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PCA9306: PCA9306DCUR outputs voltage on VREF1

Nanjunda M
Expert 2485 points
Part Number: PCA9306

Hi,

I'm using PCA9306DCUR in my design for I2C level translation from 3.3V to 1.8V. The connection is as shown below.

With this circuit, even if I have not given the 1.8V supply, I'm observing a 1.8V on the VCC_1V8 voltage net.

Can anyone explain me, what may be the reason for this behavior.

Thanks & Regards,

Nanjunda M

  • 0
    TI__Guru 62910 points
    Hi Nanjunda,

    If the VREF1 side is left open then it is possible for some of the VREF2 voltage to couple through the device and show up on the VREF1 side - that is most likely what is happening here.

    One thing I noticed on your schematic is that VREF2 is pulled up directly to 3.3 V. We recommend tying VREF2 and EN together and then using a shared 200-kOhm resistor to pull both pins to the pull-up voltage (see Figure 8 in the datasheet). This resistor then helps to limit the current flowing into VREF2.

    Regards,
    Max
  • 0 in reply to Max Robertson
    Expert 2485 points
    Hi Max,

    Thanks for the response.
    1. What are the reasons for voltage coupling from VREF2 to VREF1 ? Is this common in all the devices or only on this device ?
    2. Why VREF2 to be shorted to EN and then to pull-up voltage through 200K resistor ? Why not individual connection to pull-up voltage ?

    Regards,
    Nanjunda M
  • 0 in reply to Nanjunda M
    TI__Genius 17225 points

    Hello Nanjunda,

    In your schematic the device is not correctly connected.  It should be connected as follows:

    We specifically call out that it should be connected this way in the datasheet.  The reason why isn't as obvious, but I will explain.  The Vref2 and EN pin setup the threshold voltage that will be used by the other pass fets.  It is setups by the connections and the 200k resistor allows for enough current to pass to allow biasing and it also limits current during turn on/power up.  You could experience damage is the 200 k resistor wasn't there.

    Let me know if that isn't clear.  We also have other posts on E2E that show how to connect the device and what happens if you don't.  Please try using the search tool at the top of the forum to do searches for these types of questions. 

    -Francis Houde

  • 0 in reply to fhoude
    Expert 2485 points
    Hi Francis,

    I do agree that, that was an error in the design.
    Thanks for the info.

    Regards,
    Nanjunda M
  • 0 in reply to Nanjunda M
    TI__Genius 17225 points
    Hello Nanjunda,
    In the future, if you want use to review schematics, lets us know. We are more than willing to review and make sure we ask the right questions and find errors to make sure you get first pass success.
    -Francis houde