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LSF0204: Bad signal integrity at the output signal with a 750kHz signal.

Francisco Zamudio
Prodigy 40 points
Part Number: LSF0204
Other Parts Discussed in Thread: TXU0204, TXU0304, SN74LXC8T245, SN74AVC8T245, SN74AXC8T245

Hello team,

I am using the LSF0204 as a level shifter from 3.3V to 5V but we have experienced an unexpected behavior at the output. We tried different pull-ups and the output is not good. This is our circuit.

and this is the output

Output signal using 1k and 1.8k pull-ups

As you can see, the output is not reaching the 5V and the signal looks bad. The frequency is not really high, it is lower than 1 MHz. We have changed the value of the pull-up and there is a similar behavior, just a different offset. Another detail is that the maximum level is not reaching the 5V, just if the resistor is smaller according to the datasheet values. 

Could you please advise about it?

Thanks

Francisco

  • 0
    TI__Mastermind 22275 points

    Hi Francisco,

    Please note that the schematic provided isn't the full schematic and the waveform isn't showing the input and output behavior. However, you are using an auto-bidirectional device with signals that are not auto-bidirectional, and would further recommend TXU0204 / TXU0304 for the SPI signals.

    Also note that the LSF device used is a passive FET and pull-ups should not be required on the low side if the outputs are not open-drain or if the leakage is not >1uA. Please help see [FAQ] How do the LSF translators work? for further clarification, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 40 points

    Hello Michael,

    Thank you so much for your answer. On the VerfA I have a uC which is the source and EEPROM memory (at this moment is not used). on the VREFB side, at this moment I don't have any connection, but it is connected to a connector, and the plan is to connect to another board that will work as a slave using the same uC. According to your recommendation, that means that this LSF doesn't work for SPI signals?

  • 0 in reply to Francisco Zamudio
    TI__Mastermind 22275 points

    Hi Francisco,

    Please note that connectors would have additional capacitance impacting your signals with the recommendation for <70pF.

    However, SPI applications typically have a fixed direction similar to your application shown for B1 >, B2 > B3 <; as auto-bidirectional translators have many restrictions as they do not actually know what direction is desired and cannot act as a buffers so it is not recommended to use in applications that are not translating auto-bidirectional signals (<> for each channel),  thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 40 points

    Hi Michael,

    I forgot to mention that we also have ferrites before the connectors, do you think that this one could affect them?

    It is good to know the capacitance restriction and thanks for confirming the bidirectional effect. 

    I have a similar scenario with the TXS0108EPWR in regards to the signals, I am using a bidirectional level shifter with signals that have only one direction. Could you please recommend one that would be available?

    Regards

    Francisco

  • 0 in reply to Michael Ikwuyum
    Prodigy 40 points

    Michael,

    In regards to the comment about the pull-up should not being required on the low side, does this apply to both sides? so, that means that I should remove the R37-R39 or R37, R38, and R40?

    Regards

    Francisco 

  • 0 in reply to Francisco Zamudio
    TI__Mastermind 22275 points

    Hi Francisco,

    I believe the ferrites should be resistive helping with reflections and not so much capacitive, so that should not be a problem. However the decoupling capacitors should also be near / closer to the device (and should not be separated from the device with a ferrite bead if applicable).

    Francisco Zamudio said:
    In regards to the comment about the pull-up should not being required on the low side, does this apply to both sides? so, that means that I should remove the R37-R39 or R37, R38, and R40?

    This is for the lower supply voltage i.e 3.3V side. Hence, the pull-ups on the A-ports.

    For the 5V application, please double confirm if you are driving with a high or a low impedance driver by any chance; as the device is designed to drive a capacitive (high-impedance) load. Also double confirm the drive strength of your driver to help verify the output VOH which is very much so dependent on the strength of the RC pullup circuitry due to the device being a passive switch.

    For the TXS application, the TXU recommendation also holds for all fixed directions. However, are all 8 channels used? If so, direction controlled buffers like the SN74LXC8T245 should work fine, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 40 points

    Hi Michael,

    We just found the issue. It was the TVS capacitance value. Could you please recommend a good TVS for this Level shifter?

    In regards to the other recommendation SN74LXC8T245, I couldn't find the stock, that's the reason that I've used the TXS0108EPWR.

    Regards

    Francisco

  • 0 in reply to Francisco Zamudio
    TI__Mastermind 22275 points

    Hi Francisco,

    Thanks for sharing. 

    Francisco Zamudio said:
    We just found the issue. It was the TVS capacitance value. Could you please recommend a good TVS for this Level shifter?

     Any should work okay as long as the total parasitic capacitance for the loading isn't excessive per >70pF.

    Is the main purpose of the TVS for ESD protection? If so, please note that the device also has device-level ESD protection intended for protecting the IC while in an ESD safe manufacturing environment. See Design considerations for system-level ESD circuit protection  further clarifying the differences between device-level and system-level ESD protection.

    TI also offers a wide variety of ESD protection devices that will prevent damage to this device if placed appropriately. See  https://www.ti.com/interface/circuit-protection/esd-protection-and-tvs-surge-diodes/overview.html for system-level ESD protection devices.

    Francisco Zamudio said:
    In regards to the other recommendation SN74LXC8T245, I couldn't find the stock, that's the reason that I've used the TXS0108EPWR

    Please also see available LXC8T devices. SN74AXC8T245 / SN74AVC8T245 should also work as their direction can be controlled, thanks.

    Best Regards,

    Michael.