Other Parts Discussed in Thread: LM61480
Hello,
I recently designed an LM61480-Q1 based DC-DC converter (actually for powering the display board of a DLP3010-LC EVM, but that is another story) and built up my first board based on the schematic attached below. The 3.3v power supply based on a different switcher (and downstream of the LM61480) is a system I've used many times before, and works satisfactorily. In the first test board, the LM61480 did not appear to be regulating at all; in poking around on the board, it seems to be switching with very nearly a 100% duty cycle at about 100kHz, and is just passing the supply voltage straight through.
The behavior I'm seeing out of the test board is simple; if I apply say, 12V to the board, the output side of the regulator rises linearly to ~200mV below that supply and stays there. It's possible that I just killed the device somehow during my initial bring-up, but I was careful to limit input current below 50mA (and never saw it draw more than 30mA, mostly when I loaded the 3.3v downstream supply).
I wanted to check that my basic design is sound before I populate another board, since if I've made a gross design error somewhere and need to re-spin the boards, then I'd like to do that before I cook another populated board. Do you see any major issues in the schematic I should look at? On digging through my design and the datasheet, my component values (in particular the inductor, which is this unit from Wuerth) differ some from table 9-2 in the datasheet, but I think that is based on me mistaking the frequency configuration; I must have read the top row of the table (for 400kHz) vs the 2.2MHz line. Is my inductor selection to blame?
Thanks,
Sam