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LSF0108: Using Two LSF0108s to downconvert then upconvert I2C and SPI

Andrew Elliott
Prodigy 30 points
Part Number: LSF0108
Other Parts Discussed in Thread: TXU0304, LSF0102, TCA9517-Q1

Hello,

I am currently designing a test board that allows a user to hook up a MSP432's I2C and SPI lines to a device under test (DUT) at a different voltage level selected by the user. The best configuration we found seems to be using two of the LSF0108s to downconvert the 3.3V lines of the MSP432 to 1.2V, which should then allow a user to upconvert to whatever to voltage between 1.2V and 5V they want for the DUT. It seems to work in theory for I2C, but we are not confident that this should work for the SPI lines which are push-pull.

Would this usage of the LSF0108 work efficiently (up to 32 MHz) for I2C and SPI? Or will it only work for I2C?

  • +1
    Guru 317060 points

    The LSF is a passive switch and requires pull-up resistors that are strong enough for the desired speed. The LSF supports any pull-up voltage higher than Vref_A on all A/B pins, so you can simply use a single LSF.

    SPI signals are unidirectional, so a unidirectional translator like the TXU0304 would be more appropriate, and has no speed problems. For I²C, you could use the LSF0102.

    With a 5 V DUT, the total I²C pull-up current in this circuit is 6.5 mA, which exceeds the 3 mA limit of the I²C specification. If you want to isolate the two buses, use a buffered translator like the TCA9517-Q1.