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Original question:

TCAN1044A-Q1: Can the VCC power supply be removed in standby mode to save leakage?

TCAN1044A-Q1: How to use the low power mode.

TAKESHI KASAHARA
Prodigy 25 points
Part Number: TCAN1044A-Q1

Tool/software:

I found a discussion in another thread about reducing the standby current of the TCAN1044AV.

The suggested method involves supplying voltage only to Vio, while setting Vcc (+5V) to 0V.

To achieve this, we are using the TPS70950QDBVRQ1, a 5V LDO, and controlling its Enable pin to turn Vcc on and off.

However, we are encountering an issue:
When the CAN bus enters the dominant state, we want to trigger CAN Wakeup. The dominant state is detected by an external circuit, which then wakes up the MCU. After wakeup, we enable the LDO, supplying +5V to the TCAN1044AV.

At this point, for some reason, the CAN commands are not being received properly.

One possible cause we suspect is that although +5V is supplied to TCAN1044AV and STB is deactivated, the voltage ramp-up may be too slow, or the power-up time of TCAN1044AV may be too long, causing the beginning of the CAN command to be missed by the MCU.

For reference, a 3.3µF capacitor is connected to the +5V supply.

If anyone has insights into this issue, please share your knowledge.
Thank you!

  • 0
    TI__Guru 50886 points

    Hi Takeshi,

    Yes, your suspicion is correct as the device needs to fully power up and be ready for operation after 5 V is applied and before CAN frame starts I.e., MCU will not decode the message correctly if the start of frame is missed. You may confirm by observing the LDO output's 5 V rise time on an oscilloscope, observe the CAN bus state when 5 V is applied as it should be recessive briefly and re-initialize the CAN module in the MCU after wake-up. I.e.,

    • It would seem the TPS device has a soft-start feature which may cause a slow ramp-up of Vcc. The 3.3 uF on 5 V can further slow down the ramp with Vcc taking too long for the device to catch the first CAN frame. Would recommend to check theTPS startup time and consider a faster LDO if needed, while reducing the 3.3 uF to 0.1 uF.

    • Furthermore, if CAN bus is already dominant due to another node transmitting or wake-up logic holding it low for example, the device may not sync correctly when powered on. Would further recommend ensuring the CAN bus has an idle period and also ensuring external circuits release the CAN bus to recessive briefly after power-up, allowing the MCU to sync correctly.

    • Note that, some controllers require a reset or re-initialization when the transceiver is powered on after the MCU and a short delay may be added before enabling CAN communication. For example, you may wait 1-5 ms after enabling the LDO, before letting the MCU start communication, thanks.

    Best Regards,

    Michael.

  • 0 in reply to Michael Ikwuyum
    Prodigy 25 points

    Thank you for your answer.

    I plan to review the capacitor values and other factors to shorten the LDO startup time.
    Also, thank you for your guidance on the Wakeup sequence.
    It was very helpful.

    Best regards