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SN74CBT3305C: Using SN74CBT3305C to switch 50 Ohm termiantion

Karo Aivazian
Intellectual 420 points
Part Number: SN74CBT3305C
Other Parts Discussed in Thread: SN74CB3Q3305, TS5A21366

Hi;

May I use SN74CBT3305C to turn ON/OFF 50 Ohm termination resistors on a TTL line?

Regards;

Karo

  • 0
    TI__Mastermind 45370 points

    Karo,

    You may use signal switches like the SN74CBT3305C to switch in and out resistors as long as the signal passing through the switch is within the recommended operating conditions.  Keep in mind that you will be adding small series resistance through the switch ~3-8ohms as seen in the Ron parameters in the datasheet.

    What is the reason you are terminating 50 ohm resistors on your truth table logic line?

    Here is another example of using a signal switch to switch in and out resistors.

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Intellectual 420 points

    Hello Adam;

    Thank you for the response, I was planning to use this switch in a Bidrectional TTL 50 Ohm application, so in input mode; 50 Ohm is connected to the line, but according to the graphic on the page 3 of the scdb006a Ron depends from the voltage levels (it would have been helpful if this graphic was part of SN74CBT3305C data sheet), now I'm thinking to go with SN74CB3Q3305, everything looks OK, except the current capability, I don't think it can handle 50 Ohm load. 

    SN74CB3Q3305 has two channels, can we parallel the channels to get more current capability?

    Regards;

    Karo

  • 0 in reply to Karo Aivazian
    TI__Mastermind 45370 points

    Karo,

    I'm sorry we don't have the Ron vs I/O voltage in the older datasheets and appreciate your honest feedback.  In our newer signal switches we include this graph as standard practice like the TS5A21366 datasheet.

    Now I see the issue if you have a 5-V Transistor-Transistor Logic (TTL) signal  on the I/O pin of the switch connected to 50 ohm termination you will pass 100mA through the switch.  5V = (100mA)(50 ohm)

    If you look at the absolute maximum ratings of the  SN74CB3Q3305 device you can see that the device will support up to 128mA through the switch.  

    I you would like to search for a 2-channel 1:1 SPST device on our TI.com switch selection tool you have the ability to filter on I/O continuous current.   

    It is possible to parallel the 2 channels of the SN74CB3Q3305 to decrease the amount of current that flows through the signal path but I don't think you will need to do this with your application and TI's large portfolio.

    Thank you,

    Adam

  • 0 in reply to Karo Aivazian
    Intellectual 420 points
    Hello Adam;
    We are going with TS5A21366.
    Thank you very much.
    Regards;
    Karo