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LSF0204-Q1: Using LSF0204-Q1 for Voltage Translation doesn't work.

Anonymous
Anonymous
Guru 17045 points
Part Number: LSF0204-Q1
Other Parts Discussed in Thread: SN74AVC4T774, SN74LVC3G17, SN74LV1T34, SN74LVC3G34

Hello,

I would like to use the LSF0204-Q1 for level translation in an SPI communication.

Therefore I would like to use the following circuit:


That's why I designed the following PCB design:
(Tx/Rx can be ignored)

For pure test purposes I use an Arduino Uno, which outputs 5V (digital) pulses. These should then be set to 3.3V.
VrefA = 3.3V large, VrefB = 5V.
If I now set 5V pulses to B1 without connecting the Vdds first, B1 will not output any signals.
If I now connect Vdd, then B1 always takes the signal from Vdd, as it can be seen in the following picture:

I have tested all possible configurations. So to set the EN pin high with different levels, to connect Vdd and Vref with each other, to use only Vref for the Rpu. No success. Either B1 remains zero or it assumes the respective level, but doesn't pulse slightly delayed with 3.3V.
Can you tell me what I'm doing wrong?

Best regards

  • Hey Clemens

    I have now installed a 200kOhm resistor. Unfortunately the result remains the same.

    Best regards,

    Hendrik

  • Sorry, the 200 kΩ resistor is needed only for LSF010x devices; the LSF0204-Q1 integrates it.
    I see no reason why your circuit should not work. It might be possible that the chip got damaged.

    For SPI, TI recommends the SN74AVC4T774.

  • Hi Hendrik,

    I'll be happy to help you with this issue. Let me review your schematic and layout and get back to you tomorrow on this.

  • Anonymous
    0 Anonymous in reply to Clemens Ladisch

    Clemens Ladisch said:

    For SPI, TI recommends the SN74AVC4T774.

    The problem with this IC is that it cannot translate a 5V level into a 3.3V level.

  • Anonymous
    0 Anonymous in reply to Dylan Hubbard

    Dylan Hubbard said:

    I'll be happy to help you with this issue. Let me review your schematic and layout and get back to you tomorrow on this.

    Thank you!

  • Translators with automatic direction sensing must make compromises in drive strength, noise sensitivity, and/or power usage. (Which might or might not matter for you.) So for SPI, it is recommended to use unidirectional or direction-controlled translators.

    The problem with this IC is that it cannot translate a 5V level into a 3.3V level.

    Oops, sorry.

    Assuming that 5 V is the master side, you can downtranslate CS/CLK/MOSI with buffers with overvoltage-tolerants inputs (e.g., SN74LVC3G34 or SN74LVC3G17), and uptranslate MISO with a single unidirectional shifter (e.g., SN74LV1T34).

  • Hi Hendrik,

    Nothing stands out in your schematic or layout that could cause issues like this. I'd like to ask a few more questions to help clarify a few things so we can get this issue debugged.

    Can your device sink the amount of current going through the pull-up resistor? If you are using the values from the fist image then that would be about 16.5 mA which is quite high.

    You are sending a 5 V square wave into the B port when the device is powered and enabled, but the A side just sits at a DC 3.3 V correct? I'm a little confused on this since you only talk about the B port but I see a 3.3 V signal in the waveform.

    Have you tried to input a 3.3 V signal on the A side to see if you had the same signal shifted up to 5 V on the other side?

    Lastly, are you able to provide waveforms that include the input, output, and EN signals at the same time?

    Here are some training videos on the LSF devices: https://training.ti.com/introduction-voltage-level-translation-lsf-family

    I advise watching them as you might see something stand out that can be probable cause for this issue.