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SN74CBT3251: Output voltage ranges

Part Number: SN74CBT3251

Hi,

I have a question regarding SN74CBT3251D:

1.The datasheet doesn't seem to specify what the Vout max is(when using a 5V supply), nor what the Vout min is.

In an application we're seeing the output voltage range to be 1.1 to 4.2V (when 0 to 5V is expected).

Is there something limiting the voltages a part from the I*R drop of the passing FET?

Thank you!

Best regards,

Alberto

  • Alberto,

    Since these are passive FET switches, Vout is not a valid spec since the voltage on the output of the switch is dependent on the input voltage and the on-state resistance of the FET.   

    There are three main architectures for bidirectional passive FET signal switches. 

    1) NFET

    2) NFET in parallel with PFET

    3) NFET with the gate biased by a charge pump

    1) will pass signals from 0 V to Vcc - ~1V

    2) will pass signals from 0V to Vcc

    3) will pass signals from 0 V to slightly beyond Vcc

    This application note selecting the right TI signal switch goes into more detail on the different switch types and the reason the on-state resistances are different.

    The SN74CBTXXXX devices are the NFET type which means the on-state resistance gets increasingly high as the input voltage approaches Vcc.  This is why you are only seeing about 4.2 V at the output with input at 5V and Vcc at 5V.  If you would like to support 0 to 5V you will need to use the SN74CB3Q3251.

    What are you connecting to the signal switch?  You may also be interested in the new TMUX1208.

    Thank you,

    Adam

  • In reply to Adam Torma:

    Hi Adam,
    thank you, that clarifies and helps with the solution.
    Is there a way to see which type of pass-FET is being used? (as far as I can see the give-away is that the part can pass 5V signal from a 3.3Vcc)
    p2p compatible is required, therefore the TMUX1208 won't be a fit.
    Thanks!
    Regards,
    Alberto
  • In reply to Alberto Doronzo:

    Alberto,

    The closet p2p device in a 16 pin SOIC package I found that will support voltages from 0 to Vcc is the SN74CBTLV3251.  This device supply voltage goes up to 3.6 V.

    Thank you,
    Adam

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