Version 5.0.1 has been built. It replaces the PAM2305 + LDO with a MP2149GJ dual buck regulator. One half of it makes the 3.3 volts for the controller and GPS, the other half makes ~2.3v for the display.

I'm a bit torn, though. The MP2149GJ is a little pricey in comparison to the older design. I could fairly easily use either a pair of PAM2305 adjustable units or a single adjustable and a single 3.3v fixed (or an LDO - like for v5.0) for less money in terms of the BOM price. But the MP2149GJ reduces the BOM quite a bit - one chip, one inductor twice, one ceramic cap three times, three feedback resistors (one common value between the two sides) instead of (for v5.0) two chips, an inductor, two different pairs of ceramic caps, a third ceramic cap as part of the feedback network and two resistor values.

I'd still rather have some sort of constant current supply for the LED segments. If I did that, I could feed the TBD62783 from either 3.3v or 5v (depending on the LED Vf), and introduce a current regulator on each of the anode lines. Classically, this would be in the form of a JFET and resistor. But having to do that 8 times with discrete components would be pretty silly.

There are LED driver chips intended to do the job, but most of them are low-side drain switches, which means that they'd be designed for common anode displays, and most of them use serial input of one form or another instead of parallel. Serial input is nice for saving pins, but I don't really need to save pins - the XMega_E5 has plenty for what we want to do.