I created an LED driver board based on PAM2863ECR http://www.diodes.com/_files/datasheets/PAM2863.pdf
The board includes three identical driver circuits to drive three LEDs. I'm not able to wire the three LEDs in series in this application. There is also a regulator and switch to adjust intensity. The board is typically powered from 6 NiMH cells, so the voltage is normally 6-8V.
The board works fine and intensity changes as expected, however I'm not getting the efficiency I expected. I never expect to get the max value claimed on the front page of the datasheet (97% for this part), but I expected at least 80% which is what I based the battery life estimates on. Instead I'm only getting about 64%.
Running a single LED at full output draws 930mA at 1.54V (1.43W), while drawing 340mA @ 6.6V from the input (2.24W), giving just under 64% efficiency. This is about the same efficiency throughout the input voltage range. I also changed the inductor to different values of the same series part, from 22uH to 68uH, and the result didn't change. This is a 2-layer board, and I thought maybe 4 layers with ground and power planes might help, but I can't see it making that much difference.
I'm attaching a PDF of the schematic (in some readers you can click on the parts to see part numbers), and images of the PCB layout.
Any suggestions on what I could try to improve the efficiency? If not, any suggestions on replacement driver chips that you've seen good efficiency from?
EDIT 2: I have done some additional testing. First I tried adding caps across the LED. It gave no measurable improvement in efficiency.
I then ran a sweep of the input voltage and saw efficiency drop by about 1% with Vin at 9V compared to 6V.
I rewired the LEDs in series and saw a huge increase in efficiency, up to about 86%.
Finally I changed the 47uF inductor with a 33uF type having 50mOhm DC resistance. This brought the efficiency up a little more, close to 90%.