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TCAN1044V: Standby mode behavior with VIO left floating but STB is supplied with 3.3V

Abhishekh Kumar Prasad
Prodigy 40 points

Part Number: TCAN1044V
Other Parts Discussed in Thread: TCAN1044A-Q1

Tool/software:

Hello TI Team,

I am using the TCAN1044AVDRQ1 and would like to know the supply current on VCC behavior under following conditions:

  1. VCC = 5V, VIO=3.3V
  2. STB is changed from 0V to 3.3V.
  3. VIO is cut and left floating (effectively 0V).

I have noticed that if MCU is shut, it will source from the CANRX/CANTX pins (approx. 20mA), and I want to avoid this. So, during standby, if VIO is left floating, will the CAN IC be standby mode?

Regards,

Abhishekh

  • +1
    TI__Guru 64727 points

    Hi Abhishekh,

    Please see section 8.3.6 of the data sheet. 

    The device will be in a protected state and the data sheet section recommends not to send messages during the state.

     Undervoltage on VIO forces an unpowered, high-impedance state I.e, essentially off, and waiting for VIO (or VCC) to rise back above the valid range.


    Furthermore, camped I/O pins will still source current into the floating VIO rail I    .e, the external TXD/RXD pins still have ESD-clamp diodes back up to VIO. If you hold STB or TXD at 3.3 V (from your MCU) while VIO is at 0 V, current will flow through those clamp diodes into the VIO node. You have already measured that this is on the order of 20 mA, exactly what you would typically expect from a 3.3 V driven I/O clamped down through a diode.


    True standby mode requires VIO present and STB = VIO.

    The data sheet’s low-power standby spec (ICC_STBY) is only guaranteed when both STB and TXD are driven to the valid I/O voltage (VIO) and would recommend VIO to a defined voltage (preferably to the same domain as your MCU’s I/O when its off, e.g. 0 V). Hence, the clamp diodes have no forward bias or switch STB and TXD to high-impedance (tri-state) before cutting VIO, so there is no 3.3 V on those pins when VIO goes away.


    If you must leave STB or TXD driven, add a series resistor (e.g. 1 k to 10 k) in each line to limit the clamp-diode current.

     As an alternative, consider the non-VIO version (TCAN1044A-Q1), thanks  

    Best Regards,

    Michael.