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Variation of TXS VIH and VIL input thresholds

Binupandian S
Prodigy 180 points
Other Parts Discussed in Thread: TXS0108E

Hi,

We have the following design using TX0104E.

 

The VIL/VIH does not seem to match in our design.

But we found a reference design that uses TXS0108E for the same device (1.5V to 3.3V translation, but the above cases still valid).

Is the VIL/VIH limit different for different cases?

How to validate VIL/VIH in our design?

Thanks and Regards,

Binupandian S.

  • 0
    Guru 287370 points

    Your device's VOL is probably specified for a higher current, not for the 500 µA that would be used here. What is your device's output impedance?

  • 0
    TI__Expert 6335 points

    Binupandian,

    I am not sure if I understand your question. Let me know if the information below is what you need:

    Binupandian S said:

    Is the VIL/VIH limit different for different cases?

    For VIL - No. It is fixed to 150 mV.
    For VIH - Yes. It is depedent upon the supply voltage.

    This is documented in the Recommended Operating Conditions in the datasheet (shown below).

    For your setup: With VCCB = 3.30V, VIH >= (3.30V - 0.40V) = 2.90V and with VCCA = 1.80V, VIH >= (1.80V - 0.20V) = 1.60V

    Best Regards,
    Nirav

  • 0 in reply to Nirav Patel
    Prodigy 180 points

    Hi,

    Thanks for the reply.

    So, I understand that VIH and VIL are both fixed.

    The issue is that the device side VOH and TXS side VIH don't seem to match(refer original post).  Yet, I have a reference design with the same setup.

    But, as per post, this seems to be a device side issue.  Is it right?

    Regards,

    Binupandian S.

  • 0 in reply to Binupandian S
    TI__Expert 6335 points
    Binupandian,

    Yes I would consider this to be a limitation of the device connected to the TXS0108E. Can you specify the device that you are using?

    Best Regards,
    Nirav
  • 0 in reply to Nirav Patel
    Prodigy 180 points
    Hi,

    We are using Xilinx FPGA Kintex-7.

    Part No.: XC7K325T-1FBG676


    Signalling standard used is LVCMOS18 with drive strength 12mA.


    The AC/DC characteristics can be found in the following datasheet link:

    www.xilinx.com/support/documentation/data.../ds182_Kintex_7_Data_Sheet.pdf


    Regards,

    Binupandian S.
  • 0 in reply to Binupandian S
    Guru 287370 points

    The VOL/VOH values of the Kintex 7 are guaranteed for the specified load, i.e., 12 mA.

    The TXS does not load the output that much; for a low signal, the two 10 kΩ pullup resistors to 1.8V/3.3V result in a current of about 0.5 mA.

    Assuming that the voltage drop is caused primarily by the RDS(on) of the output FETs (the datasheet does not have a nice graph to confirm this, but it's generally true for all CMOS devices), we can simply scale it with the current, so the maximum VOL value at 0.5 mA should be somewhere near 0.45 V × (0.5 mA / 12 mA) ≈ 0.02 V. Even with a large safety factor, this is still enough for the TXS.