Level shifting from +5V to 3.3V with 74ALVC164245

[caius]

Hi all,
I have the need to translate some (not bidirectional) +5V signals (from an old TTL hardware) to 3.3V (MAX10 FPGA), I chose a 74ALVC164245 bus transceriver.Are the attached schematics correct?I have some doubts about the VCC(B) power supply pins (if they must tied to 3.3V like I did or +5V instead).Thanks in advance.

[Psi]

If you want to have 5V logic on the B side you need 5V on the B side VCC.

[MarkF]

If you only need 6 or less lines, the CD74HC4050 Hex Buffer is a simpler and smaller chip.
It has high voltage tolerant input protection.  You just tie Vcc to 3.3V to drop the levels.

   

[Psi]

If it's non-critical a simple voltage divider would probably work fine. but the fact you're working with a FPGA means you should probably do it properly with an IC.

[Infraviolet]

A BSS138 mosfet circuit with pullups can also work for 5V - 3V3 level shifting, both for translating down a 5V signal to 3V3 and magnifying up a 3V3 to feed to 5V circuits. But would probably lead to a larger part count (mosfet + two resistors, per channel) than using the single IC methods.

[SiliconWizard]

VCCB is 3V to 5.5V while VCCA is 2.3V to 3.6V.
So in your case, you must use B as the "input", 5V side, and power VCCB with 5V, and use A as the output, 3.3V side, and power VCCA with 3.3V.

As you did, if you power VCCB with 3.3V, you're going to potentially damage the chip as the input voltage on the B side is spec'ed at VCCB max. It's in the datasheet.

For a similar use case as yours, I've used the SN74LVC16245A instead, which requires only one voltage rail and is 5V-tolerant. So you'd power its VDD with 3.3V and it will tolerate 5V inputs with no problem. (Of course, it would not translate from 3.3V to 5V, but that's not what you need here as far as I can tell.) I recommend using it, especially if you don't have a 5V rail available on your board.

[c64]

For 6 signals only, can use hex inverters from 74LVC (5v tolerant) family

[Buriedcode]

For 6 signals only, can use hex inverters from 74LVC (5v tolerant) family

And LCX.  The VHC family are also 5v tolerant but can also work up to 5.5V.  The exception is the 74VHC245 - only the control signals are 5v tolerant, the IO aren't (since its bidirectional).  For unidrectional 5v -> 2-3.3V, I go with the LCX/LVC.