Simple DC-DC converter with design tool
The MC34063A is an easy to use DC-DC converter that can be used to step battery voltage up or down to run your processor, small servos or to make an efficient LED spotlight. The photo shows a kit I bought from Oatley Electronics using this IC and has parts for stepping up or down on the one board. In the photo I'm driving 6x0.5W 10mm LEDs from two AA batteries (3V). This kit is set up to regulate current by regulating the voltage across a shunt resistor but only minor modification is needed to use it as a 5V regulator.
The Datasheet also has several sample application circuits complete with board layouts that can step up or down or even invert to supply a negative voltage rail for opamps etc.
Best of all, I've found a simple design tool that displays the apropriate schematic and calculates component values for you. If you get some strange values, adjust your frequency or ripple voltage.
You will need soldering skills to assemble the kit otherwise just get the components from your local supplier and breadboard it. The hardest part may be the inductor although premade inductors bought from a local electronics shop like the one in the picture seem to work well enough. The datasheet does specify how to construct one.
This circuit above should give better than 80% efficency at 500mA load. Using a standard regulator you would get about 56% efficiency. This can be the difference between win or loose in an endurance competition. This is a general purpose robot power supply I have designed based on the datasheet and design tool. After experimenting with the design tool using different input voltages, frequencies etc. and noting that some values given were min. values I've chosen values that should work in a wide range of situations.
As you can see I haven't changed the component values even though a 12V battery is being used.
I've used a large output capacitor to minimise output ripple as much as possible but a smaller value could work depending on how tollerant your robot is of ripple in the 5V supply. I've also added two small 0.1uF capacitors even though none are shown on the datasheet to ensure com noise from motors isn't a problem. Remember that sample applications in datasheets show the bare minum of components needed as they cannot allow for every possible situation. I've also gone for the largest inductor shown in the datasheets for maximum flexability as the values given by the design tool are minimum values.
Since every robot is different I suggest testing your circuit out on a breadboard first. You may find that with the design tool as your guide that you can tweak the values shown here to improve efficency. The datasheet does list the formulas used for circuit design which I believe is what the design tool is based on. These may be needed if you want higher output currents and choose to experiment with the schematics shown in the datasheet using an external switching transistor.