What Is a Bus Switch?
In electronic circuits, a bus switch is a device that electrically conducts (turns ON) or interrupts (turns OFF) a signal. This is achieved using MOSFETs. The basic role of a bus switch is the same as that of a mechanical switch.
Since it is a switch, it naturally operates in both directions, so there is no need to worry about the direction of the signal.
In the case of a mechanical switch, it only conducts the signal by turning it ON or interrupts it by turning it OFF. On the other hand, some bus switches have a built-in function to amplify the signal or, conversely, to reduce its level.
Uses of Bus Switches
Devices that turn signals on and off in electronic circuits include bus switches and analog switches.
Analog switches are devices designed to perform switching operations in analog circuits. On the other hand, bus switches are devices for digital circuits and are inserted between the CPU and peripheral ICs.
Bus switches are used not only to conduct and disconnect signals but also to switch the connection destination. In the simplest case, for example, if outputs B and C are placed in relation to input A, the bus switch switches from conduction between A and B to conduction between A and C.
Bus switches used in electronic circuits are designed for use in relatively high-speed circuits due to their high-speed switching characteristics.
Principles of a Bus Switch
A typical bus switch is made by combining two MOSFETs: a P-CH MOSFET and an N-CH MOSFET with their gates directly connected and their drains directly connected in parallel.
By inputting a signal from the gate, either the P-CH or N-CH MOSFET will always be in a conducting state, and this is used as the output signal.
A device similar to a bus switch is a bus buffer. A bus buffer has a large fanout number (high drive capability), but there is a delay in switching.
The choice of which device to use should be determined based on the application in the circuit.
Types of Bus Switches
In addition to the aforementioned type that consists of P-CH and N-CH MOSFETs connected in parallel, there are also N-CH type and charge pump type bus switches.
The N-CH type switches ON/OFF by applying a specified voltage to the gate and inputting a signal between the drain and the source.
The charge pump type is made using only N-CH MOSFETs to improve the output signal voltage range.