Switching converter: trading noise for efficiency
A switch turns on and off quickly, storing energy in an inductor and releasing it to change the voltage. The principle of buck and boost, and PCB cautions.
Buck converter (step-down)
When the switch is on, the input sends current through the inductor to the output; when it is off, the inductor keeps the current flowing through the diode (or the low switch). The output becomes the average.
Vout ≈ Vin × D (D = duty cycle)
For 12 V → 3.3 V, D ≈ 0.28. Efficiency is usually 85 to 95%, so it runs far cooler than an LDO under the same conditions.
Boost converter (step-up)
When the switch is on, energy builds up in the inductor; when it turns off, the inductor voltage is added on top of the input voltage and delivered to the output. Vout ≈ Vin / (1 - D). Used to make 5 V from a single battery cell.
RoadmapSee where this note sits in Electronic Circuits→Why noise appears
Current is interrupted and restored at hundreds of kHz to several MHz, which creates sharp voltage changes. Output ripple and radiated noise can disturb analog circuits.
PCB placement principles
- Make the fast current loop formed by the input capacitor, switch and diode as small as possible
- Keep the switching node (SW) trace short and wide, and sensitive signals far away
- Put the feedback resistors close to the IC and the feedback trace away from the SW node
Most converter IC datasheets include a recommended layout drawing. Following it as it is is the safest course.