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TCA9802: Undershoot at falling edge

NAOKI AOYAMA
Guru 11215 points
Part Number: TCA9802


Hello,

Our customer use the TCA9802 as I2C repeater.

When the slave device drive High to Low, there is an undershoot as -1V.

I attach the scope shot.

TCA9802_undershoot.pptx

For work around, the customer will add a damping resistor.

Is it possible to add a damping resistor between B-side of the TCA9802 and the slave device?

or

If the drive capability of the slave device is less than 20mA, is this violation permitted?

Best Regards,

Naoki Aoyama

  • 0
    TI__Guru* 78111 points
    Hello Naoki,

    When you see the -1V, is the TCA9802 driving the low from A side to B side or is the slave the one pulling low and causing the undershoot?

    What is the Rdson of the slave? You can measure this by looking at the VoL on B side of the overshoot when the slave pulls low. Take VoL/IoL (this is 2.2mA from the TCA9802). This will provide the Rdson. The damping resistor cannot be larger than RilCmax of TCA9802 minus Rdson of the slave.

    For example, TCA9802 has a RiLc of 150 ohms. If Rdson of the slave is 100 ohms then the damping resistor cannot be larger than 50 ohms.

    Thanks,
    -Bobby
  • 0 in reply to BOBBY
    Guru 11215 points
    Hello Bobby,

    Thank you for your reply.

    The customer see the -1V when the slave is pulling low.
    Please look at the SCL signal of scope shot. There is no undershoot.
    So the TCA9802 is not causing the undershoot.

    I will check the Rdson of the slave.

    Thank you for telling me how to think about the damping resistor.

    If I get a new information, I will let you know.

    Regards,
    Naoki Aoyama
  • 0 in reply to BOBBY
    Guru 11215 points
    Hello.

    I checked the Rdson of the slave device.
    VoL = 0.2V max.
    IoL = 3.6mA
    Rdson = 0.2V/3.6mA=55.5 ohms

    It may be possible to use the damping resistor less than 90 ohms.
    I will ask the customer to evaluate the undershoot using a damping resistor as less than 90 ohms.

    If I get the results, I will let you know.
    But it is summer holiday in Japan, so the reply will be next weekend.

    Regards,
    Naoki Aoyama
  • 0 in reply to BOBBY
    Guru 11215 points
    Hello Bobby,

    The customer added the damping resistor.
    The issue is resolved.

    Thanks,
    Naoki Aoyama
  • 0 in reply to NAOKI AOYAMA
    TI__Guru* 78111 points

    Good to hear!

    Thanks for getting back to us as well. This will help any other people who search the forum and find this question + answer.

    -Bobby