While I was waiting for the band-aid PCBs to arrive I got to thinking about a better approach for the timing circuits. The LTC6993-1 is cute, but at $3.50/each it doesn't seem like very good value. Plus the PCB space that the one-shot, DFF, and LDO consume doesn't justify the function. I set about to see if Microchip had any simple micro-controllers in a SOT23-6 package -- yes they do!

The PIC10F202 is a pretty bare bones µP -- just a watchdog timer, a clock/timer, and four I/O pins. A big plus is the 1% accurate 4MHz internal clock. It has a supply range of 2V-5.5V and uses TTL logic levels. Best of all, it's only $0.36 in low quantities -- and eliminates more than $4 of discrete components. I can also trade 3 SOT23 packages and a smattering of SMD components for a single SOT23-6 and two SMD passives. I'm sold.

Here's the new improved schematic:

The small PIC simply replaces the one-shot, DFF and LDO. There is no noise filter on the SHUTDN input -- the PIC can provide noise filtering in the code.

The layout is a lot smaller (1.05 inch x 1.275 inch):

The input output pads are arranged to accommodate 3.5mm terminal blocks so that all of the connections don't require soldering. The terminal blocks are mounted on the back side.

The balance leads are beefier (on the back side) and should support the 2.5A output load if you're going to employ 2 supercaps -- that was missing from earlier versions.

The option to use the larger 12x12 inductor is gone. This layout should work with most 6x6 or 7x7 inductors with adequate current/heat ratings. The inductor I'm using costs ~$0.09/each.

PCBs will be ordered soon.