After looking at my mega 2560's input voltage specs, it looks like i can just use 12v to power the board, and eliminate the sepic for the dimmer. So all i need to figure out, is how to limit the current that flows through the 12v pwm dimmer, and its ~5-12v output that gets converted to linear 0-5v using the optoisolator circuit. Thats simpler and more effecient than adding a dc/dc converter to bump 9v up to 12v for the dimmer, when drawing the power input for the dimmer from the arduino input supply.