• State TI Thinks Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 3 answers
  • Subscribers 17 subscribers
  • Views 167 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN74CB3Q6800: SN74CB3Q6800PW Specification of BIASV

Angelo Kako
Prodigy 20 points
Part Number: SN74CB3Q6800
Other Parts Discussed in Thread: SN74CB3Q3384A, SN74CBTLV3861

Hi, 

I am looking to use the SN74CB3Q6800PW bus switch IC with a PCI bus to allow for 3.3V and 5V IO voltages to be used in a new design. 

Looking through the datasheet, it looks like BIASV can be tied to Vcc, ground or left open. I am wondering if each configuration is meant for a different purpose and if so, what would be the recommended configuration for my purposes? 

Also, in the test circuit provided (Page 7) in the datasheet:

  • a 50 ohm resistor is used on the Vin and Vi lines.
    • Is a 50 ohm resistor going to be required for each Vi line?
  • a capacitor (Cl) and two resistors (Rl) are used on the Vo lines. 
    • Are these required for each Vo line? 

Is there an application note that gives a sample circuit? 

Thank you,

  • 0
    Guru 260250 points

    Precharging is needed only for hotplugging. In that case, the bias voltage should be equal to the switching threshold of the other devices on the bus.

    The 50 Ω resistors are used only for testing. (This is the characteristic impedance of BNC cables.) CL and RL simulate the actual load.

    Just connect the switch directly to whatever signals you're using. (Like with any other digital chip, VCC needs a decoupling capacitor.)

  • 0
    TI__Genius 10974 points

    Hi Angelo,

    The BIASV pin is used to precharge the outputs in order to reduce any glitching/spikes seen when used in a hot/live insertion application. Normally, this voltage would be equivalent to what you would expect to see on your output voltage of the application.

    In addition, you will not need those extra resistors and capacitors seen in the datasheet for normal operation. They are just test conditions for a given measurement setup for certain parameters.

    Lastly, there is an app note here that can help give more information on this device and its characteristics:

    https://www.ti.com/lit/an/scda009/scda009.pdf?ts=1637250356789&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FSN74CB3Q6800

    Hope this helps!

    Thanks!

    Bryan

  • 0 in reply to Bryan Marshall
    Prodigy 20 points

    Hello, 

    This is all great information, thank you for your replies. 

    Additional questions: 

    • If I am not planning to use the hot/live insertion application, should I tie BIASV to ground or leave it as a NC? Or, is there a direct replacement without a BIASV pin in the package? 
    • If not all input and output pins are being used on the IC, should those lines be NC to tied to ground? 

    Thank you again for the help.

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    Hello, 

    This is all great information, thank you for your replies. 

    Additional questions: 

    • If I am not planning to use the hot/live insertion application, should I tie BIASV to ground or leave it as a NC? Or, is there a direct replacement without a BIASV pin in the package? 
    • If not all input and output pins are being used on the IC, should those lines be NC to tied to ground? 

    Thank you again for the help.

  • 0 in reply to Angelo Kako
    Guru 260250 points

    This device connects BIASV to the outputs through 10 kΩ resistors. You can simply leave it open.

    Unused switches could be left open, but for high-frequency signals, should be connected with 50 Ω to GND.

  • 0 in reply to Angelo Kako
    TI__Genius 10974 points

    Hi Angelo,

    Yes, you can just leave it floating or tie to GND if it is not being used.

    For other options, take a look at the SN74CB3Q3384A or the SN74CBTLV3861 as alternative 10 channel, 1:1 devices that could also work in your application (not pin to pin, but same functionality).

    Lastly, if not using some I/Os, you can leave them floating or tie them to GND with a 50 ohm resistor if using in a high frequency application to prevent noise coupling to those pins.

    Thanks!

    Bryan