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Tool/software:
Hello -
I understand the UVLO startup requirements of the TCAN455x (>5.9V worst case to assure proper startup). Also, understand that you can 'bypass' VSUP by applying ~5.2V or greater to the VCCout terminal on the TCAN4550.
A couple related questions:
1) Can you also bypass the LDO by applying 5.2V or greater to the VCCfltr terminal on the TCAN 4551? And that way the CAN bus power will be derived from that terminal rather than VSUP?
2) Before Vio is applied, is this the proper figure to use for Isup? ~25-42uA?
3) Assuming Vio is applied, SPI active, and CAN bus with ~55 ohm loading, VSUP = 24V, VCCout/VCCfltr applied with external 5.2V, can you help us to estimate Isup? I don't see a figure or way to calculate that.
Thank you & Happy Thanksgiving,
Leon
Hi Leon,
Thank you for pointing out the e2e thread you are referencing. I had not seen that post, but I understand the premise of your question. However I take exception to the content of this post and respectfully do not agree that this is possible, or recommended. In my opinion, this post should be taken down to avoid future confusion.
The TCAN455x devices were designed to be completely supplied from the VSUP pin with exception of the clock circuit (OSC1/2) which uses the VIO supply pin. The device uses the 5V LDO to step down the VSUP voltage and then additional LDOs step the 5V down to additional internal voltage rails to operate the digital core and to be used for the nWKRQ pin etc. This 5V LDO can also be used as a supply for other circuitry for through the VCCOUT pin in the TCAN4550 version of the device.
This 5V LDO can't be disabled and the under-voltage threshold for the VSUP pin is set to ensure there is enough overhead voltage to create a regulated 5V from this LDO.
The device uses this internal 5V LDO as part of it's Power On Reset (POR) and mode control functions.
The Device implements a Hardware or Software Reset by disabling this internal 5V LDO which in turn disables the other internal digital LDOs creating a Power On Reset event. Forcing the device to sink current from an external 5.2V source could prevent the device from needed Reset Events.
The Internal 5V LDO also is enabled/disabled for mode changes, particularly when entering and exiting Sleep Mode. Forcing the device to sink current from an external 5.2V source could prevent the device from entering the low power Sleep Mode, and have other mode changes.
The device uses the 5V LDO to create the 2.5V bias voltage for the CAN signals. Bypassing this LDO with a 5.2V source will also have impacts to the CAN bus voltage levels. Guaranteed current limits in a fault condition would also be bypassed and there could be large current flow through the external 5.2V supply.
All of my comments are just from a theoretical consideration of your question and the possible problems with this request. I do not have specific test data from trying to bypass the internal 5V LDO with and external supply, but these are some reasons why I think this is problematic. I also do know know what type of possible leakage paths could exist by trying to back feed 5.2V when the device is trying to disable the internal 5V LDO and whether there could be any threat of damage to the device.
I hope my concerns with this approach are clear. But specifically to your question about to calculate or estimate the Isup current, I don't have any information to share.
Most of the device operates off of one of the internal LDOs. Because the 5V LDO is the main core LDO, likely most of the device current could be estimated to flow through the 5V supply. Some circuits related to the INH, WAKE, and nWKRQ pins operate directly off of the VSUP. The CAN transceiver can be used as a wake-up source so it also has some circuitry that operates off of VSUP in Sleep Mode. However, I don't know how to isolate the amount of current that would flow through VSUP instead of an external supply. If you would still like to pursue this, I would suggest a direct measurement would be required to determine this.
Regards,
Jonathan
