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PCA9306: Asking for the PCA9306 issue's comment

Part Number: PCA9306
Other Parts Discussed in Thread: ISO1540

HI Team,
custome is designing the PCA9306 on the I2C level shift from 1.8V to 3.3V.

The PCA9306 SCL/SDA output was connected to ISO1540.

However, the SDA pin's cannot go to low. ( VIL is too high around 747mV)

Could you please provide your comment and suggestions for this issues?

Here are the schematic, waveform and block diagram for reference in ISO1540 word file. the passport is "ti"

ISO1540 ISSUE.zip


Thanks,
SHH

  • Hi SHH,

    thank you for reaching out! We've been looking into your problem and I think I get your point.

    First of all, I would like to ask, when do you specify VIL= 747mV, do you actually mean VOL of the ISO1540? As a matter of fact by looking into your waveform I can see that the direction of the communication is happening from the ISO to the PCA and that the VOL of the ISO1540 goes to 0.7V. This is a normal behavior of the device as you can verify on the datasheet in paragraph 6.9 VOL is between 650mV and 800mV. This is a limit of the device itself ISO1540 and it's within its specifications.
    So I believe the problem that you are encountering doesn't depend on the PCA9306 but on a physical limit of the ISO1540.

    What I can recommend is:
    - Find a different isolator device, always from the same family of the ISO which has a lower VOL so that the a low value can go through the PCA9306 and directly read by the master. In order to do so, I can suggest to post on the ISOLATION forum, they can recommend you a better device.:
    e2e.ti.com/.../


    -Decide to use a third device between the PCA9306 and the ISO1540; this in order to have some kind of a buffer which is able to lower even more the VOL coming from the ISO1540, and making it enter the spec of the VOL of the Master.

    Let me know if this helps and if you need help with anything else.
    Best regards,
    Adrian
  • Hey SHH,

    Just to clarify,

    "Could you please provide your comment and suggestions for this issues?"

    -The PCA9306 does not affect a transceiver's VoL by much because the Ron between side 1 and side 2 is very low. The high Vol you are seeing at 750mV looks like it is due to ISO1540's diode.

    When a low is seen on side 2, Side one will turn on the FET which will allow the diode to conduct. This generates the voltage you are seeing at 747mV (it's forward voltage).

    -Bobby