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TCAN4550-Q1: external load for Vccout

Part Number: TCAN4550-Q1

Hi Expert,

My customer is using TCAN4550-Q1 in their new PEPS platform. Right now, they are designing the schematic. I have two questions would like to check with you. 

  • Vccout of TCAN4550-Q1 have total available current of 125mA. I would like to check if customer can use Vccout as a MCU 5V power supply? The max current consumption of MCU is ~25mA. Is it feasible? 
    • If there have some CAN BUS shorted fault happen, the internal LDO will have current limit capability. Does Vccout still can keep 5V output when LDO current limit is trigger by CAN BUS shorted fault? 
  • Customer's system requirement: 'When CAN device enter sleep mode, customer can use INH pin to disable the EN of DCDC/LDO to turn off 3.3V. And when leave sleep mode, CAN device can wake up the MCU'. Due to the limited GPIO resource of MCU, so customer would like to save the GPIO resources as much as possible. 
    • Question is that, customer would like to NC NINT pin. So for the interrupt function, does customer can also monitor all the interrupt flag through the SPI? 

Thanks!

Ethan Wen

  • Hi Ethan,

    1.Yes you can use the LDO to power your MCU, but keep in mind when this device goes into sleep mode the LDO will be turned off and therefore your MCU will be turned off. For some users this is actually a good thing since you are conserving current consumption when your CAN transceiver goes into sleep mode. However, if this is not your intended use case then you should not use the LDO to power your MCU.

    2.Can you point out where you read that the CAN bus will limit the LDO current during the a CAN bus fault. I just want to make sure we are talking about the same part in the datasheet. The device does limit the amount of current that the CAN driver can short circuit during a dominant state because it has a dominant state timeout. So the device can only drive dominant for so long. This lowers your average short circuit current.

    3.Yes, the device has the ability to do exactly what your customer is describing. When the device receives a wake event on the CAN bus or the Wake pin it will automatically enter standby mode. There the INH pin will go high and your LDO (VCCOUT) will be turned on.

    4.That is also correct, you do not have to connect the nINT pin. This may make your coding a little more tricky since you will have to constantly check the interrupt registers to see when you have a new interrupt. In practice this may seem easy but actually realizing that code may become difficult. Every time a new message comes in through the MCAN you will have to one read the interrupt to know a new message has come in and then you will have to read it from the RXFIFO/Buffer. If multiple messages have come in you will also have to read the status register to find out how many new messages have come in in that time so that you can read them all. We detail all of this in the software users guide for the TCAN4550.

    Best,

    Chris

  • Hi Chris,

    Thanks for your detailed reply. 

    2.Can you point out where you read that the CAN bus will limit the LDO current during the a CAN bus fault. I just want to make sure we are talking about the same part in the datasheet. The device does limit the amount of current that the CAN driver can short circuit during a dominant state because it has a dominant state timeout. So the device can only drive dominant for so long. This lowers your average short circuit current.

    Sorry for the confusion, we talked TCAN4550-Q1 here. I mean before the dominant state timeout, when the short circuit on BUS happens, the internal LDO current will be limited in 100mA. If I understand correctly, 5V will be pulled lower. If decrease too low, it will exceed the operating range of MCU power supply. I would like to check with you if 5V output will be affected during BUS short circuit case, to make sure the MCU can work normally even in this case. 

    1.Yes you can use the LDO to power your MCU, but keep in mind when this device goes into sleep mode the LDO will be turned off and therefore your MCU will be turned off. For some users this is actually a good thing since you are conserving current consumption when your CAN transceiver goes into sleep mode. However, if this is not your intended use case then you should not use the LDO to power your MCU.

    Another question, if customer use the LDO to power MCU, how to evaluate the maximum current can be sourced from the internal LDO? 70mA? Or we need to consider the CAN BUS shorted circuit case?  

    Thanks!

    Ethan Wen

  • Hi Ethan,

    The VCCOUT current capabilities are separate from the CAN transceiver supply current, so the LDO output can remain stable during a CAN bus fault. It is still possible to draw the full 70mA out of VCCOUT during a CAN bus fault without the output voltage dropping below the specified dropout value. This current draw will most likely not be thermally sustainable (depending on board characteristics and ambient temperature), so the device relies on the dominant timeout and other CAN protocol safety mechanisms to prevent it from overheating during this potentially large current draw condition. 

    Regards,
    Eric Schott