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TCA9803: Connecting VCCB to EN Pin

Matt Mottier
Prodigy 210 points
Part Number: TCA9803
Other Parts Discussed in Thread: SN74LVC1G07

My I2C controller will directly connect to about eight I2C devices.  I plan on adding TCA9803 devices with port A connected to the I2C controller side and the four level shifted/isolated devices would be on port B.  I would like to have VCCB controlling the EN pin, such that the device is in lowest power state when the device VCCB is not up yet.  I know EN has internal pull up to VCCA.  Can I tie VCCB directly to the EN pin or should I use an open-drain, non-inverting buffer between VCCB and EN.

As a side note, all TCA9803 devices VCCA will be 3.3V.  The VCCB side of each device varied from 1.8V to 3.0V to 3.3V (isolation reasons).  Is there any issue with this?  From what the spec said, I woudl say no, but just double checking.

Thanks,

Matt

  • +1
    TI__Mastermind 29906 points

    Hey Matt,

    This configuration would be OK. The recommended operating range of VCCA/ VCCB goes up to 3.6V, and similarly on the EN pin as well. However, note that the EN pin is not shorted internally to VCCA, so the open drain driver is not necessary in between VCCB and EN. 

    Your understanding is also correct- there isn't any biasing requirements for the power sources, so VCCA can be greater than, equal to, or less than VCCB (as long as it is within the recommended operating conditions).

    Regards,

    Jack 

  • +1
    Guru 292490 points

    The EN pin is referenced to VCCA, so to be able to reach its VIH, the voltage at the EN pin must be at least 0.7 × VIH = 2.31 V. So for the 1.8 V signal, you indeed need an open-drain driver (e.g., SN74LVC1G07).

    The internal pull-up is about 250 kΩ. Without an open-drain buffer, a small current might flow into VCCB. This might or might not be a problem in your circuit.