Hi Tom,
I got my board running this afternoon with the DPAK package. I did not see any issues with a 4A load so we need to look closer at what the differences are between what we're testing. I also ran a PSPICE simulation this morning and didn't find any issues. You're right the difference in lead inductance should not make a difference when compared to the TO220 package. I'm really trying to rule everything out since this design should work from what I have looked at so far.
I'm starting to look very closely at the layout again. Nothing has jumped out me as a clear culprit of your issues yet, although I do see some minor differences in the ground return paths. I've attached the layout of the PMP4540 we have been comparing to for your reference. Please find it below.
(Please visit the site to view this file)
How do the analog circuitry components compare to the BOM of PMP4540? Such as the components connected to the VDD pin, the ISNS pin, COMP and RC. Are the caps a high enough voltage rating and are they a higher grade dielectric (X5R, X7R, etc)?
I'm going to try tweaking a few things on my board tomorrow.
Best Regards,
Anthony