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TCA9800: oscillations on SDA pin

Vincenzo Pizzolante
Mastermind 22776 points
Part Number: TCA9800

hi all,

a customer of mine started using the TCA9800 that was placed before an isolator on an I2C bus; but when he tried out the configuration an oscillation appeared on the SDA line. He figured out that the isolator output voltage is somewhere 600mV and the TCA9800 didn't really like it, as it started to pull up and down the voltage on SDA line above 100mV.

To avoid this problem he looked search for anoter isolator with low output voltage, but unfortunately he didn’t find one.

After this the solution became a simple voltage level translator in between, and it works perfectly.

I think the problem with TCA9800 is the active current source which is a good feature but in this configuration it is not necessary for us and unfortunately it causes problems.

may you help me undertanding the root cause of the problem?

thanks a lot in advance

KR

Vincenzo

  • 0
    TI__Mastermind 49370 points

    Hello Vincenzo,

    "a customer of mine started using the TCA9800 that was placed before an isolator on an I2C bus....He figured out that the isolator output voltage is somewhere 600mV and the TCA9800 didn't really like it"

    -Sounds like the isolator uses a static voltage offset. This is known to cause problems with buffers connected to another buffer with a SVO.

    "To avoid this problem he looked search for anoter isolator with low output voltage, but unfortunately he didn’t find one."

    The issue here is the isolator uses a static voltage offset which is connected to our TCA9800 which uses a static current offset. These two static offsets don't play nice with each other.

    "may you help me undertanding the root cause of the problem?"

    - TCA9800 watches the current source to see if the device is sinking current (occurs when A side pulls low) or if the device is sourcing current (occurs when a device on the B side is pulling low). The static voltage offset of the isolator is confusing the TCA9800's detection algorithm, from what your describing it sounds like when the isolator pulls low then TCA9800 momentarily sees it is sourcing current and pulls low then after some time it stops sourcing (may have to do with impedance on the isolator's end changing during the short sourcing period).

    Two ways we could fix this is to either flip the TCA9800 so A side is connected to the isolator or to flip the isolator (I need to look at the isolator to verify if this would work).

    Thanks,

    -Bobby