[Editor's Note] Generally, when we think of semiconductors, we tend to think of semiconductors that are familiar to the general public, such as the CPU and memory of computers. On the other hand, MCUs (Micro Controller Units), which are core semiconductors used to operate electronic products, are semiconductors that are still unfamiliar to the general public, even though they are commonly used in all electronic products that we easily come across. These MCUs have recently been in the media due to the semiconductor shortage, and have begun to attract the attention of the general public. Accordingly, our magazine has prepared a place to learn about MCUs through a series of articles by Yuji Kawano, Manager of ST Microelectronics, a company specializing in MCU semiconductors.

■ Bypass capacitor, reduces MCU radiated noise and bypasses incoming noise

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

A decoupling capacitor for connecting the MCU power line is also called a bypass capacitor. It is 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.

Ceramic capacitors and tantalum electrolytic capacitors are commonly used as MCU decoupling capacitors because they are small in size and provide excellent frequency characteristics as well as low impedance characteristics.

The decoupling capacitors are placed as close to the MCU power terminals as possible. The size of the capacitors to which terminals are connected is determined by the internal power circuitry of the MCU. For information on recommended circuits, refer to the user manual or application notes for the relevant MCU. You can also learn more by using the evaluation circuit boards from the MCU manufacturer.





■ Classification by 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 article), tantalum electrolytic capacitors, aluminum electrolytic capacitors, and film capacitors, and they are classified according to their dielectric materials and structure.

Electrolytic capacitors are used in large-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 the capacitive component, the actual capacitor contains resistive and inductive components. Since ceramic capacitors have small resistive and inductive components, the impedance decreases as the frequency increases. Ceramic capacitors have excellent noise reduction characteristics because the noise components generally have high frequency characteristics.

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

The ST website provides an application note: 'AN3320: Getting started with STM32F20xxx/21xxx MCU hardware development'. Most MCU manufacturers provide such documents, so contacting the MCU manufacturer you plan to use will give you information on which capacitors to choose.

For the ST application note mentioned above, the section on decoupling provides information on specific capacitor types and capacitance values. Here, we see: “... Each power supply pair should be decoupled via a filtering ceramic capacitor (100 nF) and a single tantalum or ceramic capacitor (minimum 4.7?F and typically 10?F)…” This section also recommends a circuit similar to the one shown in Figure 2.




This section does not explain how many ceramic capacitors are required, but you can check this by referring to the circuit diagram for the STM32F2 evaluation board (STM3220G-EVAL). Looking at the diagram, you can see that the MCU has 176 pins, 14 of which are power pins, and it is equipped with 15 100nF ceramic capacitors and one 4.7?F tantalum electrolytic capacitor. This means that at least one capacitor must be connected to a single power pin. If you are using a package with a smaller number of pins, you will have to reduce the number of ceramic capacitors to match the number of pins.

Since the noise types and power supply voltage fluctuation characteristics do not appear the same for the entire user system, you do not necessarily have to use the circuit recommended by the MCU manufacturer. It is provided for reference only. Ultimately, you must decide which design you use.

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




In electronic circuit diagrams, all decoupling capacitors are usually 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 should be placed as close as possible to the MCU power terminals (see Figure 3(b)).

For BGA (ball grid array) packages, the terminals are placed under 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 place them as close to the MCU as possible.

■ Other Considerations and Reference Information

(1) Capacitor smoothing for built-in regulator

Today's MCUs incorporate built-in voltage regulators, and most of their internal circuits operate at voltages lower than the MCU supply voltage. In such cases, the actual power source for the MCU's internal circuits is the built-in regulator output power line. Some MCUs provide a special pin for an external capacitor to smooth the regulator output voltage. However, a basic approach to 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, as described in 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 onboard regulator input (note that this is not the case for all MCUs; you should contact the relevant MCU manufacturer for more information). If you know which power terminal is closest to the onboard regulator input, connecting a larger or more capacitors to that terminal rather than to the other terminals can provide better results in reducing noise and smoothing the supply voltage (see Figure 4).

If you contact the MCU manufacturer, they will be happy to tell you which power terminal is closest to the internal regulator. Alternatively, if you can identify a power terminal that uses more capacitors in one of the MCU manufacturer's application circuits, that terminal is closest to the input of the internal regulator.