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PCA9306: Question for PCA9306 specifications

Jacky Wang(QD)
Genius 11530 points
Part Number: PCA9306
Other Parts Discussed in Thread: TCA9406

Hi Experts,

Our customer is evaluating the PCA9306 and having below questions about this part, can you help to answer them? Thanks a lot!

1. In the datasheet overview section, we mentioned the I2C bus capacitance limit of 400pF, but for PCA9306, we can keep this limitation on both side of this device, that means we can isolate the capacitance on I2C bus. Can you help to explain some details reason for this? It will be better that we can provide an internal equivalent circuit.

2. Do we have the internal I/O circuit structure or block diagram for SDA1/SCL1, SDA2/SCL2?

3. There is no detail info for VIH and VIL specification for SDA1/SCL1, SDA2/SCL2 in our datasheet, can you help to check for them?

  • 0
    TI__Guru 53555 points

    Hi Jacky,

    1. In the datasheet overview section, we mentioned the I2C bus capacitance limit of 400pF, but for PCA9306, we can keep this limitation on both side of this device, that means we can isolate the capacitance on I2C bus.
    This device does not isolate bus capacitance. As stated in the highlighted section, the system designer must ensure the sum of both sides of this device stays under 400pF. The reason for this is due to the pass-FET design of this device. When propagating a LOW signal, PCA9306 effectively connects boths sides together allowing the side that is pulling low to pull the other side low as well. During this time, both sides of the bus are effectively the same and thus the total bus capacitance is the sum of both sides. Let me know if this makes sense or if you'd like further explanation.

    2. Do we have the internal I/O circuit structure or block diagram for SDA1/SCL1, SDA2/SCL2?
    These I/Os operate using a pass-FET architecture. Practically this can be thought of as a switch which connects both sides of the device when one side falls below an internal threshold voltage created by the device. For reference, TCA9406 operates using the same pass-FET method but also includes integrated pull-ups and RTAs. 

    3. There is no detail info for VIH and VIL specification for SDA1/SCL1, SDA2/SCL2 in our datasheet, can you help to check for them?
    Because the internal switch on the SDA/SCL pins references the internal bias voltage, typical VIH/VIL specs are tricky to define. In short: the device will begin to propagate a low (VIL) when either side of the device falls below the internally generated bias voltage. This value will be very close to Vcc1, so once side-1 begins to fall, it will connect side-2 to allow it to be pulled down as well. However, because PCA9306 is only connecting the two sides and is not actively pulling either side to ground, side-2 will only fall to the same voltage as side-1. The issue with defining a VIL spec for this side means that the resulting VOL spec will always be higher. Instead, we define the Ron characteristics of the switch so that the difference between input and resulting output voltage can be calculated based on the current draw of the individual system. 

    I realize these answers are not as straight forward as you may have liked. The pass-FET architecture of PCA9306 means it's tricky to specify parameters typically associated with I2C buffers or similar devices. Please let me know if you'd like further clarification on any of these points.

    Regards,
    Eric

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