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TCA9517A: Alternative with switchable internal pull-up

js
Prodigy 140 points
Part Number: TCA9517A
Other Parts Discussed in Thread: TCA9803, TCA9509

Hi,

I am looking for an I2C buffer alternative for TCA9517A, which has 

  1. internal pull-ups on one side which get disconnected or high impedance when the VCC is off
  2. no internal pull-ups on the other side
  3. powered-off high-impedance I2C bus pins
  4. preferably pin compatible with TCA9517A

Issue with TCA9517A in my application is that when the VCC is off, the I2C lines cannot be used by the other board because of the external pull-ups at TCA9517A at connected to VCC.

Does TI has such alternative?

Thanks

  • 0
    TI__Guru 54438 points

    Hi Js,

    I don't understand the problem with this statement. Do you have a block diagram? 

    js said:
    Issue with TCA9517A in my application is that when the VCC is off, the I2C lines cannot be used by the other board because of the external pull-ups at TCA9517A at connected to VCC.

    Are you saying that the external PU resistors are also connected to VCC? Thus there is a leakage path to GND, causing SDA/SCL to be connected to GND while TCA9517A is off? 

    Can you use something like TCA9803? It requires no external PU resistors on B-side. However my question still remains, how are you continuing communication on the powered off side without external PU resistors attached? I2C is open-drain. 

    Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 140 points

    Hi, 

    The block diagram looks like this:



    Device B is powering both VCCA and VCCB. The pull-ups on device B side cannot be removed.
    Unfortunately, device A doesn't provide power supply output and device A already has a master and a slave device in it with switchable pull-ups.

    If the device B is off, as you said, I2C lines get pulled to powered-off VCC, those sub-modules inside device A cannot communicate.

    Therefore, I a looking for a solution which has internal pull-ups on device A side and the IO lines also go to high impedance when the buffer is powered off.

    TCA9803 provides power-off high impedance but only on side A. On the side B, it says: There must be no external pull-up resistor on the B-side to contribute to IEXT-I.

    which is a problem because there are switchable pull-ups inside device A too.

  • 0 in reply to js
    TI__Mastermind 30180 points

    Hello JS,

    Please expect some delays with our response since Tyler is current out of office. Thank you for understanding. 

    Regards,

    Josh

  • +1 in reply to js
    TI__Guru 54438 points

    Hi Js,

    I don't think we have a proper device for this application. 

    If you supply VCCA=VCCB, even if the buffer provided internal PU resistors on one-side, there would still be a leakage path to GND. 

    Closest device we can offer if TCA9509. It requires external PU resistors on B-side, but has an internal current source on A-side that provides pull-up current without external PU resistors in place. A-side is required to have no PU resistors. The current source still pulls HIGH when the device is disabled. When VCCA=VCCB=GND, the current source on A_side will be disabled.

     Regards,

    Tyler

  • 0 in reply to Tyler Townsend
    Prodigy 140 points

    Sorry for the late follow-up.

    TCA9509 seems suitable as we already have external PUs on device B and we can disable the PUs on device A.
    But the specification not quite clear. Table 5.3 in datasheet specifies both VCCA and VCCB up to 5.5V, but table 5.5 says VCCA ≤ (VCCB – 1 V).
    So then, can it be used with VCCA = VCCB = 3.3V?

  • 0 in reply to js
    TI__Guru 54438 points

    Hi Js,

    While I have used the device on bench for VCCA = VCCB = 3.3V, the datasheet was not characterized for voltages at that level. The electrical characteristics were completed when VCCA <= (VCCB - 1 V). If you want the guarantee of the EC table, you would need to stay within those voltage ranges. 

    Regards,

    Tyler