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LSF0108 question

Peter Chen(ICP_FAE)
Intellectual 2480 points
Other Parts Discussed in Thread: LSF0108

Customer  would like to use LSF0108 to perform Low voltage to high voltage level shifter(ie 3.3V to 5V). the results is showing the output can not up to 5V. please help review the test circuit and results then supporting this application.

LSF0108 questtion.docx

  • 0
    TI__Genius 16510 points
    Hello Peter, I've tagged the appropriate engineer for this one.
  • 0 in reply to Jonathan Valdez
    TI__Guru 62190 points
    Hi Peter ,

    I will review and provide you with the feedback .
  • 0 in reply to ShreyasRao
    TI__Guru 62190 points

    Hi Peter ,

    I do not see the pull up resistor at the B8 output . The pullup resistor needs to be connected to 5V .

    The LSF is a passive switch which does the translation by having external pullup resistor pulling the output to the desired voltage level when the switch is off . when the A signal is low , the switch is on , thereby having B at the same level .

    The driver at the A side must be able to handle the current flowing through the pullup resistor and the small ron of the LSF switch itself when in low state (refer to the IOL of A side driver) .

    I will also attach an app report and EVM user guide which will be very helpful in understanding the device and your application design too .

    LSF_EVM_sdlu003a.pdf

    LSF_slva675b.pdf

  • 0 in reply to ShreyasRao
    Prodigy 20 points
    Hi ShreyasRao
    Thanks,your reply.
    But I have another question which I put pull-high R to result to Square wave's low level will be shifted up 0.8V~0.4V.
    E.g
    VrefA input 3.3V , one A port inputs Square wave(high level 3.3V ,low level 0V ,2)
    VrefB input 5V
    Through R(360 ohm) pull 5V on LSF0108 B port ,to measure B port wave which low level is shifted up 0.8V.
    Through R(1K ohm) pull 5V on LSF0108 B port ,to measure B port wave which low level is shifted up 0.4V and high level becomes slope.

    Are these E.g normal?
    Why are these situation?
    How to improve?

    thanks
  • 0 in reply to ShreyasRao
    Prodigy 20 points
    Hi ShreyasRao
    I have another issue
    E.g
    RefA=3.3V, RefB=5V, A port input Square wave ( High leve is 3.3V ,low level is 0V, 2MHz)
    Through R(360 ohm) pull up 5V on B port, To measure B port wave wich low level is shifted up 0.8V.
    Through R(1K ohm) pull up 5V on B port, To measure B port wave wich low level is shifted up 0.4V and high level becomes slope.

    Are these E.g normal?
    Why do occur these situations?
    How to improve?

    thanks
  • 0 in reply to Lee Kloud
    TI__Guru 62190 points
    Hi Lee ,

    The issue here is the value of R you choose on the pullup side . During the low level of the input , the current flows from the pullup voltage through the pullup resistor into the A side driver . The A side driver must be large enough to handle this IOL current . eg. 5/1k = 5mA and 5/0.4k = 12.5mA . If the A side driver doesn't have this IOL current specified , the input voltage Vil tends to rise up . This is what is seen on the B side output signal.
    Also , the R must be small enough so as to not cause the large rise time for faster switching signals as you are seeing . So , this is the kind of balance which needs to be maintained on the resistance at the B side .
    The app note explains in detail on the pullup resistor sizing .