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SN74CBT3257C: Is SN74CBT3257C bidirectional?

Part Number: SN74CBT3257C
Other Parts Discussed in Thread: SN74CB3Q3257

Hi team,

The customer uses SN74CBT3257C. I check the FAQ about the  signal switches and multiplexers in the below link:

It shows"all analog and digital bus switches are bidirectional". 

And it also shows the bilateral FET switch in the figure 1 of the attach.

But the SN74CBT3257C  datasheet shows the FET switch in the figure 2 of the attach. There is only one

 FET in the figure 2. So, would you help me to verify if SN74CBT3257C is the bilateral FET switch? 

If it is the bilateral FET switch, why does the datasheet show only one FET switch?

                                                     Figure 1

                                                             Figure 2

Best Wishes,
Mickey Zhang
Asia Customer Support Center
Texas Instruments

  • Mickey,

    Thank you for the detailed question.

    There is a difference between the word bi-directional and bilateral and they do not mean the same thing for switches.  

    All TI passive FET switches are bi-directional meaning a signal or current can be passed in either direction.  There are three different types of passive FET switches, single NMOS FET, NMOS FET in parallel with PMOS FET, and signle NMOS FET with the gate biased by a charge pump.  All three of these switch types are bidirectional.

    Bilateral switch refers to the switch type that is NMOS FET in parallel PMOS FET.  Bilateral switch, T-gate, and Transmission gate switch architectures all mean the same thing.  

    The SN74CBTXXXX devices are all single NMOS FET switches

    The SN74CBTLVXXXX devices are all NMOS FET switches in parallel with PMOS FET

    The SN74CB3QXXXX devices are all single NMOS FET with their gates biased by a charge pump

    The app note below goes into more detail on the performance differences of the 3 different types of bi-directional switches

  • Hi Adam,

    Thanks for your help.

    Compare with the single NMOS FET switch and the single NMOS FET with their gates biased by a charge pump,

    I would like the advantage for the single NMOS FET with their gates biased by a charge pump, this switch can allow

    the signal voltage can be above VCC to be passed. Is this correct? Is there any other advantage?
  • Mickey,

    You are correct, FET switches with their gates biased with a charge pump may pass signals that are higher than Vcc but you should always follow the recommended operating conditions for the device.

    Here is the SN74CB3Q3257 recommended operating conditions.

    There are potentially other advantages like Ron flatness and bandwidth performance that are discussed in the above application note.

    Thank you,

    Adam