Image Credit: Texas Instruments | Logic Signal Switches

 

 

Logic signal switches are electronic devices which activate or deactivate a signal once an event has occurred. They are sometimes called digital signal switches, and are for the switching of digital logic signals which typically consist of high speed on/off signals. The chip is designed to pass or isolate digital signal levels but may also pass analog signals. They are a high-performance, low-power replacement for standard bus-interface devices when signal buffering is not required.

 

How Logic Signal Switches Work

Signals are electrical or electromagnetic means of conveying and transferring information. Logic signal switches are binary transducers, meaning they take these electrical or electromagnetic signals and convert them to binary logic (digital) signals in the form of 1s and 0s. The switching action in the device determines whether this generated signal is isolated or passed through. 

 

Unlike normal electrical switches which physically connect or disconnect a circuit to switch between on and off, logic signal switches do not handle power. Instead, they handle logic signal levels. A binary input of 0 (false) sets the switch to "off" (open) where it operates with high resistance to create almost perfect signal isolation. A binary input of 1 (true) sets the switch to "on" (closed) where the resistance is much lower, allowing the signal to pass through. A reasonable amount of resistance, up to a few hundred ohms or so, can be tolerated in the "on" state when the signal is passed. The switch should be able to control signals at any reasonable voltage level and not be tied to ground or the positive supply.  

 

Digital vs. Analog

Logic signal switches are considered digital switches, meaning they process digital (binary) signals. However, some digital signal switches are able to pass analog signals as well. Thus, it is important to understand and distinguish the terminology surrounding switch function.

  • Digital (logic) signal switches pass or isolate digital signals. Some logic signal switches are also able to pass analog signals.
  • Analog signal switches pass or isolate analog signals. Many analog signal switches can also handle digital signals.
  • Bilateral switches can have one of two functions:
    • They pass signals in either direction through the switch
    • They can pass and isolate both analog and digital signals
  • Bus switches are digital switches designed for multi-bit switching in computer applications.

Specifications

There are a number of specifications to consider when selecting logic signal switches. The most important are the number of switches, power rating, and operating temperature. Others include the maximum current rating and supply voltage. 

  • Number of switches - the number of logic switches built into the device. This determines the number of inputs and outputs available for switching functions.
  • Power rating- the power required to operate the chip; also referred to as the power dissipation. It is typically expressed in watts or milliwatts. This rating is important for proper incorporation with the provisions of the rest of the system or circuit board.
  • Operating temperature - the ambient temperatures at which the switch is designed to operate. This is important to consider when operating in extreme temperature environments.
  • Maximum current rating- the maximum current that can be supplied for proper chip operation, expressed in amps (A) or milliamps (mA).
  • Supply voltage - the source voltage range for the device, expressed in volts (V).

Package Type

The package type affects a number of the logic signal switch's properties, including how the device is mounted on a circuit board. There a number of package types to consider.

  • Dual in-line packages (DIPs) can be installed either in sockets or permanently soldered into holes extending into the surface of the printed circuit board. The pins are distributed into two parallel lines along opposite site of the rectangular package. There are several types of DIP packages, such as Ceramic Dual in-line package (CDIP), Plastic Dual in-line package (PDIP), and Shrink Plastic Dual in-line package (SPDIP).
  • Ceramic dual in-line package (CDIP) consists of two pieces of dry pressed ceramic surrounding a "DIP formed" lead frame. The ceramic / LF / ceramic system is held together hermetically by frit glass reflowed at temperatures between 400° - 460° centigrade.
  • Plastic dual in-line package (PDIP) is widely used for low cost, hand-insertion applications including consumer products, automotive devices, logic, memory ICs, micro-controllers, logic and power ICs, video controllers commercial electronics, and telecommunications.
  • Small outline transistor 23 (SOT23) is a rectangular, surface mounted, small outline transistor (SOT) package with three or more gull wing leads. SOT23 features a very small footprint and is optimized for the highest possible current. Because of its low cost and low profile, SOT23 is used in home appliances, office and industrial equipment, personal computers, printers, and communication equipment.

Other typical package types for signal switches include:

  • Small outline integrated controller (SOIC).
  • Small outline package (SOP).
  • Thin shrink small outline L-leaded packages (TSSOP).
  • Quarter size outline package (QSOP).

For protection against the environment (water, dirt, dust, temperature, stress), users need to consider the strength, durability, and sealing properties of the packaging materials.

 

Materials

The materials used to form the semiconductor and housing play a large role in determining the properties of the device. The chart below lists the properties of a number of different materials used in both packaging and connections.

 

Table Credit: National Semiconductor

 

References

 

National Semiconductor - Semiconductor Packaging Assembly Technology (pdf)

 

Selecting the Right Texas Instruments Signal Switch (pdf)