I have an LM3478 DC-DC converter (with external FET) that I'm using to generate 9V @ 1A from a LiPo battery. This is an upgrade from an earlier LM2735 circuit. The circuit works quite well at full load and exhibits performance quite close to my Webench sims.
But I have a problem with light-load performance, i.e. less than a few 10's of ma output. The earlier 2735 design worked fine right down to no output current. However, the 3478 circuit seems to go into burst mode below about 40ma output (with a 4.7uh inductor). This in itself isn't a problem, except that the output exhibits a saw-tooth on the output at about 80Hz and 400mv pk-pk. This is causing some problems with my load, an audio circuit, and it's going to be very difficult to filter out due to the very low frequency.
I tried running the 3478 in DCM by changing the inductor to 0.68uh; all that does is reduce the frequency of the light-load saw-tooth, not what I had in mind.
Is my observation of the LM3478 "burst-mode" correct? Is this a design "feature"? I didn't see it discussed in the data sheet. I would like to get the same "non-burst" behavior I saw in the 2735 circuit. I realize there will probably be an efficiency hit, but I can live with that at light loads.
Am I missing something? If not, is there a 3478-like chip that does not exhibit this burst characteristic?
Thanks for any help.
