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TCA9406: Series application

Alex J
Prodigy 10 points
Part Number: TCA9406
Other Parts Discussed in Thread: TCA39306

Tool/software:

Hi,

I'm planning to connect two TCA9406 level shifters in series, like in the below picture.

Do you think this setup would cause any issues?

If this configuration is not recommended, could you suggest a better combination of devices for this kind of use case?



Thanks!

  • 0
    Guru 316690 points

    This is allowed, as long as you respect the VCCA/VCCB restriction.

    Please note that this device already has internal pull-up resistors. Those external pull-up resistors add up to a pull-up current of about 5.4 mA, which exceeds the limit specified in the I²C specification and makes it unlikely that your 1.8 V device can drive the signals correctly.

  • 0
    TI__Mastermind 47328 points

    Hi Alex,

    I agree with Clemens here. 

    TCA9406 is a pass-through device and not a buffer. The 1.8k PU on the master side, 1.8k PU between the 9406's, and the 2.2k PU between the 9406 and target device will appear in parallel. The equivalent resistance is those three PU resistors in parallel, plus the parallel resistance of each 10k internal to the TCA9406. Since there are 2 x TCA9406 in series, this means there are an additional 4 x 10k PU resistors on the I2C bus. See the functional block diagram in the datasheet: 

    Recommendation is to use weaker PU resistors. You might find that removing the PU resistors (1.8k , 1.8k ,2.2k) entirely would be beneficial since the internal PU resistors already exist. 

    If you don't want a level translator with a one-shot inside the device, or you don't want the internal 10k PU resistors, you could opt for a simpler level translator such as TCA39306 which is a pass through level translator like the TCA9406, but removes the one shots and internal PU resistors. 

    Regards,

    Tyler