Adjustable 9v - 15v 1.5A DC Switching Power Supply from 6 D batteries

Question

I am starting a project that needs a way to adjust a 9v supply to anywhere from 9v - 15v at 1.5A. The power is coming from 6 D cell batteries so the voltage will drop as the batteries wear down.

Does anyone have any schematics, pointers, or suggestions where I can look? I have browsed around Linear's website and found a couple chips that might work but don't have enough experience to be certain. I have built a couple switching supplies in the past, but I always followed a schematics in the datasheet.

After some browsing, this is the regulator I picked out. Will it work? http://www.linear.com/product/LT1377

Bonus: I also need to power a 3.3v micro-controller off the same set of batteries. If the circuit could also supply 3.3v that would reduce the overall number of components.

Answer 1

Yes, that part should work fine. You can follow the example circuit in the datasheet here too (use the SEPIC version if you need to vary below and above the input voltage) If the voltage needs to be adjustable you can use a fixed resistor on the bottom half of the FB pin divider, and a fixed reistor in series with a potentiometer for the top half (calculate values to give an adjustment from 9V-15V for a full sweep of the pot. Something like 5K for bottom, a fixed 30k in series with a 25k pot for the top will get close assuming a 1.25V Vref)
Be sure to read the datasheet well for the details on recommended coil value, divider resistance, compensation, layout, etc.

The 3.3V output will have to be regulated separately if you want the first supply to be adjustable (otherwise you could use a transformer with two secondary windings and feedback from one of the outputs)
A simple fixed buck regulator can do this (Microchip make a few nice cheap ones, just make sure the input voltage and current ratings are suitable)