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SN74LVC3G17: Level Shifter

Brian Wang0928
Genius 12190 points
Part Number: SN74LVC3G17
Other Parts Discussed in Thread: SN74AHCT125, SN74LXC2T45, SN74LXC1T45, SN74LV1T34

Is SN74LVC3G17 able to do the below two scenario for level shifter application in MD sector?

VCC=5V, Input High Level=3.3V (I think this should be ok)

VCC=3,3V, Input High Level=5V (I'm not sure if this is ok or not?)

Regards

Brian

  • 0
    Guru 262920 points

    LVC inputs are overvoltage tolerant, so you can use it for shifting down. But the LVC switching thresholds do not allow 3.3 V signals with a 5 V supply; use a device wither lower thresholds (e.g., SN74AHCT125) for that. If you need Schmitt-trigger inputs, use the SN74LXC1T45/SN74LXC2T45.

  • 0 in reply to Clemens Ladisch
    TI__Genius 12190 points

    Hello Clemens,

    Thanks for your comments.

    1. It looks that the LVC is CMOS process so that the thresholds do NOT allow 3.3V signals with a 5V supply, correct? We need to use TTL process. 

    2. Customer would like to add pullup resistor externally as below circuitry with shifting down and up. I think it's also not able to achieve the level shifter function, correct? The reason is also because threshold of the VT+ could be over 3.3V level, correct?

    3. if above are not workable, Is there p2p device with TTL process to meet customer requirements?

    Thanks for your support.

    Brian W  

  • +1 in reply to Brian Wang0928
    TI__Guru* 96681 points

    Hi Brian,

    Brian Wang0928 said:
    It looks that the LVC is CMOS process so that the thresholds do NOT allow 3.3V signals with a 5V supply, correct? We need to use TTL process. 

    Yes, that is correct.

    They could also use a TTL-compatible CMOS device, such as the SN74AHCT125 that Clemens mentioned.

    The SN74LV1T34 is also designed with reduced input thresholds to handle a 3.3V to 5V conversion with a single supply. Using either this device or the AHCT family (TTL-compatible) will use more power than a dedicated dual-supply translator though.

    Brian Wang0928 said:
    Customer would like to add pullup resistor externally as below circuitry with shifting down and up. I think it's also not able to achieve the level shifter function, correct? The reason is also because threshold of the VT+ could be over 3.3V level, correct?

    That is correct - the addition of a pull-up resistor won't change the function of the device -- the input will still have the same thresholds, and the output will still be the same voltage. The buffer drives the line and will overpower the pull-up resistor.

    The best solution would be a dual supply voltage translator.

    I would recommend using SN74LXC1T45, which is a direction controlled dual supply voltage translator. The direction can be fixed by holding the DIR pin either high or low. low = B -> A direction, and high = A -> B direction.