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TCA9517: Behavior of the IC if VCCB=0,VCCA=3.3V & EN is in floating/0

Blessing Selvakumar
Prodigy 115 points
Part Number: TCA9517
Other Parts Discussed in Thread: TCA9617B

Tool/software:

Dear team,

We are using TCA9517DR in our design.As per our design, during initial power ON the VCCB will be 0, VCCA will be 3.3V and EN will be floating/0 in this condition we are accessing a I2C device where the I2C communication is not through TCA9517DR but the bus will be connected only to the SDAA & SCLA of TCA9517DR.In this condition we are unable to access the I2C device.

While probing it is found that the SCLA Input high level voltage is 1.8V instead of 3.3V. The SCLA input high level voltage issue is solved when we provide supply to VCCB and enable pin(3.3V).

Is it mandatory to provide supply for both VCCA and VCCB and define the EN pin even if the communication is not through TCA9517DR but any one of the SCL and SDA pins are connected to the operating I2C bus?

Kindly clarify the behavior of the chip under this condition.

Regards,
Blessing.S

  • 0
    TI__Mastermind 46703 points

    Hi Blessing,

    The TCA9517 supports high impedance on I2C pins when the device is powered off. Since one supply is still up, the device is technically not being used correctly according to the recommended operating conditions, i.e. leakage might be expected. 

    I did see a case in another thread:

    https://e2e.ti.com/support/interface-group/interface/f/interface-forum/600849/tca9517-tca9517-leakage-current-from-vcca-to-vccb

    The EN pin is floating / 0, any chance we can place a stronger pull-down to ensure that EN is biased to GND? It looks like when EN = GND, the leakage tends to go away. A floating EN pin is not idea in system design since this pin can float to any voltage. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 115 points

    Hello Tyler,

    As mentioned in the another thread we too observed the 0.8V issue while partially powering up the TCA9517.But in our case we had also lowered the enable pin to 0V,Even-though we still observe the SCL bus connected to SCLA of TCA9517 is unable to raise beyond 1.8V.

    The issue is cleared only when the VCCB is also powered to 3.3V.Is that mandatory to power up both VCCA and VCCB even the IC is not involved in communication?

    Regards,
    Blessing.S

  • 0 in reply to Blessing Selvakumar
    TI__Mastermind 46703 points

    Hi Blessing,

    With only one power supply up, the device is being used according to the recommended operating condition in the datasheet which requires VCCB to be powered as well. 

    Powered-off impedance implies when the device is completely powered off that no leakage will translate to the other side. With one supply up, powered-off impedance seems to not apply any more. 

    According to the thread, pulling EN = 0V, should have cut off the leakage. So I am curious to know why your case is different. 

    The only things I can think of doing now is to make changes to power sequencing, or try the TCA9617B which allows for VCCA > VCCB without leakage problem. It is p2p and same package. 

    Regards,

    Tyler