I needed to add a -3.3V to -6V supply to my RIS-796A 250A pulser, analog version. The PCB didn't have much space, so I implemented a simple inverting switcher with an LM2840 buck converter. See circuit and layout on file U7=LM2840_sch+pcb.jpg in the DropBox folder:
There's also a sub-folder with relevant datasheets.
The circuit works OK, although it's a bit fragile, with ICs failing instantly when I miss-probe. The L1 ME3215 inductor has 1R5 resistance, and can deliver over 100mA, more than needed. But it has a horrible 200mV output ripple, forcing me to add an LC output filter stage.
Checkout the other file U7=AP3012_sch.jpg It's my idea for a simple SEPIC inverting converter, using a boost- switcher and a coupled inductor. The inductor feeds a continuous current into the output cap, for low ripple, so maybe the LC filter can be eliminated. Now L1 is an LPD3015, with 2R8 ohms, but it can still deliver 100mA. The coupled inductor is only 3x3mm, and I hope the parts will fit into the same space as the original circuit.
I thought about an LMR64010 for the boost IC, but its current-mode sense resistor is only 0.05 ohms, which doesn't bode well for operation at currents like 20mA. I had a hard time finding modest-current switchers in sot-23 packages, until I discovered the AP3012 by BCD, which has a 0.2-ohm sense resistor. Also, they only cost 32-cents! That's 10x less than the $3.32 LT8330 I also considered. BCD was purchased by Diodes, Inc about 7 years ago. and their parts are well stocked.