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SN74CB3T3245 / interface to 5V

Other Parts Discussed in Thread: SN74CB3T3245

Hi,

Customer is considering SN74CB3T3245 for the following system.

・VCC=3.3V

・Data rate=100MHz max

・Bidirectional translation between 3.3V and 5V

Is it possible to add external pull up resistors ? If possible, would you give us application information ?

Best Regards,

Kato

  • These switches are made for down translation but with the addidion of a pullup resistor they can go both directions.

    WIth Vcc set to 3.3V the part will translate down from 5V to 3.3V.  when going back the other direction 3.3V to 5V you will need a pullup on the 5V side.  Thes size of this resistor will partially determine the speed. So since you will be trying to run at 100Mhz the resistor must be a lower value. My guess would be around 300ohms.

    The part can translate down runnig at 100Mhz with no problems but I am not 100% sure that it can translate back up running at 100Mhz. Similar parts that operate in the same manner can reach 100Mhz so I believe this one can also. 

  • Hi Chris - san,

    Thank you for the reply. I understood we need a pull up on the 5V side when going back the other direction 3.3V to 5V.

    In this case, we have some quesitons.

    ・Which should customer power up 3.3V side device(=FPGA) or 5V side device(=optics) at first ?

    ・Is there any problem for isolation between different voltage ?

    In addition, regarding application guide, scda008,

    ・Should customer configure Schottky Diode on the 5V side ?

    Best Regards,

    Kato

  • I do nont beleive that the diode will be needed. This part does not have a clamp diode to Vcc  and has the Ioff feature wich puts all I/Os high impedance when Vcc is 0V. It should not matter which side is powered first..

  • Hi Chris - san,

    Regarding a pull up resister on the 5V side, if the port's data rate is up to 10MHz, what value resister is required ?

    In addition, would you give us the characteristic curve for Data rate vs Pull up resister value ? 

    Best Regards,

    Kato

  • Hi Kato-san

    We do not have a characteristic table because there are to many variables the effect the max frequency.

    For instance, If VrefA is very low then the resistor will have more affect than if th VrefA was higher.

    We use 350ohm  or 500ohm resistors in our test conditions and either one of these would work well at 10Mhz.

  • Hi Chris - san,

    Thank you for the explanation.

    Customer would like to use higher pull up register as possible for low power consumption. Is customer's understanding correct ?

    Best Regards,

    Kato

     

  • That is correct. The customer may be able to go to a higher vale then 500 ohms but i do not have any dat to confirm this.