Lesson 47 · Boards and embedded systems

Power supplies: steady rails for every chip

Chips need steady, exact voltages. Follow 5 V from a USB port through two kinds of regulator: a linear regulator that is simple and quiet but burns the extra voltage as heat (0.85 W in our example), and a buck converter that flicks a switch on and off and smooths the pulses with a coil and a capacitor, at about 90% efficiency. Finish with decoupling capacitors, batteries and the order rails must come up in.

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Fig 47 · Interactive

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01A linear regulator turns 5 V into 3.3 V for a chip drawing 0.5 A. How much power does it turn into heat?

P = (Vin − Vout) × I = (5 − 3.3) V × 0.5 A = 0.85 W. The chip itself gets 3.3 V × 0.5 A = 1.65 W.

02Why is a buck converter much more efficient than a linear regulator for a big voltage drop?

Fully on, the switch carries current with almost no voltage across it; fully off, it has voltage across it but no current. Either way voltage × current in the switch is small, so a buck is about 90% efficient.

03A buck converter switches 5 V on 20% of the time. Roughly what steady voltage do the inductor and capacitor give?

The coil and capacitor average the pulses: 5 V × 20% = 1.0 V (in an ideal buck, duty cycle = Vout / Vin).

04Where should a 0.1 µF decoupling capacitor go?

It hands out quick bursts of charge when the chip switches. Long wires add inductance and slow that burst, so it goes as close to the power pin as possible.

05On this board, what happens first when the power comes on?

Some chips, such as the Artix-7 FPGA, recommend the core rail first and the I/O rail last. The buck's power-good output enables the linear regulator, so the order is built into the board.

Key terms

Glossary

Rail: A wire or copper area held at one steady voltage that feeds many parts, such as the 3.3 V rail.
Regulator: A circuit that holds its output voltage steady even when its input voltage or its load changes.
Linear regulator (LDO): A regulator that drops the extra voltage across a transistor and turns it into heat. Simple and quiet, but wasteful for big drops. LDO means low-dropout.
Buck converter: A switching regulator that steps a voltage down by switching it fully on and off and smoothing the pulses with an inductor and a capacitor.
Duty cycle: The fraction of each switching period that the switch is on. In an ideal buck converter it equals Vout / Vin.
Inductor: A coil of wire that stores energy in a magnetic field and resists sudden changes in current.
Decoupling capacitor: A small capacitor, often 0.1 µF, placed right next to a chip's power pin to supply quick bursts of current.
Power sequencing: Bringing a board's rails up (and down) in a set order, often using a power-good signal from one regulator to enable the next.

Fine print

Notes and sources

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).
0.1 µF ceramic decoupling capacitors placed close to each power pin are the classic recommendation (Analog Devices MT-101, Decoupling Techniques).
USB: 500 mA (USB 2.0) and 900 mA (USB 3.x) at 5 V (USB-IF). Li-ion: 4.2 V full, about 3 V cut-off (Battery University BU-303). Core rail ±5% and power-on order: AMD/Xilinx DS181.

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