参数资料
型号: MPC875VR133
厂商: Freescale Semiconductor
文件页数: 5/84页
文件大小: 0K
描述: IC MPU POWERQUICC 133MHZ 256PBGA
标准包装: 60
系列: MPC8xx
处理器类型: 32-位 MPC8xx PowerQUICC
速度: 133MHz
电压: 3.3V
安装类型: 表面贴装
封装/外壳: 256-BGA
供应商设备封装: 256-PBGA(23x23)
包装: 托盘
MPC875/MPC870 PowerQUICC Hardware Specifications, Rev. 4
Freescale Semiconductor
13
Thermal Calculation and Measurement
7.2
Estimation with Junction-to-Case Thermal Resistance
Historically, 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 (°C/W)
RθJC = junction-to-case thermal resistance (°C/W)
RθCA = case-to-ambient thermal resistance (°C/W)
RθJC is device-related and cannot be influenced by the user. The user adjusts the thermal environment to
affect the case-to-ambient thermal resistance, RθCA. For instance, the user can change the airflow around
the device, add a heat sink, change the mounting arrangement on the printed-circuit board, or change the
thermal dissipation on the printed-circuit board surrounding the device. This thermal model is most useful
for ceramic packages with heat sinks where some 90% of the heat flows through the case and the heat sink
to the ambient environment. For most packages, a better model is required.
7.3
Estimation with Junction-to-Board Thermal Resistance
A simple package thermal model that has demonstrated reasonable accuracy (about 20%) is a two-resistor
model consisting of a junction-to-board and a junction-to-case thermal resistance. The junction-to-case
thermal resistance 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. It has been observed that the
thermal performance of most plastic packages and especially PBGA packages is strongly dependent on the
board temperature. If the board temperature is known, an estimate of the junction temperature in the
environment can be made using the following equation:
TJ = TB + (RθJB × PD)
where:
RθJB = junction-to-board thermal resistance (°C/W)
TB = board temperature (°C)
PD = power dissipation in package
If the board temperature is known and the heat loss from the package case to the air can be ignored,
acceptable predictions of junction temperature can be made. For this method to work, the board and board
mounting must be similar to the test board used to determine the junction-to-board thermal resistance,
namely a 2s2p (board with a power and a ground plane) and vias attaching the thermal balls to the ground
plane.
7.4
Estimation Using Simulation
When the board temperature is not known, a thermal simulation of the application is needed. The simple
two-resistor model can be used with the thermal simulation of the application [2], or a more accurate and
complex model of the package can be used in the thermal simulation.
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