SN74LVC2T45-Q1: Hardware review

Please note our MCU accepts only 1.8V logic level.

[Same answer in the isolators forum.]

The R34/R35 voltage divider does not generate a valid logic level. To set the B→A direction, drop R34; but you can connect DIR directly to GND.

Unidirectional downtranslation can be done with a buffer with overvoltage-tolerant inputs; you can replace U21 with a SN74LVC2G17-Q1.

CMOS inputs must not float; ground pins C and D of U22.

These three level shifters do not support isolated power supplies and require a common ground on both sides. And when you do connect all the grounds together, isolating the power supplies is pointless. If you do require isolation, then you must also isolate the three signals with a real isolator like the ISO7731-Q1. (It cannot go below 2.5 V or above 5 V, so you still need the level shifters.)

Hi Manuel,

Please find the attached updated design.

1070.GPIO's.pdf

1. The R34/R35 voltage divider does not generate a valid logic level. To set the B→A direction, drop R34; but you can connect DIR directly to GND.

Manjunath : Noted.

 2. Unidirectional downtranslation can be done with a buffer with overvoltage-tolerant inputs; you can replace U21 with a SN74LVC2G17-Q1.

Manjunath : "SN74LVC2G17-Q1" doesn't do voltage translation from 5V to 1.8V.

3. CMOS inputs must not float; ground pins C and D of U22.

Manjunath : Thanks Noted.

4. We are currently looking for Digital Isolators for our apllications.

i. We need to isolate 2 Digital inputs whose digital lines voltage will be 12V

ii. and One Digital output whose voltage levels is 5V

iii. Main MCU accepts only 1.8V level.

So, for Digital inputs, Considering from Outside world, first I need to regulate the 12V levels to 5V levels and then I need to Isolate it and then again should do the voltage translation to 1.8V. I'm I right here?

For Digital Output, I need a Isolation and then Voltage translator to 1.8V.

Please check the attached schematic and review the same.

Hi Manjunath,

For Question 2, yes that device can down translate 5 V to 1.8 V. For everything regarding isolation devices I will move this thread over to the respective product line.