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MPC8275ZQE Datasheet(PDF) 12 Page - Freescale Semiconductor, Inc |
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MPC8275ZQE Datasheet(HTML) 12 Page - Freescale Semiconductor, Inc |
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12 / 80 page  MPC8280 PowerQUICC II™ Family Hardware Specifications, Rev. 1.7 12 Freescale Semiconductor Thermal Characteristics 4.2 Estimation with Junction-to-Case Thermal Resistance 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 adjusts the thermal environment to affect 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 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. 4.3 Estimation with Junction-to-Board Thermal Resistance A simple package thermal model which 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, 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 (ºC/W) TB = board temperature (ºC) 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 by attaching the thermal balls to the ground plane. 4.4 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, or a more accurate and complex model of the package can be used in the thermal simulation. |
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