Datasheet

Y Y
JT J T JT D
:T =T + P·
Y Y
JB J B JB D
:T =T + P·
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
TPS74801-Q1
SLVSAI4B –OCTOBER 2010–REVISED JULY 2013 www.ti.com
Note: θ
JA
value at board size of 9 in
2
(that is, 3-in × 3-in) is a JEDEC standard.
Figure 29. θ
JA
vs Board Size
Figure 29 shows the variation of θ
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 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)
Where P
D
is the power dissipation shown by Equation 4, 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 (see
Figure 31).
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 30, 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 way to estimate T
J
by simply measuring T
T
or T
B
, regardless of the
application board size.
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