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Why is thermal resistance of the package not prescribed for some ICs?

Latest Updated:11/01/2007

Question:

Why is the thermal resistance of the package not prescribed for some ICs?

Answer:

In the case of microcontrollers and peripheral ICs, the power consumption is not restricted by ratings, unlike the case for power semiconductor devices, and the power consumption is determined as a result, such that maximum operating speed, voltage, ambient temperature and the package can be prescribed. Therefore, the thermal resistance of the package is not prescribed. If the input/output current of the signal lines grows large, so does the amount of heat that is emitted, but the limit voltage between the high level and the low level is prescribed with the current as a condition, and when that current value is exceeded, degradation and/or destruction risk occurring. Therefore, thermal design is not necessary, and if the operating ambient temperature exceeds the rated range, the implementation of cooling is sufficient.
In the case of discrete and power supply ICs, ASICs, etc., for which the usage method cannot be known in advance, the thermal resistance of the package is shown so as to allow thermal design.
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