MPC8347VRAGDB datasheet(87/99 Pages) FREESCALE

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MPC8347EA PowerQUICC II Pro Integrated Host Processor Hardware Specifications, Rev. 12

Freescale Semiconductor

Thermal

20.2.3

Experimental Determination of Junction Temperature

To determine the junction temperature of the device in the application after prototypes are available, use

the thermal characterization parameter (

JT) to determine the junction temperature and a measure of the

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

TJ = TT + (ΨJT × PD)

where:

TJ = junction temperature (°C)

TT = thermocouple temperature on top of package (°C)

JT = junction-to-ambient thermal resistance (°C/W)

PD = power dissipation in the package (W)

The thermal characterization parameter is measured per the JESD51-2 specification using a 40 gauge type

T thermocouple epoxied to the top center of the package case. The thermocouple should be positioned so

that the thermocouple junction rests on the package. A small amount of epoxy is placed over the

thermocouple junction and over about 1 mm of wire extending from the junction. The thermocouple wire

is placed flat against the package case to avoid measurement errors caused by cooling effects of the

thermocouple wire.

20.2.4

Heat Sinks and Junction-to-Case Thermal Resistance

Some application environments require a heat sink to provide the necessary thermal management of the

device. 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:

RθJA = RθJC + RθCA

where:

RθJA = junction-to-ambient thermal resistance (°C/W)

RθJC = junction-to-case thermal resistance (°C/W)

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

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

change the case-to-ambient thermal resistance, RθCA. 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.

The thermal performance of devices with heat sinks has been simulated with a few commercially available

heat sinks. The heat sink choice is determined by the application environment (temperature, air flow,

adjacent component power dissipation) and the physical space available. Because there is not a standard

application environment, a standard heat sink is not required.

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