MPC8349ECVVAJDB datasheet(76/87 Pages) FREESCALE

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MPC8349EA PowerQUICC II Pro Integrated Host Processor Hardware Specifications, Rev. 13

Freescale Semiconductor

Thermal

(edge) of the package is 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 = TA + (RθJA × PD)

where:

TJ = junction temperature (°C)

TA = ambient temperature for the package (°C)

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

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.

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

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