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PCA9306: PC9306 for RS485 interface

Gabriel Xu
Intellectual 2145 points
Part Number: PCA9306


Hi Teams

Customer want to use PCA9306 for MCU and RS485 voltage level shift. And have some questions about such kind of application:

  1. PCA9306 bridge MCU and RS-485 TX/RS signal between 3.3V and 5V system. We should remove the pull-up resistor in typical application diagram is that right? These resistor is majorly for I2C OD port base on my understanding.
  2. For the channel not being used, what's the most reliable type of connection?
  3. Customer have test this device under reverse power supply configuration (They connect VREF1 to 5V and connect VREF2 to 3.3V) but they found PCA9306 work normally and no fault have report during RS-485 communication. They want to know why?
  • +1
    TI__Guru* 77441 points

    Hi Gabriel,

    Gabriel Xu said:
    PCA9306 bridge MCU and RS-485 TX/RS signal between 3.3V and 5V system. We should remove the pull-up resistor in typical application diagram is that right? These resistor is majorly for I2C OD port base on my understanding.

    On the PCA9306, I would leave a 4.7k pull up on the RX sides of the device. TX sides can be have their pull up removed.

    Gabriel Xu said:
    For the channel not being used, what's the most reliable type of connection?

    I usually recommend tying them to a pull up though it doesn't matter to be honest. (I assume you mean either SDA or SCL)

    Gabriel Xu said:
    Customer have test this device under reverse power supply configuration (They connect VREF1 to 5V and connect VREF2 to 3.3V) but they found PCA9306 work normally and no fault have report during RS-485 communication. They want to know why?

    The Vref2 side is meant to be higher than Vref1 inorder to generate a higher biasing voltage internally. If you make Vref1 higher and Vref2 lower, then the device will have a lower gate voltage fed to the pass FETs. This will slightly lower the prop delays but functionality is okay. Technically you lose some 'foot room' in the sense that the 'ViL' (I don't like this term for this device) will shift lower.

    -Bobby