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LSF0102: Vref B and EN pin voltage level drop

Shu-Cheng LIN
Guru 11785 points
Part Number: LSF0102

Hi Sirs,

As title,

Our schematic

LSF0102.pdf

Our design is  Vref B connect EN, add use external 200k resistor to pull up , 0.1u cap to GND.

But after passed 200k resistor, we found Vref B only had 2.68v, not 3.3v

We think it's not normal, could you help us to explain this phenomenon and how to fix it?

  • 0
    TI__Guru* 96591 points

    Hi Shu-Cheng,

    This is exactly how the circuit is supposed to operate.  You can see a short video that explains in detail how this bias circuit operates, located here:

  • 0 in reply to Emrys Maier
    Guru 11785 points

    Hi Sirs,


    Please let us update as below


    We use 4.2V instead 3.3V through 200K to connect Vref_B

    But the Vref_B is still 2.7V

    We just want to adjust Vref_B larger, not margin pass with your Vref_A+0.8 spec. (Vref_A=1.8V)

    Also spec. recommend 1.0V higher than Vref_A but seems it can not be done

    So will possible change the spec and recommend description? or I'm got wrong information?


     

  • 0 in reply to Shu-Cheng LIN
    TI__Guru* 96591 points

    Hello,

    3.3V - 1.8V = 1.5V of difference between the supplies. This is much larger than 0.8V, and no additional margin is needed.

    I think you are making this far more over-complicated than it needs to be to operate the LSF translator.  This device is confusing to many people, thus why we created the detailed video series explaining the operation of these translators. Were you able to watch the videos?  They should give you a very detailed understanding of how this device works.

    The Vref_B pin should not be directly changed, and I'm not sure why you would try. The specified voltages are for the _supply_, not the pin.

    If you want to translate from 1.8V to 3.3V, it's as simple as connecting this circuit (figure 9 from the datasheet):

    The 3.3V enable signal is optional -- you can just connect the 200k to 3.3V directly so the device is always on.

  • 0 in reply to Emrys Maier
    Guru 11785 points

    Hi Sirs,

    We did follow your spec. SCH but 3.3V and 4.2V supply got same 2.7V at Vref_B(pin 7)

    We would like to know why VrefB always only 2.7v?

    Thanks!!

    Supply

    Vref_B(pin 7)

    3.3V

    2.7V

    4.2V

    2.7V

  • 0 in reply to Shu-Cheng LIN
    TI__Guru* 96591 points

    That is explained in detail in the videos I referred to.  Did you have trouble viewing them?

    Specifically, this video:

    At the 1:55 time code explains why this is the case.