What is a bus switch?

A bus switch is a semiconductor switch optimized for transmitting digital signals. The basic function of a bus switch is the same as that of a mechanical switch, using a MOSFET to realize the on/off function (Fig. 1).

Fig. 1 Difference between Mechanical switch and Bus switch
Fig. 1 Difference between Mechanical switch and Bus switch

The bus switch is just a switch: unlike a buffer, it cannot drive a load. The bus switch is designed specifically for high-speed digital signal transmission and utilized to turn on and off the signal flow or switch between output ports.
Toshiba provides not only simple SPST switches but also SPDT and SP4T multiplexers.

Semiconductor switches are broadly divided into signal switches and load switches (Load Switch ICs) designed to turn on and off power supply rails. The following describes signal switches.
There are two types of signal switches: those that handle digital signals and those that handle analog signals.

Load Switch ICs

Bus switches were developed to transmit digital signals. In the case of a digital signal, it is necessary to transmit its logic states (including voltage levels) with little or no delay. In contrast, analog switches require signal linearity. In other words, low capacitance and on-resistance are important for bus switches whereas analog switches require on-resistance that is low and constant over the voltage range in which a signal is transmitted.
Generally, the on-resistance of both p-channel and n-channel MOSFETs for switching applications depends on control voltage as well as the voltage of the signal passing through a switch as shown in Fig. 2 and Fig. 3. To compensate for this voltage dependence, an analog switch is composed of P-channel and N-channel MOSFETs connected in parallel as shown in Fig. 4. This makes its on-resistance constant over the operating voltage range, yet at the expense of increased switch capacitance.
In contrast, many bus switches are composed using only a P-channel MOSFET (or N-channel MOSFET) as shown in Fig. 5. Although its on-resistance is also dependent on voltage, it is low enough that it does not interfere with the transfer of digital signals.
Therefore, bus switches are ideal for high-speed digital transmission. To transmit an analog signal, use an analog switch with analog electrical characteristics.

Fig. 2 R<sub>DS(ON)</sub> – V<sub>GS</sub> curve of P-ch MOSFET (SSM3J36TU)
Fig. 2 RDS(ON) – VGS curve of P-ch MOSFET (SSM3J36TU)
Fig. 3 R<sub>DS(ON)</sub> – V<sub>GS</sub> curve of N-ch MOSFET (SSM3K36TU)
Fig. 3 RDS(ON) – VGS curve of N-ch MOSFET (SSM3K36TU)
Fig. 4 Equivalent circuit of Analog switch
Fig. 4 Equivalent circuit of Analog switch
Fig. 5 Equivalent circuit of Bus switch
Fig. 5 Equivalent circuit of Bus switch

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