Input Current Size Three-Row Scan - Electronic Circuits IV: Digital Logic Signals and Switch Networks Step by Step

Direct fanout also needs a companion scan where load count is fixed and the exact input current changes. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

The same formula also depends on input current

This scan holds load count fixed and changes the current drawn by each input. It is the companion view to the load-count scan, so both variables in the multiplication get their own rows.

I_{\text{total}}=N\cdot I_{\text{in}}

Three current rows show the second dependency

The load count stays fixed. Increasing the per-input current moves the total current from comfortably inside the limit to an over-limit request.

\begin{array}{c|c|c|c|c}N&I_{\text{in}}&I_{\text{total}}&I_{\text{limit}}&\text{state}\\4&1\ \text{A}&4\ \text{A}&10\ \text{A}&\text{PASS}\\4&2\ \text{A}&8\ \text{A}&10\ \text{A}&\text{PASS}\\4&3\ \text{A}&12\ \text{A}&10\ \text{A}&\text{FAIL}\\\end{array}

Changing either factor can cross the boundary

The earlier scan changed load count. This one changes current per load. In both cases the product, not the label, decides whether the output can drive the inputs.

8\ \text{A}\le10\ \text{A},\quad 12\ \text{A}>10\ \text{A}