MPC5200B(2006) Просмотр технического описания (PDF) - Freescale Semiconductor

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MPC5200B
(Rev.:2006)

SDRAM/DDR Memory Controller

Freescale Semiconductor 

Electrical and Thermal Characteristics

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. 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

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 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 to the printed

circuit board. This model can be used for either 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 (ΨJT) 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)

ΨJT = thermal characterization parameter (ºC/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 MPC5200B System requires a system-level clock input SYS_XTAL. This clock input may be driven

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

There is a separate oscillator for the independent Real-Time Clock (RTC) system.

MPC5200B Data Sheet, Rev. 1

12

Freescale Semiconductor

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