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Original question:

TCA9406: Not switching on the A-side

TCA9406: Overshoot on SCL/SDA

Yusuke Mukuno
Expert 7395 points

Part Number: TCA9406

Hi

Could you please refer the attached file(issue waveform)

Customer found the overshoot on SCL/SDA signals. They have to improve it, because it is over IC's ABS.

Could you please give me your advice for improvement.

TCA9406E2E_question.pdf

  • 0
    TI__Guru* 78301 points
    Hey Yusuke-san,

    Are you communicating through a cable/wire?
    How long is the wire?

    This is likely adding parasitic inductance into the line. We may be able to get rid of the overshoots by adding some series resistors (replace the 0 ohms on the board on the right) at around 30 or 60 ohms.

    To remove undershoots we can add some schottky diodes to GND on the SDA/SCL lines to clamp the negative swings.

    Any idea how much capacitance you have on the bus currently? We may be able to add some capacitors to try to help fight the swinging.

    Thanks,
    -Bobby
  • 0 in reply to BOBBY
    TI__Expert 7395 points

    Hi Bobby-san

    Thank you for your advice.

    Wire length : 105mm.

    Customer will try to modify their circuit from your advice.

    Customer tried to put an RC filter(100ohm//100pF) instead of ferrite on the left board.
    Overshoot was improved. However, Low voltage was increased to 0.6V. It don't  meet VIL requirement.
    I will reply again, if I have any update from customer .
    Thanks
    Muk
  • 0 in reply to Yusuke Mukuno
    TI__Guru* 78301 points

    Hey Muk,

    The series resistance of 100 ohms will shift the VoL (recall VoL = (Rseries+Ron) *IoL) so we may want to try 50 ohms and 200pF instead. This should give you the same affect and lower the VoL.

    Thanks,

    -Bobby