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?
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AThe smaller of (IOH / IIH) and (IOL / IIL)
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BThe larger of (IOH / IIH) and (IOL / IIL)
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CThe smaller of (VOH / VIH) and (VOL / VIL)
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DThe average of (IOH / IIH) and (IOL / IIL)
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EThe sum of (IOH / IIH) and (IOL / IIL)
Answer
Correct Answer: The smaller of (IOH / IIH) and (IOL / IIL)
Explanation
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:
Compute HIGH-state limit: N_H = IOH / IIH.Compute LOW-state limit: N_L = IOL / IIL.Overall fan-out N = min(N_H, N_L) (take the smaller result).Round down to the nearest whole input count for safe design.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)