I am designing a power supply around the LMR38020. The specifications that led to the selection of this part are:
- Input voltage range: 9VDC-36VDC (typical), 70VDC (absolute maximum peak during certain transient tests).
- Output voltage: 3.3VDC.
- Load: 200mA (typical), 300mA (used as worst-case during design calculations).
The design I arrived at is the following:
The switching inductor PN is TYA3015220M-10. Additionally there is an LC pi filter at the input:
I am obtaining the expected output voltage, the UVLO is working as intended, and noise measurements don't show anything concerning. However the efficiency of the regulator is extremely poor. The load at 3.3V was measured 192mA, with a 24V input measured 45mA -> 59% efficiency.
Attempting to verify the design against Webench, there are a couple odd behaviors of Webench that I'm fighting (trying to set the operating frequency to 300kHzm, every time I enter the value the site reverts it back to 200kHz, but it appears that efficiency should be much better - in the mid-high 80's at least. The Webench report indicates this, although as noted I cannot set it to the 300kHz operating frequency that is designed.
The layout is on a six-layer PCB, with all components on the top side (L1), and a solid ground plane underneath (L2). The inductor and bootstrap cap are close to the switching node and routed with a wide pour, the input capacitors are placed as close as possible to the input pin, and in general there are no departures from the general recommended power supply layout rules:
Compared to general rules and also the suggestions in the datasheet:
Can you please help me identify the issues that are leading to such a significant loss of efficiency and steps to correct them - help is much appreciated.