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PCA9306: connected with LVTTL device

Ryuji
Guru 20090 points
Part Number: PCA9306

Hello,

In my customer using, the LVTTL device is connected at the SDA2 side and Open Drain device is connected at the SDA1 side.
LVTTL device VOH(min) is 2.4V and VOL(max) is 0.4V.
They are considering Figure 7 connection.
I have some questions in this conditions.

(1)
Can we use this device with this condition?

(2)
If the SDA2 input is High, the internal switch is off?
From the following post, is  the internal switch always ON when the EN input is high?
https://e2e.ti.com/support/interface/i2c/f/390/p/566841/2083016#2083016

I think that we couldn't level shift if the switch is always ON.
Could you please let me know the SW turned on/off threshold voltage at SDA1/SDA2/SCL1/SCL2?
I think that it depend on EN voltage but I couldn't found the information.
 

(3)
If the SDA1 side is high(I2C device open drain output), is the internal SW of PCA9306 turned off?

Best Regards,
Ryuji

  • 0
    TI__Guru 62910 points
    Hello Ryuji-san,

    The PCA9306's internal switch becomes conductive when the voltage on either side is low (so that the low level is propagated from one side to the other) but is high impedance otherwise (so that each side is pulled up to a high level by its respective pull-up resistance). This is how the device accomplishes voltage level translation.

    Because of this, the device can be used for either open drain or push-pull signals. However, for push-pull signals the communication can only occur in one direction - you couldn't have a push-pull output on one side and an open-drain output on the other side. When the open-drain driver activates and tries to pull the line low while the push-pull driver is outputting a high level, the two drivers would contend with one another, resulting in high currents and an unpredictable voltage level.

    I hope this answers your question, and please let me know if anything is unclear.

    Regards,
    Max Robertson
  • 0 in reply to Max Robertson
    Guru 20090 points

    Hello Max san,

    I appreciate your support.
    I understood that we couldn't use PCA9306 for level shifting between open drain(I2C) and push pull(LVTTL).
    Are there any idea to level shifting between open drain and push-pull?

    Also, I have the question about the following comment.

    "internal switch becomes conductive when the voltage on either side is low"
    -->Could you please let me know the threshold of the low?  I think that there is threshold voltage of internal SW on/off.

    Best Regards,
    Ryuji

  • 0 in reply to Ryuji
    TI__Guru 62910 points
    Ryuji-san,

    The issue I mentioned earlier (not being able to pull the line low when the push-pull output is driving high) is actual a general limitation, not something that would be specific to PCA9306. In this system, is it possible to disable the push-pull output so that it only acts as a high-impedance input when it needs to receive data? If so, then the PCA9306 could be used as long as a pull-up resistance is populated on the push-pull side (in order to pull the voltage high when the output is shut off and the switch is high impedance). If not, then I am afraid there is no good solution - two digital outputs generally should not be connected to the same net unless there is some way to ensure they will never drive opposite states simultaneously.

    To answer your question about the threshold - the PCA9306 is designed such that the point at which the switch resistance starts to decrease quickly is internally regulated to be equal to VREF1.

    Regards,
    Max