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TCAN1145-Q1: unexpected over current flows out from RxD pin in the state of sleep mode.

Shunsuke Yamamoto
Expert 5436 points
Part Number: TCAN1145-Q1
Other Parts Discussed in Thread: TCAN1145EVM

Hi team,

When the RXD pin is shorted to GND in the state of sleep mode(Please refer to the red circle), the current of approximately 30mA flows out from RxD pin. My customer assumes that the current of approximately 80uA flows out from RxD pin, so what he is facing is not as he expected and wanted. Is his assumption correct or not?

Best regards,

Shunsuke Yamamoto

  • 0
    TI__Guru 69032 points

    Yamamoto-san,

    In the datasheet, you can see the expected leakage current and RXD pull-up values in sleep mode:

    Even if the path were only the pull-up value of 60k and VIO was 5V, shorting RXD to GND would at most result in 1mA. The reality is that in Sleep mode, the customer should only be seeing ~1uA, so 30mA is much higher than expected, and not what they should be seeing. 

    Is something else connected to RXD that could be drawing this much current in a short to GND condition? Has the customer tried disconnecting everything else from the device except what is needed for power and then shorting RXD to GND to see the current consumption? And can a schematic be shared so we can see everything that is connected to the TCAN1145-Q1 device?

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    TI__Expert 5436 points

    Hi Eric,

    Thank you for the reply. My customer mentioned that the rough schematic is the figure below. VIO is connected to 5V, VSUP is connected to 14V, RXD and GND are connected to GND. Other pins are not connected which means open.

    There are two procedures to measure the current, the one is the measurement in state of fail-safe mode, the another is the measurement in state of sleep mode. In both modes, my customer can observe that 30mA of the current flows from RXD to GND.

    - Measurement in state of fail-safe mode

    1. Apply 14V to VSUP pin.

    2.  Apply 5V to VIO pin.

    3. Measure the current on the node between RXD and GND.

    - Measurement in state of sleep mode

    1. Apply 14V to VSUP pin.

    2. Apply 5V to VIO pin.

    3. Make a transition from fail-safe mode to sleep mode by SPI communication.

    4. Measure the current on the node between RXD and GND.

    Additionally, my customer has done the same measurement on the EVM(TCAN1145EVM) and is able to observe the same result.

    Best regards,

    Shunsuke Yamamoto

  • 0 in reply to Shunsuke Yamamoto
    TI__Guru 50795 points

    Hi Shunsuke-san,

    When the TCAN1145 is in sleep mode, the RXD pin is used as a wake indicator. When Vio is supplied, the RXD pin will drive high when no wake event has been detected. When a wake event is triggered, the RXD pin will drive low to indicate the wake to the MCU. Because the RXD pin is never high impedance when Vio is supplied, the RXD pin should never be shorted to ground during this supply condition. 

    What is the purpose of shorting this transceiver output signal to ground during sleep mode? This is not expected by the transceiver as this digital signal is not overcurrent protected because it only expects to drive an MCU input pin. 

    Regards, 
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 5436 points

    Hi Eric,

    Thank you for the reply. I understand why the excessive current flows from RXD pin. The purpose of shorting the transceiver output signal to ground during sleep mode is that SPI communication and CAN DATA communication are connected to different MCU, and the status below happens. During this status, RXD on TCAN1145 is connected to ground in MCU because MCU reads unknown status of TCAN1145. 

    TCAN: ON

    MCU connected to SPI communication on TCAN1145: ON

    MCU connected to CAN DATA communication on TCAN1145: OFF 

    Best regards,

    Shunsuke Yamamoto

  • 0 in reply to Shunsuke Yamamoto
    TI__Expert 5436 points

    Hi Eric,

    May I ask you an additional question?

    Are there any ways to support what my customer would like to do?

    Best regards,

    Shunsuke Yamamoto

  • +1 in reply to Shunsuke Yamamoto
    TI__Guru 50795 points

    Hi Shunsake-san,

    If the RXD output from the transceiver needs to be MUXed with another signal, there must be a way to protect the pin from other signal driver sources. This can either be done by ensuring that the pin will be in a high impedance state based on the transceiver behavior (unpowering Vio), or there must be an external component such as a multiplexer to safely combine the output with another signal source. 

    Is is possible in the customer's current design to implement a circuit with a multiplexer for this purpose? Or do you think they would be more interested in controlling the pin state through the transceiver? Note that the second option may limit some transceiver functions during the modified state. 

    Regards,
    Eric Schott