Datasheet

Y Y
JT J T JT D
:T =T + P·
Y Y
JB J B JB D
:T =T + P·
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
(RGW)
Y
JB
(RGW)
T onPCB
B
T on ofICtop
T
1mm
(a)ExampleRGW(QFN)PackageMeasurement
TPS75005
SBVS144C –NOVEMBER 2011–REVISED APRIL 2012 www.ti.com
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.
Where:
P
D
is the power dissipation shown by Equation 5
T
T
is the temperature at the center-top of the device
T
B
is the PCB temperature measured 1 mm away from the device on the PCB surface (as Figure 35
shows). (6)
NOTE: Both T
T
and T
B
can be measured on actual application boards using 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.
By looking at Figure 34, 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.
Figure 34. Ψ
JT
and Ψ
JB
vs Board Size
For a more detailed discussion of why TI does not recommend using θ
JC(top)
to determine thermal characteristics,
refer to Application Report SBVA025, Using New Thermal Metrics, available for download at www.ti.com. For
further information, refer to Application Report SPRA953, IC Package Thermal Metrics, also available on the TI
website.
Figure 35. Measuring Points for T
T
and T
B
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