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LSF0108: The poor signal integrity in B port

Lacey Lin
Intellectual 2900 points
Part Number: LSF0108

Hi team,

My customer use the LSF0108 in the server to shift the voltage of I2C signal between the 1.8V and 3.3V.

For the A port, the Ref A is 1.8V and connect 100nf cap to GND. the A port IO has 2.2k ohm pull up resistor to 1.8V.

For the B port, the Ref B is connect to the EN pin and both connect a 200k to 3.3V. Also the Ref B pin connect a 10nf cap to GND. The B port IO has 2.2k pull up resistor to 1.8V.

But they find the signal of I2C is not good.

For A port.

For B port.

The purple is the SCLK, the green is the SDA.

1. can you help to provide the cause for the issue?

2. For the B port, customer feed back the voltage in Ref B and EN pin is only 2.6V. Does this make sense?  If we want to increase the voltage, can we decrease the value of the 200k ohm resister?

Please help to look at above issue.

Thanks a lot!

Lacey

  • 0
    TI__Guru* 97046 points

    Hi Lacey,

    Can you provide a schematic?

    From the above scope shot, there are two likely issues:

    (1) The input signal is causing a very significant undershoot - it looks to be about 1V.  This could cause a localized shift in ground that will adjust the thresholds of the device, causing the internal switches to briefly turn on and transfer the input signal to the output. How long is the input remaining below 0V? I can only see a short spike in this scope shot, but I expect this is a significant amount of time (>20ns).

    (2) There is a lot of cross-talk, which usually indicates that the capacitor that should be from EN to GND is misplaced (typically the capacitor is incorrectly placed from VCCB to GND).  Your statement above says it is placed correct, but a 10nF cap was chosen instead of a 0.1uF cap (100nF).  I would recommend double checking the capacitor location and to try replacing it with a standard bypass cap (0.1uF).

  • 0 in reply to Emrys Maier
    Prodigy 10 points
    Hi TI team,
    Thanks for your replying!
    Firstly,correct a mistake for the Lacey's descriptioS "the B port IO has 2.2k pull up resistor to 3.3V".
    For your replying:
    the first ,your remark picture is the output of B port .
    the second ,I will change the location and value of the capacitor ,then try again .
    but there is a another question,The LSF0108PWR is just SW or with itself's driver for the IO port ?
    looking forward for your replying,many thank!
  • 0 in reply to user5795712
    TI__Guru* 97046 points

    The LSF0108 is a passive switch only.  You can see exactly how it works in the video series "Understanding the LSF Family of Bidirectional, Multi-Voltage Level Translators"


    Please let me know if I can be of further assistance.