Fan-out

This article deals with the output load capacity of electrical components. The fanning out of the paper in offset printing is treated under the fan-out effect .

Fan-out (or fan out or fan-out ) is a measure of the ability of a logic gate output gear (engl. Logic gate ) inputs of other components of the same logic family to control, i. H. to ensure the required electrical voltages and currents for error-free operation. It must between fan-out and fan-in (or fan in or Fanin be distinguished). Fan-out is a measure of the performance of an output, while fan-in is a measure of the output required by an input.

The output of a logic component can, depending on the design, actively drive an H level ( electrical current flows out of the component) or an L level (current flows into the component). For the driver capability at the output, fan-out is a standardized parameter that relates to the load case of a standardized component of the respective logic family .

An L level and an H level can be applied to an input of a logic component. At an H level, current flows into the component, while at an L level, current flows out of the component. Fan-in is a standardized parameter for this behavior at the input of a logic component.

The fan-out and fan-in can be specified for logic gates , flip-flops and other integrated logic circuits .

Fan-out

Load behavior

Typical fan-outs

Output type	Fan-out
TTL to TTL	10
TTL to CMOS	50
CMOS to TTL	1
RTL to RTL	3

Only a maximum number of additional component inputs can be connected to the output of an individual component so that the levels for high and low can be maintained (for TTL components, these are, for example, 0 V to 0.8 V for "low", and 2.4 V to 5 V for "High"). This number of connectable component inputs is called the fan-out of a component.

Most TTL gates can control up to 10 digital gates (or devices with the same load behavior) at their outputs. A typical TTL gate therefore has a fan-out of 10. These values ​​depend on the logic families and on the individual component types of a logic family.

If the maximum fan-out is exceeded, the very low output impedance in connection with the pull-down resistor can no longer reach the high state of the logic signals. This means that subsequent devices (gates) can no longer correctly evaluate the incoming signals (from the output). This can be avoided by inserting an additional driver component.

calculation

The fan-out is calculated from the ratio of the output current and the input current . The rounded value of the quotient divided by the fan-out gives : ${\displaystyle I_{\mathrm {out} }}$ ${\displaystyle I_{\mathrm {in} }}$ ${\displaystyle I_{\mathrm {out} }}$ ${\displaystyle I_{\mathrm {in} }}$

${\displaystyle {\text{Fan-Out}}=\left\lfloor {\frac {I_{\mathrm {out} }}{I_{\mathrm {in} }}}\right\rfloor ={\text{floor}}\left({\frac {I_{\mathrm {out} }}{I_{\mathrm {in} }}}\right)}$

Note: is a Gaussian bracket and means that it is rounded down to the next smaller whole number. ${\displaystyle \lfloor \;\rfloor }$

Exceeding the fan-out

Let us consider the case when the maximum permissible fan-out of a component is exceeded. Each digital component has a maximum current driver capability at the output for an L level (the output is switched to ground) and for an H level (the output is switched to the supply voltage). If the maximum current at the output of the transmitting component is exceeded, the current can be overloaded and, in the worst case, the component can be destroyed.

If the component is not damaged, however, an overload has repercussions on the switching behavior of the component. If a component is overloaded, changes in the logic level can occur. For example, let's consider the voltage range for an L level of a component. If the component sends an L signal, a voltage in the range between 0 V and 0.8 V is present at the output, for example. In the event of an overload, the output level may be greater than the maximum value of 0.8 V. With the following components there is a risk that this excessive voltage level is in the undefined input voltage range. The same case can occur with the H level. For example, a voltage range of 2.4 V to 5.0 V is specified for the H level. In the event of an overload, the actual output voltage value can be below 2.4 V, which in the case of the receiver component can also be in the undefined input voltage range and thus can result in interference.

A further consequence of this can be a very slow voltage rise, which manifests itself in a poor edge steepness . An insufficient edge steepness of the transmitted signal can also lead to interference in the receiver, since the undefined input voltage range of the downstream component is crossed too slowly during the switching process.

Oversized fan-out

Let us consider the second case in which the fan-out is many times higher than the required fan-out. Very powerful components with a large fan-out usually have a high current drive capability. This has particular advantages for the signal level, because with these components the actual output voltage value at the L level is mostly at the lower definition limit of 0.0 V and at the H level mostly at the upper definition limit at the level of the supply voltage (analogous to the above example mostly slightly below 5.0 V). This has a positive effect on the absolute voltage level.

Furthermore, in this application, there are usually very fast switching processes, which is reflected in a very high edge steepness. Because of these very fast transients, interference signals can be emitted (lack of electromagnetic compatibility ). In addition, there may be transients in the output signals . Due to the design of the circuit (mostly in the form of printed circuit boards ), these parasitic influencing variables ( capacitors and coils ) have an increased effect during rapid switching processes and cause interference.

Fan-in

At the input of a logic component, as described above, current flows in with an H level and current flows out with an L level. Depending on the type of component, these currents can be different. Fan-ins are used to standardize these flows. This indicates how large the actual current of a specific component is compared to a standardized comparative component of the same family.

In the case of a standardized component of the logic family, the fan-in is 1. If the fan-in for a chip family (e.g. transistor-transistor logic) is the same for all component types, each component with a fan-in (input load factor) of 1 are accepted.

Components of the logic family under consideration, which for example have double the input currents due to their design, have a fan-in of 2 (based on the standardized component of the logic family). When calculating the load behavior at the output of the upstream driver component, this double current load must be taken into account.

Components with a high fan-in result in higher input currents. This results in slower switching processes and a greater load on the transmitting component. Furthermore, this reduces the voltage value for an H level, while the voltage value for an L level is increased. In extreme cases, the voltage level can be in the undefined input voltage range.

literature

• Klaus Beuth: Digital technology . 10th edition. Vogel, 1998, ISBN 3-8023-1755-6 .

Individual evidence

1. ^ Klaus Beuth: Digital technology 10th edition, Vogel-Verlag, 1998, p. 122 f.