MPC8358E datasheet(87/96 Pages) FREESCALE

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MPC8358E PowerQUICC™ II Pro Processor Revision 2.1 PBGA Silicon Hardware Specifications, Rev. 1

Freescale Semiconductor

Thermal

many natural convection and especially closed box applications, the board temperature at the perimeter

(edge) of the package will be approximately the same as the local air temperature near the device.

Specifying the local ambient conditions explicitly as the board temperature provides a more precise

description of the local ambient conditions that determine the temperature of the device. At a known board

temperature, the junction temperature is estimated using the following equation:

TJ = TB + (RθJB × PD)

where:

TJ = junction temperature (°C)

TB = board temperature at the package perimeter (°C)

RθJA = junction to board thermal resistance (°C/W) per JESD51-8

PD = power dissipation in the package (W)

When the heat loss from the package case to the air can be ignored, acceptable predictions of junction

temperature can be made. The application board should be similar to the thermal test condition: the

component is soldered to a board with internal planes.

23.2.3

Experimental Determination of Junction Temperature

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

23.2.4

Heat Sinks and Junction-to-Case Thermal Resistance

In some application environments, a heat sink will be required 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:

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