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How can I calculate fan-out?

The fan-out can be calculated by dividing the output current, IOH/IOL, of the driving IC by the input current, IIH/IIL, of the driven IC. However, if the driven ICs are CMOS devices, there is hardly any fan-out restriction in terms of direct current because of the very high input impedance of CMOS.

When calculating CMOS fan-out limits, an increase in the rise and fall times due to capacitive loads (sum of the input pin capacitances of the driven ICs and wire capacitances) should be taken into consideration. Ensure that the input rise and fall times meet the specifications and that the device does not malfunction because of reflections on transmission lines.

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