Bus Switches

Fairchild Switch is a family of low-impedance bus switch, bus exchange, multiplexer/de-multiplexer and analog switches that provide high-speed switching. The low "ON" resistance of these FET switches allow inputs to be connected to outputs without adding propagation delay or generating additional ground bounce noise. Fairchild switches are ideal for high-speed, bidirectional interfaces between mixed-supply buses, and in applications requiring isolation and protection.

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Features & Benefits

  • Near zero propagation delay provides high performance
  • Low "ON" resistance and input capacitance to minimize bus loading
  • Low power consumption
  • Options with UHC™ undershoot protection for reliable hot-swap isolation
  • Options with integrated diode for voltage translation
  • Options with integrated series resistors for noise suppression
  • Options with precharge to allow live insertion

Product Models

Featured Products

Sample Application Diagrams

FST16211 Bus

FST16211 switches are ideal to provide isolation and/or  voltage translation in applications that involve hot-swapping.  Multiple switches will be required for some applications such as PCI (and its derivatives) in personal computers and notebooks.

FST3125 Bus

FST3125 when used as a load isolation switch, can greatly reduce the capacitive load seen by an ASIC.  In the example above, the total load on the ASIC has been reduced form 570pF to 172pF using the FST3125.

Frequently Asked Questions

1. What’s the difference between a digital switch (or bus switch) and an analog switch?
A. The Bus switch is an NMOS only and the Analog switch is an NMOS and PMOS in parallel (transmission gate).

2. Can Bus switches be used to pass an analog signal?
A. Yes. When using a Bus switch to pass an analog signal allowance must be made for the voltage translation or clamping attributes of the NMOS only switch. The Vcc supply available, input signal swing levels, desired output levels, application bandwidth and Ron/Con characteristics have to be considered. (For additional information, please read applications note AN-996.)

3. Can a bus switch be used to perform voltage translation?
A. The FSTxxxx Bus switch family of products achieves voltage translation by utilizing the VTn of the NMOS. The output is in effect clamped to Vcc - VTn  so if the input signal is less than this value the input is transferred directly to the output. If the input is greater than Vcc - VTn then the output is clamped to Vcc - VTn.  For example, Vcc = 5V and VTn = 1V then for a 3.3V input the output will be 3.3V. For a 5V TTL signal then the output is clamped to 4V. Hence, adjusting Vcc will change the output translated voltage.

4. Can bus switches be used for hot swapping (live insertion)?
A. Bus switches are ideal for hot swapping applications as there is no parasitic path to be turned on during live insertion with an NMOS only architecture.

5. What is undershoot protection?
A. Undershoot protection may be needed where a fast HL edge can undershoot by 2V or more. This undershoot can create a false trigger on DDR memory or other devices. Special undershoot protection circuitry compensates to prevent such a false trigger of other devices. Undershoot protected devices are designated with “U” code (for example, FSAU3157). (For additional information, please read applications note AN-5008.)