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LSF0108: Schematic Review

Jim CHEN
Expert 4795 points
Part Number: LSF0108
Other Parts Discussed in Thread: SN74LV4T125, , SN74AVC2T244

Dear Team,

For new platform application, the SPI bus level of SoC is 1.05V, so need a translator to do level shift to 3.3V for SPI Flash.

Currently we design in LSF0108PWR translator which no direction.

Is there any recommendation or notice for SPI application?

Does anyone can help to review the below schematic?

Thank you.

  • 0
    TI__Guru* 96862 points
    Hi Jim,
    Short answer - please see page 17 of the datasheet for a full schematic example.

    Specifics to the above schematic:
    Pin 19 (Vref_B) in the above schematic needs to be connected directly to pin 20 (EN) with the 200kohm resistor separating _both_ pins from the P3V3_STBY rail. Only one external capacitor is required.

    Pull-up resistors are required on the 3.3V side for all up-translation lines.

    It is highly recommended to avoid series resistors and pull-down resistors with the LSF series of translators.

    *****

    In this case, you might want to consider a different translator. SPI is typically unidirectional, so you should be able to use a translating buffer (like SN74LV4T125) rather than an auto-bidirectional translator like the LSF0108.
  • 0 in reply to Emrys Maier
    TI__Expert 4795 points
    Hi Sir,

    Thanks for the feedback. So what is the reason you do not recommend to use the auto-bidirectional translator if the bus is unidirectional?
    Since their lower voltage is 1.05V. It seems like SN74LV4T125 cannot support such voltage, right?
    I search on the TI website and found SN74AVC2T244 which can support lowest 0.9V. Is that ok?

    Thank you.
  • 0 in reply to Jim CHEN
    TI__Guru* 96862 points
    There are a lot of trade-offs associated with using an auto-bidirectional translator that are not necessary for a unidirectional translation.

    I did miss the low voltage requirement here - I thought that was 1.5V to 3.3V.

    If you want to get 1V to 3.3V, there are only a couple options.

    LSF can do it -- I am going to be releasing a full video series in about a week that will explain exactly how LSF works and should clarify a lot. Hopefully my previous comments are also helpful.

    You could also use GTL2014. This is a 4-bit direction controlled translator - typically used for unidirectional translation from low voltage processors to peripherals. Your application would require 2 devices (1 for up-translation and 1 for down-translation).