If you power mobile solutions, you most likely need to convert a battery voltage into something “usable”. Most likely into 5V or 3.3V. An additional difficulty comes with a battery voltage that changes over time.
So, what to do? Feedback systems to the rescue. DC DC converters provide a rather constant current at different output voltage from its input. Here is a collection of links that might clarify some concepts:
A datasheet of a an advanced DC DC converter: Interesting here is the amount of digital logic that goes into a DC DC converter. The logic is controlled with I2C. A simpler but widely popular DC DC converter is the MC34063.
An intro to BOOST converters: DC DC converters can be from type step-up or step-down. This tutorial shows some ideas for building your own Boost converter.
Linear regulators: These converters provide an accourate scaled voltage (or voltage) based on a resistor. However, resistors dissipate power, so for mobile uses they are not the first choice. In some cases, it is simpler to use this type of DC Dc converter.
Discussion about building a Buck converter: Buck or step-down converters provide a scaled version of the input. There is a nice discussion about using those in the forum of the EEVblog.
Role of a DAC in a DC DC converter: This link provides some information about using feedback in a Buck converter.
Energy in an LC circuit: In some DC DC converters power is stored and provided from an LC circuit. Here is some information about energy in that.