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PCA9306: Direction detection; Output method; One-shots?

Hideki Hayashi1
Expert 8420 points
Part Number: PCA9306
Other Parts Discussed in Thread: TXB0106

Hi experts,

Customer is having trouble to make PCA9306 works.

Therefore customer has three question.

1, Which Is PCA9306 outoput, CMOS or Nch open-drain?

2, How does PCA9306 decide which direction, master or slave?

3, Does PCA9306 include function like One-shots (on TXB0106) ?

Best regards,

Hideki

  • 0
    TI__Guru 62920 points

    Hello Hideki-san,

    PCA9306 is a FET-based switch which conducts when the voltage on either side is at VREF1 or lower. Otherwise, it is high impedance (allowing each side to be pulled to an independent voltage level). There are no one-shots. Please let us know if you have further questions.

    Regards,

    Max

  • 0 in reply to Miguel Robertson
    TI__Expert 8420 points

    Hi Max,

    Thank you for explanation.

    Let me ask further question.

    Could you please confirm my understanding. Sorry it's hard to understand.

    When small voltage (<Vth) is applied on gate of FET at I2C bus master side, PCA9306 is high impedance. So bus is pullup-ed, and each side of I2C bus is logic high.

    When the voltage over Vth is applied on gate of FET at master side, PCA9306 conducts. So I2C bus is logic low. 

    Could you please explain deeply or show me any literature if my understanding is not correct.

    Thanks and best regards,

    Hideki

     

  • 0 in reply to Hideki Hayashi1
    TI__Guru 62920 points

    Hideki-san,

    You are correct.  I2C uses open-drain signaling.  If the master side pull-down FET conducts, it will pull the SDA or SCL line down to a low voltage.  This will cause the PCA9306 switch to conduct, making the voltage on both sides low.  If the master side pull-down FET does not conduct, then the SDA or SCL line will be pulled high externally by a pull-up resistance.  This will cause the PCA9306 to become high-impedance, allowing both sides to be pulled up to their respective pull-up voltages.  Please let me know if this information is not clear or if you have further questions.

    Regards,
    Max

  • 0 in reply to Miguel Robertson
    TI__Expert 8420 points

    Max,

    I think I understand how a translator works roughly.

    The reason the PCA9306 switch to conduct is that Vgs of FET in translator exceeds Vth of that because of master side pull-down FET. Is it correct?

    In that case, I don't understand the PCA9306 is bidirectional. Are there two switches?

    Customer now is using side 1 for slave and Vdd is 1.8V, is using side 2 for master and Vdd is 3.3V.

    In that situation, are there any differences from Figure 6. in 8.1.2 in the datasheet.

    Regards,

    Hideki

  • 0 in reply to Hideki Hayashi1
    TI__Guru 62920 points

    Hideki-san,

    It sounds like your understanding is correct.  There are not two switches, though.  The internal FET switch has a gate voltage that is regulated to VREF1 + Vth.  Whenever either side's voltage drops below VREF1, then, the FET begins to conduct.

    Regards,
    Max

  • 0 in reply to Miguel Robertson
    TI__Expert 8420 points

    Max,

    Thanks for kindly explanation.

    Regards,

    Hideki