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PCA9306: What would be a good approach to level-shifting I2C bus when VCC1 is +3.3V, and VCC2 is varying between +2.8V and +4.2V ?

Part Number: PCA9306
Other Parts Discussed in Thread: TCA9517A, TCA9617A

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 peripheral 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 PCA9306 requires that VREF1 < VREF2 .

Any suggestion, insight or reference is really appreciated!

Sincerely,
- Nick

  • Hey Nick,

    PCA9306 can be used even if Vref2>Vref1. The only two downsides I can think of are slower prop delays and foot room when trying to pass larger VoLs.

    Alternatively you could use TCA9517A and have B side face the fixed 3.3V side.

    Thanks,

    -Bobby

  • i Bobby,

    Thank you for the reply.

    > PCA9306 can be used even if Vref2 > Vref1.

    If I'm reading the datasheet correctly, the PCA9306 normally works when Vref2 > Vref1 .  The datasheet mentions Vref2 = Vref1 + Vth in several places.  At the same time, more broadly, Vcc2 > Vref2 = Vref1 + Vth > Vref1 = Vcc1 .  So, Vcc2 > Vcc1.  I'm trying to figure out if the PCA9306 will work with Vref1 ≥ Vref2 = Ven , or what do I need to do to make that work.

    By the way, I will not be trying to use the PCA9306 as an  externally controlled bus switch.

    I really like the PCA9306.  It has low quiescent currents.  I can make it work with varying supply rail, it should fulfill all my needs.

    > Alternatively you could use TCA9517A and have B side face the fixed 3.3V side.

    The TCA9517A or TCA9617A could work.  The thing that puts me off a little is their quiescent current (Iccb) in the mA range.

    Cheers,
    - Nick

  • Hey Nick,

    The PCA9306 will work when Vref2<Vref1 but will have a lower "ViL" (quotes becomes this is the only term I can think of to describe what I'm talking about, the device doesn't have a true ViL because it does not redrive the input signal). This is what I meant when I said the footroom is lower. Translation will still work but you may see the prop delays move by ~50ns.

    If you don't need to redrive the signal and only way the device for level translation then the best device to use would be PCA9306.

    -Bobby

  • Hi Bobby,

    Thank for the insight.  I appreciate it.

    Just for fun, I've put together a quick&dirty low fidelity Spice simulation: 4-terminal NMOS transistors, current limiting resistor for Vref2 and EN, pull-up resistors.  It works for both Vref2<Vref1 and Vref1<Vref2 .

    - Nick