56F8167 datasheet(177/184 Pages) FREESCALE

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Thermal Design Considerations

56F8367 Technical Data, Rev. 7.0

Freescale Semiconductor

177

Preliminary

Part 12 Design Considerations

12.1 Thermal Design Considerations

An estimation of the chip junction temperature, TJ, can be obtained from the equation:

TJ = TA + (RθJΑ x PD)

where:

The junction-to-ambient thermal resistance is an industry-standard value that provides a quick and easy

estimation of thermal performance. Unfortunately, there are two values in common usage: the value

determined on a single-layer board and the value obtained on a board with two planes. For packages such

as the PBGA, these values can be different by a factor of two. Which value is closer to the application

depends on the power dissipated by other components on the board. The value obtained on a single-layer

board is appropriate for the tightly packed printed circuit board. The value obtained on the board with the

internal planes is usually appropriate if the board has low-power dissipation and the components are well

separated.

When a heat sink is used, the thermal resistance is expressed as the sum of a junction-to-case thermal

resistance and a case-to-ambient thermal resistance:

RθJΑ = RθJC + RθCΑ

where:

R θJC is device-related and cannot be influenced by the user. The user controls the thermal environment to

change the case-to-ambient thermal resistance, R θCA . For instance, the user can change the size of the heat

sink, the air flow around the device, the interface material, the mounting arrangement on printed circuit

board, or change the thermal dissipation on the printed circuit board surrounding the device.

To determine the junction temperature of the device in the application when heat sinks are not used, the

Thermal Characterization Parameter (

JT) can be used to determine the junction temperature with a

measurement of the temperature at the top center of the package case using the following equation:

TJ = TT + (ΨJT x PD)

where:

= Ambient temperature for the package (oC)

RθJΑ = Junction-to-ambient thermal resistance (

oC/W)

= Power dissipation in the package (W)

RθJA

= Package junction-to-ambient thermal resistance °C/W

RθJC

= Package junction-to-case thermal resistance °C/W

RθCA = Package case-to-ambient thermal resistance °C/W

Thermocouple temperature on top of package (oC)

Thermal characterization parameter (oC)/W

Power dissipation in package (W)

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