Other Parts Discussed in Thread: DRV8350, LM5009A, LM5008A, LM5008
if the Input voltage on the Vin is above 44V the switching node shows an unexpected behaviour. Can you explain it?
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Hi Michael
I did the simulation with the 5008A; without any errors
https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=08069858D45EF611
Why schould I do it with the 5009A?
Hi Rolf,
I meant to write 5008A sorry. I am not that familiar with eh 5008A so I will refer your question to the relevant expert.
Regards,
Michael W.
Rolf,
I will forward your post to the team handling the LM5008A. They will be able to debug this with you.
Regards,
-Adam
You are seeing ringing on switch node of the converter.
That is to be expected, especially with leaded package.
Parastics of package present a RLC network which resonate at the frequency you are seeing.
I would like to make sure you are measuring the Sw node correctly with low inductance measurement.
The ringing amplitude looks high if I am interpreted the scope capture correctly.
Hi
I measured the SW node across the diode with a cal tip and parallel with a selfmade tip. guess you can't measure any better
Thank you for sharing.
You are major concern is with switch node ringing?
Can you share the layout highlighting the location of the input capacitors to the device?
What changes in the second waveform (in comparison to the first) you shared to demonstrate SW node dipping?
Can you replicate this waveform again?
Yes, major concern is ringing, emc emissions, and an unwanted switching behaviour
Layout see attached file
change from case 1 to case 2 was a higher input voltage. from 42V to 48V0116.DRV8350_TI_Support.docx
Let's first focus on schematic entry. Thanks for sharing schematic.
How did you come up with your ripple injection circuit?
Here is good resource for ripple injection.
Hi Marshall
I/We came up with the ripple injection circuit by a) using the LM5008 QickStartCalculator (http://www.ti.com/tool/LM5008QUICK-CALC
Rolf,
I do not suspect that is the root cause but I would still double check your ripple injection as Marshall suggested.
This looks like an issue with the boot strap circuit, BST turns on the high-side FET (turns SW on) and if the boot charge is not available the FET will not turn on correctly. I think the BST capacitor vias are parasitic inductances into the Boot-SW path, and interfering with the boot voltage that keeps SW on.
The placement you have of the boot cap is not great. In the regular LM5008A, CBST is placed directly next to SW and BST pins, but I can see in this DRV device its not practical. The VCC cap placement is also poor.
Can you run these test at 48VIN to test my theory?
1) Probe VIN-GND at the device pins to confirm the VIN voltage is steady 48V during this condition.
If that is good then,
2)Probe BOOT-SW (Gnd of probe on SW and disconnect other probes!) and see if BOOT-SW voltage is solid 7V.
Let me know!
-Orlando