TPS7A80xx
SBVS135A –JUNE 2010–REVISED JUNE 2010
www.ti.com
ESTIMATING JUNCTION TEMPERATURE
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Using the thermal metrics ΨJT and ΨJB, 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 qJC,Top parameter is listed as
well.
YJB
YJT: TJ = TT + YJT · PD
6
YJB: TJ = TB + YJB · PD
YJT
4
(6)
2
Where PD is the power dissipation shown by
Equation 5, TT is the temperature at the center-top of
the IC package, and TB is the PCB temperature
measured 1mm away from the IC package on the
PCB surface (as Figure 35 shows).
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Board Copper Area (in2)
Figure 34. ΨJT and ΨJB vs Board Size
NOTE: Both TT and TB can be measured on actual
application boards using a thermo-gun (an infrared
thermometer).
For a more detailed discussion of why TI does not
recommend using qJC(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.
For more information about measuring TT and TB, see
the application note SBVA025, Using New Thermal
Metrics, available for download at www.ti.com.
By looking at Figure 34, the new thermal metrics (ΨJT
and ΨJB) have very little dependency on board size.
That is, using ΨJT or ΨJB with Equation 6 is a good
way to estimate TJ by simply measuring TT or TB,
regardless of the application board size.
Figure 35. Measuring Point for TT and TB
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