Terminology
Step-down - A converter where output voltage is lower than the input voltage (like a Buck converter).
Step-up - A converter that outputs a voltage higher than the input voltage (like a Boost converter).
Continuous Current Mode - Current and thus the magnetic field in the inductive energy storage never reach zero.
Discontinuous Current Mode - Current and thus the magnetic field in the inductive energy storage may reach or cross zero.
Noise - Since all properly designed DC-to-DC converters are completely inaudible, "noise" in discussing them always refers to unwanted electrical and electromagnetic signal noise.
RF noise - Switching converters inherently emit radio waves at the switching frequency and its harmonics. Switching converters that produce triangular switching current, such as the Split-Pi or Ćuk converter in continuous current mode, produce less harmonic noise than other switching converters. Linear converters produce practically no RF noise. Too much RF noise causes electromagnetic interference (EMI).
Input noise - If the converter loads the input with sharp load edges. Electrical noise can be emitted from the supplying power lines as RF noise. Which should be prevented with proper filtering in the input stage of the converter.
Output noise - The output of a DC-to-DC converter is designed to have a flat, constant output voltage. Unfortunately, all real DC-to-DC converters produce an output that constantly varies up and down from the nominal designed output voltage. This varying voltage on the output is the output noise. All DC-to-DC converters, including linear regulators, have some thermal output noise. Switching converters have, in addition, switching noise at the switching frequency and its harmonics. Some sensitive radio frequency and analog circuits require a power supply with so little noise that it can only be provided by a linear regulator. Many analog circuits require a power supply with relatively low noise, but can tolerate some of the less-noisy switching converters.
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