Buck Converter Primer and Reference I

The most basic and ubiquitous topology of any power distribution network is the buck converter since in embedded systems you usually want to downconvert a USB or Li-Ion battery voltage to supply a microcontroller. See Wikipedia or one of the hundreds of app notes on the subject for the full derivation, but it basically works by switching on and off the output at some percentage to scale down the input voltage.

Suppose for instance you wanted the output voltage (Vout) to be half of the input voltage (Vin). The converter would turn the output on and off 50% of the time for every cycle. During the remainder of the time, the energy stored in the inductor and capacitor keeps the output at the correct voltage until the cycle repeats. By selecting the correct feedback resistors R1 and R2, the converter will automatically switch the output at the appropriate ratio to get the desired output voltage.

Here is a standard implementation of a buck converter. The block contains power switches and the control circuitry to make the switches switch at the right time.

Usually the vendor of the power converter IC will tell you in the datasheet exactly what supporting components to use and there are only a few things you will need to calculate.

This is the relationship between Vout, R1, R2, and Vref, which is given in the datasheet:

Here are some useful spreadsheets to help design a buck converter circuit:







 When selecting the feedback resistors, make sure the current they draw from the output is much greater than the current drawn by the fb pin in order to make the output as accurate as possible. This spreadsheet snippet helps to solve for this: