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using the LSF0102 level translator for i2c & SPI interface

John Hu
Prodigy 40 points

I have an application where on onside of the LSF0102YZTR is a micro-controller which will be programmed to communicate i2c or SPI master the other side will be either i2c or spi slave.  The micro-controller will be 3.3 connected to the "b" side of LSF0102YZTR.  the "a" side will be vddio = 3.3 - 1.2V.  pin 7 & 8 are connected to 3.3v through a 200Kohm resistor.  Pins 3 &4 are pull up independently with 332 ohm resistor to vddio

Will there be any issues with using this part in "spi mode"

thanks!

  • 0
    TI__Guru 57390 points

    Hi John ,

    How is your Vref A and Vref B setup ? Your Vddio varies from 3.3V to 1.2V ?
    If the VrefA is set to 1.2V and B1 is at 3.3V , the Vpu on the A side can be 1.2V to have 3.3V to 1.2V translation. Vpu /Vref A on the A side cant be same as the B side input .
    It is undesirable to have Vddio varying from 3.3V to 1.2V . The device wont work when both sides are 3.3V .
    Can you use an active translator such as LV1T family ? This can translate 3.3V to Vddio connected to Vcc of the device.

  • 0 in reply to ShreyasRao
    Prodigy 40 points

    the application intent is to have the Vddio to be tolerant to "any voltage" similar to a FPGA programming pod which has a wide I/O voltage range.  The problem with an active translator would be contention when in "i2c mode".

    i need the VDDIO side to be able to accept 3.3 v.

  • 0 in reply to John Hu
    TI__Guru 57390 points

    Hi John ,

    I am not very confident that you can use LSF in this application . You might consider VrefB at 5V, VrefA connected to Vddio. The Vil from the B signal will pass through the LSF , the Vih level at 3.3V at B side will turn off the LSF and the pullup should translate it to Vddio but the trouble comes when Vddio is 3.3V . It would not work when both sides of the source and drain is at 3.3V with gate at 4V(VrefA+Vth =3.3V +0.7V ).

    I have attached app note for LSF and an EVM guide which might be helpful .sdlu003a_LSF_EVM.pdf

    4403.slva675b_lsf010x.pdf

  • 0 in reply to ShreyasRao
    Prodigy 40 points

    Just to confirm my understanding if i connect VrefB and enable to 5v through a 200k resistor and pull up I/Ob to 3.3 I can use this device in both open drain and push pull application on the Vrefa side? Where Vrefa  = 1.2V -> 3.3 V.

    Thanks,

    John

  • 0 in reply to John Hu
    TI__Guru 57390 points
    No John . The problem here is Vddio is varying from 1.2V to 3.3V .
    With Vddio at 1.2V or 1.8V fixed , there was no problem for translation . But if its varying and it reaches 3.3V , the LSF internal switch is turned off . providing no translation . you can take a sample and first try 3.3V to 1.2V , which should work without issues . then vary the 1.2V to 3.3V and see if it still works as intended . I will try here at my end too but it wont be anytime soon .
  • 0 in reply to ShreyasRao
    Prodigy 40 points

    Hi There,

    In the case below is your statement true if Vrefb =5 with port b pulled up to 3.3 and vrefa at 1.2 - 3.3 with porta pulled up to Vrefa?

    "The problem here is Vddio is varying from 1.2V to 3.3V .
    With Vddio at 1.2V or 1.8V fixed , there was no problem for translation . But if its varying and it reaches 3.3V , the LSF internal switch is turned off . providing no translation . you can take a sample and first try 3.3V to 1.2V , which should work without issues . then vary the 1.2V to 3.3V and see if it still works as intended . I will try here at my end too but it wont be anytime soon ."

    Thanks,

    John

  • 0 in reply to John Hu
    TI__Guru 57390 points
    Hi John ,

    The VrefA actually sets the gate voltage of the switch . so VrefB at 3.3V or 5V should both not be relevant for the discussion. I say not relevant since both of 3.3V and 5V at VrefB should be sufficient enough to bias the switch right with VrefA at 1.2V .
    Please read the LSF EVM and app note I had attached in the previous post which further explains the working of the switch . there are FAQs at the end which might answer few questions about the usability as well.
    I know there are many questions on its ability and functioning based on various VrefA /VrefB /Vpullups combinations available in this case . I would try the combinations in the lab myself and provide you the scope shots at a later stage but my theory on this application would suggest it would not work ONLY because the Vddio varies up to 3.3V .