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TCA9406: TCA9406 VIL spec.

user4979899
Expert 3080 points
Part Number: TCA9406


TCA9406 spec. mention the VIL max is 0.15V

My understanding is once the Vi voltage higher than 0.15V, TCA9406 can’t recognize the voltage as Low level, right?

Because we have an issue, SMBus can’t be detected low level when Vi low voltage is 0.6V that is higher than 0.15V.

However, we rework our schematic to provide a Vi low voltage, 0.34V. It works, TCA9406 works well and not problem on detecting low voltage. could you please help check if the VIL max is 0.15 or higher.

Thank you

  • 0
    TI__Guru* 78081 points
    Hello Anne,

    "Because we have an issue, SMBus can’t be detected low level when Vi low voltage is 0.6V that is higher than 0.15V."
    -What is VccA of our device? Do you have scope shots?

    "However, we rework our schematic to provide a Vi low voltage, 0.34V. It works, TCA9406 works well and not problem on detecting low voltage. could you please help check if the VIL max is 0.15 or higher."

    TCA9406 uses a pass FET architecture so this means as long as the gate to source voltage is lower than Vth. The low should still pass from one side to the other side. This does result in a slightly high VoL on the other side though. I believe the reason why we specify ViL of 0.15V is because our device has a RTA that can be triggered and if the voltage is close to that threshold then noise can accidentally trigger it. By specifying a low ViL we can avoid tripping the RTA.

    Thanks,
    -Bobby
  • 0 in reply to BOBBY
    TI__Expert 3080 points
    Hi Bobby,

    Thanks for your explanation.
    However, standard SMBus Vol max spec is 0.4V, it means even a device fully compliance with SMBus spec.
    The device still can’t meet your Vil max requirement 0.15V. We are curious if TCA9406 can support SMBus or not?
  • 0 in reply to user4979899
    TI__Guru* 78081 points
    Anne,

    You should be able to pass a low from one side to the other as long as the low voltage generates a Vgs larger than Vth. 0.15V is not the ViL max.

    I believe what the ViL max is stating is if you generate a low on one side of the device of 0.15V and pull 1mA of current then the other side will be a max voltage of 0.4V. This is to meet I2C spec (and SMBUS spec). The ViL is basically relating to VoL. Look at VoLA and VoLB in the datasheet in section 6.5, you can see the test conditions.

    Remember what my first post said, "This does result in a slightly high VoL on the other side though" the Rdson of the pass FET will generate a larger VoL. This Rdson is correlated to the voltage at the gate which if lower, means the Rdson is larger resulting in a larger VoL shift from one side to the other.

    "standard SMBus Vol max spec is 0.4V, it means even a device fully compliance with SMBus spec."
    This is in reference to a current. 3mA through a pass FET must generate 0.4V or less. This is actually specifying a max Rdson for a pull down FET not a VoL max. (Remember we have buffers which generate more than 0.4V because of a static voltage offset but are still I2C/SMBUS compliant because the pull down FETs meet the Rdson requirement)

    "We are curious if TCA9406 can support SMBus or not?"
    -It can. Have the customer measure the VoL on both sides of the device and they should see a small difference between the two. (pull low on side A and measure the low on side A and side B then repeat but pull low on side B instead).

    I suspect if you generated a low on one side of 0.4V you should see something like 0.45V or so on the other side though it depends on how much current you are sinking and the voltage on the gate of the pass FET.

    Thanks,
    -Bobby
  • 0 in reply to BOBBY
    TI__Expert 3080 points
    Hi Bobby,

    Recap your description, can I say that if one side low voltage is A(Vgs larger than Vth), another side’s low voltage output will be higher than A?
    In my case, our A is about 0.6V, another side Vol will be higher than 0.6V, right?

    Our application, TCA 9406 is a level shift from 3.3V to 1.8V.
    @ 3.3V side, 0.6V input voltage . @ TCA 9406 1.8V side, it will output a Vol that is higher than 0.6V, and the 0.6V can’t meet Vil of 1.8V system(end device).
    Is the situation possible?
  • 0 in reply to user4979899
    TI__Guru* 78081 points
    Hello Anne,

    "Recap your description, can I say that if one side low voltage is A(Vgs larger than Vth), another side’s low voltage output will be higher than A?"
    That is correct, the other side will ALWAYS be larger though the amount varies with Vg and IoL.

    "In my case, our A is about 0.6V, another side Vol will be higher than 0.6V, right?"
    Correct though the amount higher varies with IoL and Vgate.

    "Is the situation possible?"
    That is possible but you can fix this easily by just making the pull up resistor on the 3.3V side larger. This will make IoL lower and make VoL lower as a result. If our device is enabled, you will also be able to pull up fast using the rise time accelerator.

    If VccA is 1.8V then you are also above the the point where the RTA triggers. RTA for this device triggers at 30% of VccA (0.54V and ur VoL is 0.6V).

    -Bobby