Fan-out calculation for logic families In digital design, fan-out is determined by calculating how many standard inputs an output can drive in both the HIGH and LOW states, and then taking which result and from which ratios?

Introduction / Context:Fan-out tells you the maximum number of identical logic inputs that a single logic output can drive without violating guaranteed logic-level specifications. It is a fundamental specification for interfacing devices across TTL, CMOS, and mixed-logic systems. Correctly computing fan-out prevents marginal logic levels, excessive current draw, and reliability issues in production designs.

Given Data / Assumptions:

• Definitions: IOH = output current available in the HIGH state, IOL = output current available in the LOW state.
• IIH = input current required by one input when HIGH; IIL = input current required by one input when LOW.
• Logic family input thresholds and currents are per datasheet “worst-case” values.

Concept / Approach:

You must satisfy output drive requirements for both logic states. Compute two separate limits: HIGH-state fan-out = IOH / IIH and LOW-state fan-out = IOL / IIL. The system must meet both simultaneously, so the overall fan-out is the smaller (i.e., the more restrictive) of the two values. Using worst-case currents ensures sufficient margin under temperature, voltage, and process variations.

Step-by-Step Solution:

Verification / Alternative check:

Check with datasheet example values; the calculated minimum of the two ratios will match the manufacturer’s specified fan-out figures for the family standard load.

Why Other Options Are Wrong:

Taking the larger ratio or the average can overestimate capability and cause failures.

Using voltage ratios (VOH/VIH, VOL/VIL) ignores current loading limits.

Summing ratios has no physical meaning for drive capacity.

Common Pitfalls:

Using typical instead of worst-case currents; ignoring that IIH in CMOS is tiny but VIH may be higher, requiring level considerations; forgetting temperature derating.

Final Answer:

The smaller of (IOH / IIH) and (IOL / IIL)