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TCA9617B: TCA9617B - level shifting between 3.3V and 1.8V @ 800K speed

Matty Toker
Prodigy 20 points
Part Number: TCA9617B

Hi,

I'm using the TCA9617B as a level shifter between Aardvark (3.3V level) to a Slave device (1.8V level).Since side B can't support 1.8V level I connected the Slave device to Side A and the Master device to Side B.Is it a valid connection? What would be the recommanded typical pull-up resistors for using 800K speed?

Regards,

Matty  

  • 0
    TI__Guru 73181 points

    Hey Matty,

    "Since side B can't support 1.8V level I connected the Slave device to Side A and the Master device to Side B.Is it a valid connection?"

    I assume you are asking this to make sure the master is allowed on the B side? TCA9617B is bidirectional so master can be on either side.

    "What would be the recommanded typical pull-up resistors for using 800K speed?"

    You can find the app note below which walks through how to calculate the pull up resistor.

    Rise time in your case is 120ns and you will have to estimate your bus capacitance. I assume with small trace lengths, 1 master, and 1 slave on the bus plus our device (the capacitance will be buffered on each side), it will be around 20~30pF each side. You should be able to calculate the max pull up resistor from this.

    Thanks,

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 20 points

    Hi,

    Thank you for the information-now it is much clear.

    Yet,for the resistors calculation,in most examples the values are around 1K while in the example inside the datasheet of the

    TCA9617B device

    there are resistor's value of 240 Ohm.

    Therfore I still need your answer about a typical resistor's value : will it be close to 1K or closer to 240 Ohms?

    Regards,

    Matty

  • 0 in reply to Matty Toker
    TI__Expert 6955 points

    Matty,

    Using the equation above from the App Note that Bobby provided in the previous response with rise time = 120ns and bus capacitance = 30pF, the maximum pull up resistor value will be about 4.7k. I would choose a resistor value around 20% lower than the max to account for resistor variance due to manufacturer tolerance and changes in resistance due to different operating temperatures.

    -Lucas