56F8357 View Datasheet(PDF) - Motorola => Freescale

Part Name

Description

Manufacturer

56F8357

16-bit Digital Signal Processor

Motorola => Freescale 

Freescale Semiconductor, Inc.

Thermal Design Considerations

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:

TA = Ambient temperature for the package (oC)

RθJΑ = Junction-to-ambient thermal resistance (oC/W)

PD = Power dissipation in the package (W)

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 = RθJΧ + RθCΑ

where:

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

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 (ΨJT) 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:

TT = Thermocouple temperature on top of package (oC)

ΨJT = Thermal characterization parameter (oC)/W

PD = Power dissipation in package (W)

56F8357 Technical Data

153

Preliminary

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