[Editor's Note] Generally, when people think of semiconductors, they tend to picture components familiar to the general public, such as computer CPUs and memory. On the other hand, the Micro Controller Unit (MCU), which serves as a core semiconductor for driving electronic products, is used in virtually every electronic device we encounter, yet remains an unfamiliar semiconductor to the general public. Recently, however, MCUs have begun to attract public attention as they have been frequently mentioned in the media due to the semiconductor shortage. Accordingly, this publication has organized a series of articles by Manager Yuji Kawano of STMicroelectronics, a company specializing in MCU semiconductors, to provide a professional look into MCUs.

■ Bypass Capacitor: Reduces MCU Radiated Noise and Bypasses Incoming Noise

I am designing an MCU power circuit. What is the recommended type of decoupling capacitor for connecting the MCU power lines, and how should they be connected?

Decoupling capacitors for MCU power line connections are also called bypass capacitors. They are inserted between the power terminal and the GND terminal to reduce noise radiated from the MCU and bypass incoming noise. It is also used to smooth the power supply voltage by blocking voltage ripple (see Figure 1). To reduce noise and smooth the power supply voltage, the circuit impedance must be sufficiently low over a wide frequency range.

Generally, ceramic capacitors and tantalum electrolytic capacitors are used as MCU decoupling capacitors. This is because they offer not only excellent frequency characteristics but also low impedance characteristics while being small in size.

Decoupling capacitors are placed as close as possible to the MCU power terminals. The size of the capacitor and the terminal to be connected depend on the MCU's internal power circuitry. Please refer to the relevant MCU user manual or application notes for information on recommended circuits. Additionally, you can learn more by using the MCU manufacturer's evaluation board.





■ Classified according to capacitor, dielectric, and structure

First, let's look at the basics of capacitors. There are various types of capacitors, such as multilayer ceramic capacitors (referred to as 'ceramic capacitors' in this text), tantalum electrolytic capacitors, aluminum electrolytic capacitors, and film capacitors, which are classified according to their dielectric materials and structure.

Electrolytic capacitors are used in high-capacity power systems (e.g., power rectifiers), film capacitors are used when stable capacitance is required (e.g., oscillator circuits), and ceramic capacitors are used as MCU decoupling capacitors because they can handle a wide frequency range.

In addition to capacitive components, actual capacitors contain resistive and inductive components. Because ceramic capacitors have small resistive and inductive components, their impedance decreases as the frequency increases. Ceramic capacitors exhibit excellent noise reduction characteristics because their noise components generally have high-frequency characteristics.

Next, we will examine the design of an actual power circuit and explain several preventive measures using the STM32F2 series, a 32-bit MCU produced by STMicroelectronics, as an example.

The ST website provides the following application note: 'AN3320: Getting started with STM32F20xxx/21xxx MCU hardware development'. Since most MCU manufacturers provide such documents, you can obtain information on which capacitor to select by contacting the manufacturer of the MCU you intend to use.

In the case of the ST application note mentioned above, the section on decoupling provides information regarding specific capacitor types and capacitance values. Looking at this, “... It explains that “each pair of power supplies must be decoupled through a filtering ceramic capacitor (100 nF) and a single tantalum or ceramic capacitor (at least 4.7 µF, typically 10 µF).” This section also recommends a circuit similar to that shown in Figure 2.




This section does not explain how many ceramic capacitors are required, but this can be verified by referring to the circuit diagram for the STM32F2 evaluation board (STM3220G-EVAL). Looking at the diagram, the MCU has 176 pins, 14 of which are power terminals, and 15 100nF ceramic capacitors and 1 4.7µF tantalum electrolytic capacitor are mounted on them. This means that at least one capacitor must be connected to a single power terminal. If a package with a smaller number of pins is used, the number of ceramic capacitors must be reduced to match the number of pins.

Since noise types and power supply voltage fluctuation characteristics do not appear identically across all user systems, you do not necessarily have to use the circuits recommended by the MCU manufacturer exactly as they are. They are provided for reference only. Ultimately, you must decide which design to use.

When placing decoupling capacitors on an actual printed circuit board (PCB), keep in mind that they should be placed as close as possible to the MCU. Figure 3 is an example layout for the LQFP64, 64-pin package for the STM32F2 series.




In electronic circuit diagrams, all decoupling capacitors are typically connected exclusively to the power supply area. However, this does not mean that they must be mounted together in a single location on the actual PCB (see Figure 3(a)). They simply need to be placed as close as possible to the MCU power terminals (see Figure 3(b)).

In the case of a BGA (ball grid array) package, the terminals are placed below the MCU. When using a BGA package, it is recommended to mount the decoupling capacitors on the opposite side of the board (e.g., directly opposite the MCU) to position them as close to the MCU as possible.

■ Other Considerations and Reference Information

(1) Capacitor smoothing for internal regulators

Modern MCUs integrate built-in voltage regulators, and most internal circuitry operates at voltages lower than the MCU power supply voltage. In such cases, the actual power supply for the MCU's internal circuitry is the output power line of the built-in regulator. Some MCUs provide a special pin for an external capacitor to smooth the regulator output voltage. Nevertheless, the basic approach to the layout is to place capacitors with good frequency characteristics and low impedance characteristics (e.g., ceramic capacitors and tantalum electrolytic capacitors) close to the MCU terminals, in accordance with the relevant MCU user manual (see Figure 4).




(2) Power terminal on the power line closest to the built-in regulator

Even if an MCU has multiple power terminal pins, in most cases, the internal power circuit is connected to the built-in regulator input (note that this does not apply to all MCUs; you should contact the relevant MCU manufacturer for more detailed information to verify this). If you know which power terminal is closest to the built-in regulator input, connecting larger or more capacitors to that terminal rather than the others can provide better results in reducing noise and smoothing the power voltage (see Figure 4).

If you contact the MCU manufacturer, they will gladly provide you with information on which power terminal is closest to the built-in regulator. Alternatively, if you can identify a power terminal using more capacitors in one of the MCU manufacturer's application circuits, that terminal is closest to the built-in regulator input.