Why regulate
Every chip needs its own steady voltage, called a rail. Here the I/O chip wants 3.3 V and the core chip only 1.0 V, held within about 5%. A power supply turns whatever comes in into those exact rails.
Power comes in through USB at 5 V. A USB 2.0 port promises up to 500 mA (half an amp); a USB 3 port, 900 mA. 5 V is too high for both chips, so it must be stepped down.
A lithium-ion cell reads about 4.2 V when full and falls toward 3.0 V as it empties. The chips cannot follow that slide. A regulator holds its output steady while its input moves: regulate means hold steady.
Linear regulators
A linear regulator is the simple way. Inside, a transistor acts like a resistor that keeps adjusting itself to drop exactly the extra voltage: 5 V in, 3.3 V out. Its output is clean and quiet, with no switching noise.
The dropped voltage turns into heat: P = (Vin − Vout) × I = (5 − 3.3) V × 0.5 A = 0.85 W. The chip gets 1.65 W, so only 66% of the input power is used.
Switching regulators
For the 1.0 V core, a linear regulator would waste 4 of every 5 watts. A buck converter instead flicks a switch fully on and off, on 20% of the time. Real ones switch hundreds of thousands of times a second; here it is slowed.
The inductor, a coil of wire, resists sudden changes in current. The capacitor holds the voltage between pulses. Together they average the pulses: 5 V on 20% of the time gives a steady 1.0 V.
A switch that is fully on or fully off wastes little power, so a buck is about 90% efficient. Feeding the core 1 W costs about 1.11 W from USB. A linear regulator would draw 5 W for the same job.
On the board
Chips draw current in sudden bursts each time their transistors switch. Small decoupling capacitors, often 0.1 µF, sit right beside each power pin and hand out charge instantly, so the rail does not dip while the regulator catches up.
Some chips need their rails in order. Here the 1.0 V core rail comes up first. When it is good, the buck sends a power-good signal that enables the linear regulator, and only then does 3.3 V appear.
In detail
;; step 1/10 · need Rails on this board 3.3 V I/O chip (pins that talk to the outside: I/O = input/output) 1.0 V core chip (the processor core) Example: an AMD Artix-7 FPGA core rail is 1.00 V, allowed 0.95-1.05 V (DS181). Too high can damage a chip; too low and its logic misbehaves.
The board is a teaching model: the 0.5 A and 1 A loads and the two chips are chosen to show the idea. The 90% buck efficiency is the worked estimate in TI SLVA477, not one part's datasheet figure. Linear regulator loss is (Vin − Vout) × Iout plus a small ground-pin current × Vin, which this lesson ignores (TI SLVA079). The switching shown is slowed down so you can see it. Real buck converters switch hundreds of thousands to millions of times a second (Erickson and Maksimović, Fundamentals of Power Electronics).