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PCA9306: Supplying same voltage to VREF1 and VREF2

g.f.
Guru 15520 points
Part Number: PCA9306

Hi,

I have a question about PCA9306.

Can I supply same voltage against VREF1 and VREF2?
The level translation are not required and I'm want to supply 3.3V to VREF1 and VREF2.

In the following E2E post, TI engineer said
"this chip operates correctly if both input and output SCl/SDA are 3.3V.
In this case you can tie the VREF1 and VREF2, and the Enable pin to 3.3V (no 200kohm pull-up needed on VREF2)".
e2e.ti.com/.../495150

But in PCA9306 datasheet, it said VREF2 must be between VREF1+0.6V to 5.5V,
and there are no specification for supplying same voltage to VREF1 and VREF2.
 
Does TI guarantee to supply the same voltage to VREF1 and VREF2 of PCA9306?
If yes, are there a plan to update the datasheet about supplying same voltage?

best regards,
g.f.

  • 0
    TI__Guru 73871 points

    Hello g.f.,

    "Can I supply same voltage against VREF1 and VREF2?"


    -Yes, you are able to supply both Vref1 and Vref2 and operate safely. However, this will affect the reference voltage on the FETs on  the gates of the SDA/SCL line. This in turn will create a small delay when switching from high to low. At the speeds I2C typically runs at, this most likely won't be an issue.

    "But in PCA9306 datasheet, it said VREF2 must be between VREF1+0.6V to 5.5V,
    and there are no specification for supplying same voltage to VREF1 and VREF2."

    -This is because the part was originally made to be a voltage translator for I2C and not intended for the use you have proposed above.

    "Does TI guarantee to supply the same voltage to VREF1 and VREF2 of PCA9306?
    If yes, are there a plan to update the datasheet about supplying same voltage?"
    -No, as this part was intended for voltage translation and the putting Vref1 and Vref2 at the same voltage will affect it's performance.

    I am curious, what application do you intend to use this device for if not translation.Are you using the Enable pin to isolate a second slave because two sensors share the same address? This information may provide more insight with helping future customers/support so if this is something you can disclose, I would love to hear it.

    -Bobby

  • 0 in reply to BOBBY
    Guru 15520 points
    Hi Bobby,

    Thank you for the reply.

    I understood that I'm able to supply same voltage to PCA9306 Vref1 and Vref2
    but this part is intended for voltage translation so that there are no gurantee for supplying same voltage.

    Actually this question was from my customer, and they are planning to use this PCA9306 as I2C line(SDA/SCL) buffer.
    So, may be I should introduce other product such as TCA980x for I2C buffer.

    best regards,
    g.f.
  • 0 in reply to g.f.
    TI__Guru 73871 points
    Hey g.f.

    The PCA9306 does not provide the ability to buffer the capacitance on the SDA/SCL lines. Any capacitance on side A will pass through and add to the capacitance on side B. I would not recommend the PCA9306 for buffering.

    Thanks,

    -Bobby
  • 0 in reply to BOBBY
    Guru 15520 points
    Hi Bobby,

    Thank you so much.
    I understood! I will introduce TCA980x to my customer.

    May I ask one more question?
    Is it allowed to connect SMbus to A side and I2C to B side of TCA980x ?

    Best regards,
    g.f.
  • 0 in reply to BOBBY
    Prodigy 70 points

    Hi Bobby, Here's a similar question, what if VREF1(3.3V) is higher than VREF2(1.8V). Can this chip still work properly? and do the level translation?
    I asked this question, because I just reversed the direction of this chip on the PCB design :(

  • 0 in reply to Jerry Zeng
    TI__Guru 73871 points

    Hey Jerry,

    A similar question was asked here, though your case is a bit more extreme version of it:

    The device will still pull up to Vref1/Vref2 but the transition from high to low will be greatly affected. In your case with that large of a difference in voltage, I don't believe the part will pull down fast enough to reach 0.3*Vref when your Vref2 side is pulls low and Vref1 tries to follow it. It may begin to pull low but when SDA is sampled during the middle of SCL, it may read high and receive the wrong data. Another issue may be the case of the SCL lines, to the device, it may only see a logic high the entire time because it may never reach 0.3*Vref1.

    "Can this chip still work properly? and do the level translation?"

    The chip may not transition fast enough and the device may read wrong data or SCL may look like it doesn't change to the device.

    The level translation will be from 3.3V to 1.8V, if that is what you wanted then yes but the device may not work as intended.

    Thanks,
    -Bobby

  • 0 in reply to g.f.
    TI__Mastermind 23406 points

    Hello G.F. 

    SMBus and I2C are both open drain architectures and bidirectional so yes you can use the TCA980x devices this way.  Keep in mind the TCA980x requires that you don't use pull up resistors on the B side.  Make sure you devices connected to the B side don't have internal pull ups.

    -Francis Houde

  • 0 in reply to BOBBY
    Prodigy 70 points
    Thank you very much for the answer, Bobby.
  • 0 in reply to fhoude
    Guru 15520 points
    Hi Francis,

    Thank you for the answer.
    I understood.

    best regards,
    g.f.