PPC5553AVR132 データシートの表示（PDF） - Freescale Semiconductor

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PPC5553AVR132

Microcontroller

Freescale Semiconductor 

Electrical Characteristics

performance. When the clearance between through vias leave the planes virtually disconnected, the

thermal performance is also greatly reduced.

As a general rule, 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

application board has one oz (35 micron nominal thickness) internal planes, the components are well

separated, and the overall power dissipation on the board is less than 0.02 W/cm2.

The thermal performance of any component depends strongly on the power dissipation of surrounding

components. In addition, the ambient temperature varies widely within the application. For many natural

convection and especially closed box applications, the board temperature at the perimeter (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 = TB + (RθJB × PD)

where:

TJ = junction temperature (oC)

TB = board temperature at the package perimeter (oC/W)

RθJB = junction to board thermal resistance (oC/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, with the

component soldered to a board with internal planes.

Historically, the thermal resistance has frequently been 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 (oC/W)

RθJC = junction to case thermal resistance (oC/W)

RθCA = case to ambient thermal resistance (oC/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 air flow around

the device, add a heat sink, change the mounting arrangement on printed circuit board, or change the

thermal dissipation on the printed circuit board surrounding the device. This description is most useful for

packages with heat sinks where some 90% of the heat flow is through the case to the heat sink to ambient.

For most packages, a better model is required.

A more accurate two-resistor thermal model can be constructed from the junction to board thermal

resistance and the junction to case thermal resistance. The junction to case covers the situation where a

heat sink will be used or where a substantial amount of heat is dissipated from the top of the package. The

junction to board thermal resistance describes the thermal performance when most of the heat is conducted

MPC5553 Microcontroller Data Sheet, Rev. 0

Freescale Semiconductor

Preliminary—Subject to Change Without Notice

7

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