TCAN4550-Q1: Internal LDO doesn't start correctly with Vccout loaded after power on.

Hi Martin,

Thanks for the scope shot and detailed description of the issue. My initial thoughts is the the inrush current needed to charge the bulk capacitance as well as supply the other ICs may be too much for TCAN4550's internal LDO during startup. Connecting the extra load after the capacitors are charged and the CAN controller is already stable may relieve this load by spreading out the inrush current (and the capacitor helping supply current rather than sinking current to charge initially. Perhaps a smaller bulk capacitor (<10uF) rather than a larger one would help relieve this strain during startup, though this may not be the final solution. 

Could you measure the supply voltage (Vsup) during this startup along with the LDO out? I'm curious if this voltage may dip during the inrush phase as well. If so, we may be able to address the issue from this side rather than on Vccout. 

Today is a holiday in the US, so we'll get back to you tomorrow with some more info. Let me know if you have any more thoughts or find out anything new in the meantime. 

Regards,
Eric Schott

Hi, I measured Vccout vs Vsup at startup. There are visible some Vdrops but not below operational level.

This is direct connection of Vccout to digital isolators VDD (with 10uF on Vccout according to datasheet)

This is connection of Vccout to digital isolators VDD via 12R serial resistor

There are no any excessive capacitive load, every isolator has only recomended 100nF ceramic cap at it's VDDA and VDDB. I already attached an 5V external LDO (IFX24401 was handy) to power isolators and it seems OK.

Hi Martin, 

I can't access that image hosting site from our internal network (I viewed the initial one off our network during the holiday). Is it possible to share the images directly? Images can be attached using the "insert" dropdown. 

It sounds like even with the limiting resistor, the inrush current during startup still exceeds the internal LDO's capabilities. The supplemental external LDO may be the easiest fix for this as it looks like you're already having success with it. I think the series resistor is a good avenue for another solution, though it seems like some more experimenting would need to be done to find a balance that will allow for inrush limitation and proper operation after startup. 

Let me know if there's anything else I can help with.

Regards,
Eric Schott

OK, I overlooked the upload option in insert image.

I made some further experiments. I loaded Vccout with that 12R to GND and it didn't pulsed, it just rise voltage to cca 2,9V still level (because of overcurrent). So it's interesting that same resistor connected to isolators VDD is not enough.

I started to increase this serial resistor and wondered that even with 1k2 it cannot startup normally. Then with 8k2 it rise to 5V as expected. So there must be some internal quirk that create some instability with capacitive load during startup. Again, when the load is connected later than Vsup it works fine at 5V without any noise, only the startup is the problem. I think some note should be added in the datasheet.

Hi Martin,

Thanks for reuploading the images. The dip in Vsup here looks significant. If you still have this setup available, could you zoom in to see how low it gets? I know you said it stays above the minimum limit, but I'm curious about the impact this rail may have on the LDO performance during startup. 

Regards,
Eric Schott

OK, I captured the 1st drop at power on and the second (no serial resistor). Vsup not going below 8.9V.

Well, I just received samples of LP2985 and wired it to the board. It works well, output voltage is smooth. So I modified my PCB design to include this LDO...