Datasheet

Y Y
JT J T JT D
:T =T + P·
Y Y
JB J B JB D
:T =T + P·
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7 8 9 10
Board Copper Area (inch
2
)
θ
JA
(°C/W)
θ
JA
(RGW)
θ
JA
(RGT)
G800
R =
qJA
+125 C T° -
A
P
D
TPS7A7100
www.ti.com SBVS189E –MARCH 2012–REVISED SEPTEMBER 2013
On the QFN (RGW or RGT) package, the primary conduction path for heat is through the exposed pad to the
PCB. The pad can be connected to ground or be left floating; however, it should be attached to an appropriate
amount of copper PCB area to ensure the device does not overheat. The maximum junction-to-ambient thermal
resistance depends on the maximum ambient temperature, maximum device junction temperature, and power
dissipation of the device and can be calculated using Equation 2:
(2)
Knowing the maximum R
θJA
, the minimum amount of PCB copper area needed for appropriate heatsinking can
be estimated using Figure 41 .
Figure 41. θ
JA
vs Board Size
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 3). For backwards compatibility, an older θ
JC
,Top
parameter is listed as well.
Where:
P
D
is the power dissipation shown by Equation 2.
T
T
is the temperature at the center-top of the IC package.
T
B
is the PCB temperature measured 1mm away from the IC package on the PCB surface (see
Figure 42). (3)
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 Application Report SBVA025, Using New Thermal Metrics,
available for download at www.ti.com.
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