The Significance of Fan-Out

When connecting standard logic ICs, care must be taken to avoid connecting too many ICs to a given output. With TTL ICs, the output current limits the number of IC connections.

The term fan-out denotes the maximum number of ICs that an output can connect to. Remember that TTL ICs consist chiefly of bipolar transistors; current is therefore required to execute switching. The fan-out of a TTL IC is the ratio of its output current to the current used by each input (see Fig. 3). If fan-out is exceeded, there is a risk that the output may be unable to maintain proper output logic levels.

In the case of CMOS ICs, where very little current flows to the input terminals, fan-out cannot be determined by looking at currents. Instead, fan-out is determined by load capacity. (See Fig. 4.) The load capacities indicated on CMOS IC data sheets are found by measuring the propagation delay. If load capacity is exceeded, propagation delay may lengthen to the point where malfunction occurs.