Welll, I worked some more and got the power supply designed, built, and assembled. Everything seems to work on it so far. The 5V and 12V outputs, both 2.8V linear LDOs, 5-12V converter (for USB in) and the battery charger seems to charge. The PCB was designed to drop in on top of the first PCB and it fits around the connectors and things perfectly. There’s some openings to allow for placement of the JTAG connector and stuff.
The supply board’s all plugged in and ready to go. It mates with a 10 pin power connector, a 2 pin signal connector and a 2 pin USB power connector. I will use standoffs and screws to join the boards eventually so they don’t come unplugged.
Top view semi closeup
Bottom view semi closeup.
The supply on this synthesizer had to fairly complicated. It had to perform the following functions: run off of 4 recharagable AA cells, run off of a 9V DC line lump, and run off of USB power. On top of that it has to charge the batteries from USB or the line lump. This is a fairly difficult thing to achieve for several reasons. The biggest problem is the output voltages (5V and 12V). The input voltage to the 5 and 12V regulators could vary between 3V (discharged AA rechargables) all the way up to potentially a bit more than 12V. So this means that the input voltage could dive UNDER the 5V output or over the 12V output… causing one supply to go out of regulation.
The solution was twin SEPIC converters which can handle this condition with ease. Running the two SEPIC converters are the two major inputs (USB power and line lump power “OR’d” together using diodes, and battery power). Both of these major inputs pass through P-channel FETs which can turn one source on at a time while isolating the two sources completely. This is done with two P channel FETs per input, wired back to back so the body diodes cannot both forward bias at the same time. I used two dual FETs for this function.
The battery charger simply connects between the power input (line lump / USB) and the battery input.
A PIC micro controls all functionality. It runs on 2.8V so it is quite low power. The PIC will run all the time, waiting for the user to press the power button if it’s totally “off”, and will control battery charging (it can control charging rate to accomodate various operational modes), control powering up of the unit, and it can take temperature of the batteries and internal environment to accomplish quick charging of the battery pack.
I have a “one wire” type interface between the PIC and the FPGA for communications. This will send USB operational modes up, and key presses and battery state, etc. down to the FPGA. There’s a second “one wire” interface between the power board and the to-be-designed capacitive touch panel board which will be the final piece of hardware for this unit.