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SN74LV4T125: Level shift for UART

Roy Chou1
Genius 12635 points
Part Number: SN74LV4T125
Other Parts Discussed in Thread: CD40109B, SN74AHCT125

Hi Team,

For customer requirement : 3.3V->5V for UART.(4bit) I suggest our SN74LV4T125. And I have questions showing as below.

1. Whether our level shift has protection function to protect processor side, like current limit?

2. Whether we have 3.3V->12V level shift function?

3. Do you have any better suggestion for 3.3V->5V UART level shifter? (dont need auto-directional function)

Regards,

Roy

  • 0
    Guru 243910 points

    1. Which one is the processor side? CMOS inputs are high impedance; CMOS outputs typically have no reasonable limit, but you can add series resistors.

    2. For voltages above 5 V, the only choice are devices in the CD4000 family, e.g., CD40109B.

    3. The high-level voltage of TTL signals happens to be near 3.3 V, so you can use any buffer with TTL-compatible inputs. The SN74AHCT125 is not really much different from the SN74LV4T125, but cheaper.

  • 0 in reply to Clemens Ladisch
    TI__Genius 12635 points

    Hi Ladisch,

    1. A port may be UART ; B port may be processor. If the port A has abnormal behavior like high voltage or high current. What happened to the IC. Does the behavior damage the processor side?

    2. Thank you for your comments.

    3. Thank you for your comments.

    Roy

  • 0 in reply to Roy Chou1
    Guru 243910 points

    When an input voltage exceeds the absolute maximum ratings, the device might get damaged. A breakdown of the isolation between inputs and outputs is one possible failure mode, and this would allow the overvoltage to be seen by the processor.