Q.When designing application circuits, which characteristics of PTC material should be especially taken into account compared to a general fix resistor?

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A. Not only part-to-part variation or temperature dependency of PTC resistance, but also voltage or frequency dependency has to be taken into account, because it has an influence on maximum currrent value that can be applied to the circuit.
Please refer to 6.1 and 6.2 of PTC general technical information: https://www.tdk-electronics.tdk.com/download/531356/728d0d379187d1dfa0831555b8c93202/pdf-general-technical-information.pdf