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Original question:

SN3257-Q1: Avoiding a level shifter

SN3257-Q1: Use of Auto-Qualified Mux for Mux function and Voltage Shifting to selectively connect uC and hardware safety circuit, 3,3V input to 5V output

John Martin4
Prodigy 80 points
Part Number: SN3257-Q1
Other Parts Discussed in Thread: UCC21710, , SN74LVC1T45-Q1, SN74LVC1G3157-Q1, SN74LVC1G97-Q1, SN74CB3T3257, TXU0104-Q1

Hi,

We are looking at using SN3257-Q1 as a multiplexer and voltage level shifter. The input is from a microprocessor (TMS570) and a seperate hardware safety circuit (for FUSA) - the mux logic is used to determine which input line is passed through to a UCC21710 gate driver. The input parts operate at 3,3V whereas the gate driver is on a 5V rail.

In our design the input signals are 0-3,3V (low-high logic) - the outputs are to the gate driver UCC21710 which requires (we believe 5V input on the gate drive signal live - datasheet shows minimum of 70% of VCC (ie. 0.7x5V = 3,5V).

Is there a design adjustment we can use to satisfactorily lift the input signal from 3,3V all the way to 5V or at least adequate to robustly drive the IN+ line on the gate driver. 

In all respects we were very attracted to SN3257-Q1 as the datasheet implied that a 1.8V input could be output at 3,3V however we are not clear on this now.

Thanks,

John

  • 0
    Guru 281350 points

    "1.8 V logic" describes only the control inputs.

    I am not aware of any device that combines muxing and level shifting.

    I'd suggest to combine a mux like the SN74LVC1G97-Q1 or SN74LVC1G3157-Q1 with a separate level shifter like the SN74LVC1T45-Q1.

  • 0 in reply to Clemens Ladisch
    Prodigy 80 points

    Hi Clemens,

    Thanks for the quick response, very much appreciated.

    Q1: So are you saying regardless of the VCC the Mux is powered by, the voltage supplied on the output will only be the same as the input?

    Q2: Are you recommending alternative Mux hardware ie. SN74LVC1G97-Q1 or SN74LVC1G3157-Q1 instead of SN3257-Q1 (which we wanted for FUSA reasons) for a specific reasoning? Do these have an advantage not in SN3257?

    Q3: My concern was to avoid specifying too many components in a chain as I was trying to keep the overall FIT rate for the transmission chain to as low as practical. Can you recommend how to minimise the failure rate?

    Thanks,

    John 

  • 0 in reply to John Martin4
    TI__Genius 10974 points

    Hi John,

    So the SN3257-Q1 does not have any capability to level shift the inputs at all and is just a passive 2:1x4 channel switch (inputs ~= outputs). Unfortunately, we do not have any automotive qualified level shifting multiplexers available at the moment. However, we do have devices that multiplex and level shift. If an automotive qualification was not necessary, you could most certainly use the SN74CB3T3257 which is a 2:1x4 channel multiplexer and level translator (but to up translate, you would need to implement 5V pullup resistors on the outputs).

    For an automotive solution, you would need to use the SN3257-Q1 multiplexer in conjunction with an automotive qualified level shifter (such as the TXU0104-Q1). In regards to minimizing failure rate, the easiest way to do this would be reducing the operating temperature you are operating the part at, as a lot of the other factors have to do with the actual transistors/devices on the die itself and how many there are.

    Thanks!

    Bryan

  • 0 in reply to John Martin4
    Guru 281350 points

    2. These devices have a single channel. If you have multiple signals, the SN3257-Q1 makes more sense.