MPC5121E datasheet(25/86 Pages) FREESCALE

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Electrical and Thermal Characteristics

MPC5121E/MPC5123 Data Sheet, Rev. 4

Freescale Semiconductor

where:

TA = ambient temperature for the package (ºC)

RθJA = junction to ambient thermal resistance (ºC/W)

PD = power dissipation in package (W)

The junction to ambient thermal resistance is an industry standard value, which 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 correct 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.

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

Eqn. 4

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. You control the thermal environment to change the case to ambient

thermal resistance, RθCA. For instance, you 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 ceramic 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 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 is 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 to the printed circuit board. This model can be used for hand estimations or for a computational fluid dynamics

(CFD) thermal model.

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)

Eqn. 5

where:

TT = thermocouple temperature on top of package (ºC)

JT = thermal characterization parameter ( º C / W )

PD = power dissipation in 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 approximately one 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.

3.2

Oscillator and PLL Electrical Characteristics

The MPC5121e/MPC5123 System requires a system-level clock input SYS_XTALI. This clock input may be driven directly

from an external oscillator or with a crystal using the internal oscillator.

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