
is the temperature at the center-top of
the IC package, and T
B
is the PCB temperature
measured 1mm away from the IC package on the
PCB surface (as Figure 30 shows).
NOTE: Both T
T
and T
B
can be measured on actual
application boards using a thermo-gun (an infrared
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.
By looking at Figure 29, the new thermal metrics (Ψ
JT
and Ψ
JB
) have very little dependency on board size.
That is, using Ψ
JT
or Ψ
JB
with Equation 6 is a good
Note: q
JA
value at board size of 9in
2
(that is, 3in ×
way to estimate T
J
by simply measuring T
T
or T
B
,
3in) is a JEDEC standard.
regardless of the application board size.
Figure 28. q
JA
vs Board Size
Figure 28 shows the variation of q
JA
as a function of
ground plane copper area in the board. It is intended
only as a guideline to demonstrate the effects of heat
spreading in the ground plane and should not be
used to estimate actual thermal performance in real
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
Figure 29. Ψ
JT
and Ψ
JB
vs Board Size
the Thermal Information table, the junction
temperature can be estimated with corresponding
formulas (given in Equation 6). For backwards
For a more detailed discussion of why TI does not
compatibility, an older q
JC
,Top parameter is listed as
recommend using q
JC(top)
to determine thermal
well.
characteristics, refer to application report SBVA025,
Using New Thermal Metrics, available for download
at www.ti.com. For further information, refer to
(6)
application report SPRA953, IC Package Thermal
Metrics, also available on the TI website.
Figure 30. Measuring Points for T
T
and T
B
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