Other Parts Discussed in Thread: LMR36500
Tool/software:
I am designing a DC/DC converter to go from 30-60V input to 3.3V to supply about 20 mA to a microcontroller and some other chips. Efficiency is key, because it needs to run for a very long time from a battery.
I choose the LRM36503-Q1 family, and in particular the LMR36503MSCQRPERQ1. I now realize that a LMR36500P3RPE would have been a better choice, the next one will be so. Anyway, I already have it built with the LMR36503MSCQRPERQ1.
It works, but I only get 45% efficiency when I would expect something over 60%.
I tried to calculate with WEBENCH, but I get bad results. The calculation says that the switching frequency will be 8.74 kHz, running in PFM mode. I don’t think it will actually lower the frequency that much, indeed, I measured 54 kHz. Also the calculation expects 90.6% efficiency, a wee bit too optimistic I think in any case.
Here is my design: LINK TO WEBENCH
What would be the ideal choice the input and output caps and inductor for this application? Is the LMR36500P3RPE indeed the best choice, or is there something that would be even better?
Also, I get this voltage oscillation on the switch voltage. Why could that be? Is that a problem?
Thank you!