Fanout Limit - Electronic Circuits IV & XII: Digital Logic and Timing Closure Step by Step

A fanout limit is a boundary test: a total current equal to the output rating is still inside the ideal budget, while any larger total fails. This page uses exact integer training currents to expose the inequality; real circuits need derating, threshold margins, temperature limits, and measured current spread.

The fanout rule is an exact current comparison, not a timing or noise-margin claim. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Equality at the limit still passes

With 5 loads, the total current is exactly 5 amperes, equal to the limit.

5\ \text{A} = 5\ \text{A}

Above the limit fails

With 6 loads, the total current becomes 6 amperes, which is above the same output limit.

6\ \text{A} > 5\ \text{A}