level-shifting I2C bus when VCC1 is +3.3V, and VCC2 is varying between +2.8V and +4.2

[kender]

Colleagues,

Is there a good method for level-translating the I2C in the following situation?

The I2C bus master and most of the slaves are powered from a +3.3V rail. One of the slave devices is powered from a different rail which can vary between +2.8V and +4.2V. Notice that this rail can be above or below the +3.3V rail.

I've noticed that the **broken link removed** requires that VREF1 < VREF2 .

Any suggestion, insight or reference is really appreciated!

Sincerely,
- Nick

 

[KlausST]

Hi,

there are ready to buy I2C level shifters.
--> Just go to your electronic part distributor an do a search.

But I assume whit the given supply voltage ranges there is a good chance you don´t need a level shifter at all.
--> Read the VIH, VIL, VImax specifications of all bus partners.

Klaus

 

[kender]

Klaus, do you think that I haven't checked existing I2C level translators ICs before posting my question here?

You have my permission to post something useful next time.

 

[KlausST]

Hi,

do you think that I haven't checked existing I2C level translators ICs before posting my question here?

I just read the informations you gave. You did not write this information.
Read through the other hundred thousands of posts in this forum and tell me who read which documents before he posted a question.
Next time when I am in doubt - I know whom expert I have to ask.

I simply wrote how I try to find a solution to a similar problem.

And for sure I give you the permission to use all informations of all the other replies to your question.

Klaus


Btw: Is your "thankful" habit your style to motivate other members to help you?

 

[d123]

Hi,

Bulky solution maybe: Could a variable gain ampifier be used, perhaps driving a BJT or MOSFET if needed?

 

[BradtheRad]

I was just about ready to post a schematic showing how a PNP transistor translates an incoming signal to the same amplitude as the new supply voltage.
Then I saw that a diode can do the job.
Output swing is to the supply rails within a fraction of a volt.

Simulation. Lefthand= 2.8v supply
Righthand=4.2v supply.



If you were to use a low-barrier (Schottky) diode, it would make output go even closer to supply rails.

 

[FvM]

do you think that I haven't checked existing I2C level translators ICs before posting my question here?

Might be the case. The first level translator that I arbitrarily selected in Digikey search tools seems already to work for up/down voltage translation: TI TCA9509.

 

[KlausST]

Hi,

Might be the case. The first level translator that I arbitrarily selected in Digikey search tools seems already to work for up/down voltage translation: TI TCA9509.

After the "thankful" post of the OP I did the same at farnell .. and came to the same result as you.

And I did some calculations with standard I2C levels and came to the result that even a "clean" passive solution is possible.

Klaus