TCAN4550 crashes when engine crank - Page 1
Forum section: electronics / projects

author: okw
author_rank: Frequent Contributor
timestamp: June 25, 2023, 12:54:22 pm
content: I have a motorcycle project where I interface the diagnostics port (CAN bus) with a TCAN4550. Every time I crank the engine, the TCAN4550 dies and has to be reset. At the moment I just reset the chip if it doesn't respond within a given time, but I would like to try to correct the problem instead.Would this be the dip in 12V supply or a large peak (negative or positive)?I have some input protection (tvs, reverse polarity (mosfet), varistor), along with all the recommended capacitors in the datasheet.How would I prevent this from happening?

author: PwrElectronics
author_rank: Regular Contributor
timestamp: June 25, 2023, 08:06:33 pm
content: It could well be the 12v battery on the motorcycle is dipping to low voltages at crank. I work in a related field to automotive (heavy duty construction and farm) and those cold cranking voltage waveforms specs can sometimes be as low as about 4.5V with a AC ripple riding on them.For things like the engine fuel injection computer, they will have a boost converter to raise the voltage to the controller above all the logic function needs and also to power the injectors through the dip.A quick look at the TCAN4550 spec shows 6V as minimum and that could well be the issue.If you can, have a blocking diode (sounds like you already have one for reverse battery). But, you will need enough bulk capacitance after that to ride out the voltage dip.If you can get a scope on the voltage to your circuit to assess the issue, that would be best.

author: Scrts
author_rank: Frequent Contributor
timestamp: June 26, 2023, 03:15:19 pm
content: I agree with the above - the voltage might be dropping and you might need to use a buck-boost power supply at the input. What do you use for VBatt->5V power supply?

author: coromonadalix
author_rank: Super Contributor
timestamp: June 26, 2023, 05:10:09 pm
content: maybe some big capacitor who could act as an battery for some seconds ??

author: okw
author_rank: Frequent Contributor
timestamp: June 27, 2023, 08:02:31 am
content: It's a very tight design so I have to avoid boost converter / big >25-50V caps / etc.Just for test, we've added a two 47uF caps (but think they will be too small to cope with the drop).It's not critical if the CAN bus is unavailable during cranking, as we are just reading motor data (not controlling any injectors, etc). But it would be nice to have it corrected in HW, rather than resetting in SW.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 27, 2023, 08:06:28 am
content: Well, the supply voltage minimum is specified at 6V so this couldn't even theoretically work during cranking except by luck. It's not a bug, it's a feature. You just need to choose whether you need a system that is alive during cranking and then design a power supply to cater for that case, or maybe you can live with a system resetting during/after the crank. There is nothing "incorrect" in the latter approach.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 27, 2023, 08:21:44 am
content: There is a standard, iirc ISO 7637 that contains the voltage test profile where your device need to main functionality in.Many vendors show this in their datasheet for "cold crank voltage" booster chipsets.Eg:https://www.monolithicpower.com/pre-booster-for-automotive-cold-crank-conditionshttps://www.ti.com/lit/an/snva741/snva741.pdfIn short, you need to be able to maintain functionality or not disturb other devices, for 20 seconds at 6V for example.This can be difficult depending on what function your device has. I know some of this from railway, but we've settled for a stopping main functionwithout loss of comms during such event followed by a hot restart. Eg: your infotainment disables the speakers/screen while cranking but resumes function quite fast.How to fix this? Big capacitors, boosters.You could only use booster for the CAN tranceiver if that's all that is important.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 27, 2023, 08:26:44 am
content: Standard electrolytic capacitors do not have enough energy density to supply power-hungry stuff such as CAN transceivers for the timescales (many seconds) needed, you would need a huge box full of capacitors. A boost converter on the other hand is orders of magnitude cheaper and smaller.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 27, 2023, 11:28:42 am
content: It doesn't say is needs to remain transmitting. Just not entering fault state. So put it in standby mode?