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SN74LVC1G57: 3.3V to 5V Translation

Rob Gaddi
Prodigy 130 points
Part Number: SN74LVC1G57
Other Parts Discussed in Thread: SN74LV1T34, SN74AHCT1G125

The SN74LVC1G57, and a couple other parts like the 1G14, list the worst-case input VH as 2.7V at 4.5V and 3.3V at 5.5V, implying 3.0V at 5.0V VCC.

Most LVC datasheets specify the worst-case input VH as 0.7*VCC, which for 5V logic is 3.5V.  Others with Schmitt trigger inputs, like the 2G14, draw a line that implies 3.4V at 5V VCC, due apparently to substantially wider variation in hysteresis.

All this implies that, as compared to the 2G14, the 1G57 and 1G14 have a slightly differently biased front-end that means they are usable as 3.3V->5V level shifters whereas the 2G14 and other, non-Schmitt LVC parts, are not.  Is that accurate?  I wouldn't want to design around this, then find out the datasheet was in error.

  • +1
    TI__Mastermind 35218 points

    Rob,

    Yes I would say it's accurate that you can use the parts you listed as voltage translators from 3.3V to 5V given that the VT+ is 3.3V when he Vcc is 5V. However, there are easier ways. The 1G57 for example would require 3 inputs for a high output or 1 input and unused inputs, Why do it this way though when there are specific translators, such as the SN74LV1T34, that are made for translating voltage levels such as 3.3->5V?

    Rami

  • 0 in reply to Rami Mooti1
    Prodigy 130 points

    Logistics; I already need logic functions on this design, and the 1G57 is already a part in my production stockroom.  If I can use 1G57s for everything then it's one fewer line on my BOM and one fewer stocking bin.  The unused inputs can be strapped to rails, it's not like I need to load my signal down with them.

    The 1G57 is also a bit faster than the single-VCC translators like the 1T34; at 5V into 50pF it's 5.1 ns max, the 1T34 is 6.5 into only 30pF.  The 1T45 with the dual-VCC gets the speed back, but then I also need a local 3.3V regulator I otherwise wouldn't.  Sure it's pennies, but that's yet another line on the BOM.  I'll do it if I need it, but if I can safely skip it all the better.

  • 0 in reply to Rob Gaddi
    TI__Mastermind 35218 points

    Makes sense. This should work with the 1G57 but will depend a bit on your system and the V_OH of the 3.3V signal being used. A direct connection at the 3.3V input and a solid 5V supply should do the trick. You could even play it a bit safer and use a good 4.5V supply. 

  • 0 in reply to Rob Gaddi
    Guru 316690 points

    Please note that applying a 3.3 V signal to a 5 V LVC input will result in high cross currents, even with a Schmitt-trigger input (see ΔICC). If that is not acceptable, you must use a device with a TTL-compatible input (e.g., SN74AHCT1G125) or a level shifter.