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LSF0102: Use LSF0102 for IO level shifting between 1.2V and 1.8V.

Tim Peng
Prodigy 150 points
Part Number: LSF0102
Other Parts Discussed in Thread: LSF0002

Hi Support

I want a part for IO level shifting between 1.2V and 1.8V. IO could be push-pull or open-drain. And I want auto bi-direction.

I found LSF0102 may fulfill the requirement. And I come across one application report called "Voltage-Level Translation With the LSF Family"

Quote: The data sheet says Vref_B must be 0.8 V higher than Vref_A. How do I translate across a small range, like 1.2 V to 1.8 V?

It says :  Set Vref_A to 1.2 V and Vref_B between 2 V and 5.5 V. Add a B-side pullup to 1.8 V. As long as Vref_B ≥
Vref_A + 0.8 V and Vref_B is within the recommended operating conditions range, the level of Vref_B
does not affect the output signal. Only the B-side pullups affect the output signal level.

Could you please help to explain more how to connect it correctly or review my circuit if I connect it right?

  • 0
    TI__Mastermind 41096 points

    Hey Tim,

    I was not able to see any attached schematic with your post, are you able to reupload? 

    Anyways, I have drafted up a quick schematic that can be used as a reference:

    By setting VREFA/ VREFB to be 0.8V apart, we can ensure that the internal FET can be biased correctly to allow for translation, even if the I/Os are only 0.6V apart. The external pullups used on B-side will ensure that the output signals reach 1.8V.  



    Regards,

    Jack 

  • +1
    Guru 317350 points

    The Vref voltages are not required to be the same as the I/O (pull-up) voltages. Just connect the Vref_B/EN resistor to some higher voltage that is available in your circuit.

  • 0 in reply to Jack Guan
    Prodigy 150 points

    Hi Jack, 

    Please help to review this circuit. The design requirements are

    (1) auto Bi-direction translate between 1.2V IO <-> 1.8V IO

    (2) IO type could be either push-pull or open-drain.

    (3) Possible application interface can be JTAG, UART.

    by the way how to decide the value of the pull-up resistor correctly? I know there are some details in datasheet. Eventually I place 1Kohm in this circuit, should I need them to ensure the circuit work properly or I can remove them?  please also help to review them.

  • 0 in reply to Clemens Ladisch
    Prodigy 150 points

    Thanks, I also see the same information from "Multi-voltage translation with the LSF family" from youtube. 

  • +1 in reply to Tim Peng
    TI__Mastermind 41096 points

    Hi Tim,

    Schematic looks OK- though pullups would still be required on the high side since the output of the LSF is open drain based.

    When you said "auto Bi-direction translate between 1.2V IO <-> 1.8V IO", it is regarding different channels sending signals to and from correct? If that is the case, the signals would be considered fixed direction since it is specific to per channel. 

    Note that the TXU0x02 devices would be a better fit for your level shifting needs since they will not require external pullups like the LSF and specific biasing requirements like the LSF0102 where power supply A needs to be 0.8V less than power supply B is not needed. 

    Regards,

    Jack 

  • 0 in reply to Jack Guan
    Prodigy 150 points

    About your question:  When you said "auto Bi-direction translate between 1.2V IO <-> 1.8V IO", it is regarding different channels sending signals to and from correct? If that is the case, the signals would be considered fixed direction since it is specific to per channel.    

    Reply: Our requirement is that each single channel can support bi-directional signal. 

    The downside of TXU0x02 for our application is that it is NOT auto-bidirection.  LSF family seems good fit. by the way, could you please also confirm 1K ohm for High side pull up is ok. please verify schematic once again 

  • 0 in reply to Tim Peng
    TI__Mastermind 41096 points

    Hey Tim,

    I see- if that is the case, then the LSF can be used to support the auto-bidirectional signals per channel. As for the pullup resistor sizing, this is dependent on the data rate needed by your application. For your convenience, please use the below equation to estimate the max data rate:

    If size is a key care-about, you may also see the LSF0002 that offers a smaller package with the removal of VREFA/ VREFB pins. 

    Regards,

    Jack