• State TI Thinks Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 2 answers
  • Subscribers 34 subscribers
  • Views 1088 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TCA9803: Slow falling edge

Johnny Guo
Expert 4910 points
Part Number: TCA9803
Other Parts Discussed in Thread: TCA9617A, PCA9306

Hi, team,

I have a question from customer. Customer changes from TCA9617A to TCA9803, no BOM is changed.

In test , they found the falling edge of TCA9803 is very slow, especially from 750mV to 400mV. 750mV is too high that may be detected as HIGH.

My question:

1. Why is TCA9803 falling edge much slower than TCA9617A, especially from 750mV to 400mV?

2. Do you have some suggestion to fix this problem?

Thanks.

Johnny

  • 0
    TI__Guru 62920 points
    Hi Johnny,

    Today is a holiday in the US, so we will need to get back to you tomorrow. Can you please let us know how the PCA9306 is connected, though? You mentioned there were no BOM changes, but PCA9306 typically requires different connections compared to TCA9617A (see the example circuit in the datasheet for reference).

    Max
  • 0 in reply to Miguel Robertson
    TI__Expert 4910 points
    Max,

    I did not mention PCA9306, customer is testing TCA9617A and TCA9803.
    Thanks.

    Johnny
  • 0
    TI__Guru 72901 points

    Hey Johnny,

    "1. Why is TCA9803 falling edge much slower than TCA9617A, especially from 750mV to 400mV?"

    You've posted a schematic of TCA9617A but are you actually using the series resistors on SDA/SCL on B side (R951 and R952)? Series resistance on an I2C line will slow down fall times of devices.

    The TCA9803 also works differently than most buffers in the sense that it has its own current source on B side so you should not have pull up resistors on this side (R949 and R950).

    "2. Do you have some suggestion to fix this problem?"

    Please remove pull up resistors on B side of the TCA9803 device and then short the series resistors (A side and B side).

    Thanks,

    -Bobby

  • 0 in reply to BOBBY
    TI__Expert 4910 points

    Hi, Bobby,

    Below is test result if removing B side pull-up resistor. The result is similar without significant change.

    Thanks.

    Johnny

  • 0 in reply to Johnny Guo
    TI__Guru 72901 points
    Hey Johnny,

    Sorry for the delayed reply.

    If removing the series resistor did not help, then the only other way to speed up the fall time is to decrease the bus capacitance (this usually isn't an option).

    What I can tell you is, the data on I2C is sampled at the middle of the high period of SCL. What voltage are you reading when SCL is half of the high period? Also extending the high period an option for the customer?

    Thanks,
    -Bobby
  • 0 in reply to BOBBY
    TI__Guru 62920 points
    Hi Johnny,

    Just checking back in on this - were you able to resolve this issue?

    Max