The v4.0 boards have arrived and I'll be building one this evening. This board has the XMegaE5 controller paired with the MAX6951. I decided on this intermediate step so that I could have a platform where the display was a known system. I can use that board to pin down the big sweeping architectural changes related to all the other parts of the design and then know that with the next board I just have to debug the software multiplexed display in isolation.

The proposed 5.0 design will use the TBD62?83AFNG switch chips to isolate the higher power required for the LEDs from the controller. The board will have an LDO and a buck converter, but I swapped their roles relative to the last log entry. The LDO will be a fixed 3.3 volt LDO for the GPS module and controller. The buck converter will be a variable voltage unit to supply just the LEDs.

The choice of how to supply the LEDs was not an easy one to make. Traditionally, one would want to design a constant current system to insure that the same amount of current was fed through each segment. This would go on the anode driver side, since those are separate. This would require 8 separate current regulators. Adding that functionality to the TBD62783A would be ideal, but that chip is more of a generic switch, so the only other choice would be to duplicate it 8 times.

Because it's untenable to have 8 separate current limiters, the only other choice is a constant voltage supply. If you look at the Vf / If graph for a typical LED, it usually has a kind of knee at the rated Vf where increasing voltage will result in dramatically increased current. Where you wind up on the vertical part of that slope winds up being kind of a crap shoot, but as long as we stay under the pulsed maximum forward current (which is generally much higher), we should be ok. Since the Vf of our red LEDs is 2.1, the design is for a 2.3 volt supply. With a constant voltage supply, it won't matter how many segments are on at a given moment - all of them will wind up seeing the same voltage and will draw as much current as they will. Using an adjustable voltage regulator will allow the design to be tweaked. In fact, one reason I haven't been able to offer blue displays before now was that the Vf is much higher than for green or red. One potential part has, in fact, a 3.8 volt Vf. This would have been impossible before without changing the design of the board so that the MAX6951 could be powered from the 5 volt rail instead of the 3.3 volt rail, but with this design, it's just a matter of changing the voltage sensing resistor divider (they're whacky expensive though, so don't look for them on the Tindie store anytime soon).

It's also conceivable that this design could wind up being the way forward for larger format digit displays. Most of the 1-2 inch digit displays use multiple LEDs in series, meaning that they simply have a much higher Vf but the same current requirements. As long as the Vf of the three types of displays (large 7 segment, small 7 segment and discrete dots for colons and AM/PM) have the same Vf value, then that voltage can be produced with either a boost or buck converter from the 5 volt rail with the entirety of the rest of the circuit unchanged.

We still have to see how this works out in practice, but I am optimistic.