(1) Introduction to MCU decoupling design
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1 Overview
We often see that one or more ceramic capacitors are often placed on the pins of microcontrollers or IC circuits. They are mainly used to enhance the power integrity PI of the MCU power supply, reduce the PDN impedance, improve the MCU's immunity to noise, and reduce the external noise of the MCU. Radiated EMI.
Later, we will briefly take a look at decoupling circuit components, including bypass capacitors, decoupling capacitors, magnetic bead inductors, and LC/PI filters in a narrow sense, to understand how to choose appropriate components to build high-performance decoupling circuits and improve circuit performance. PI/SI/EMC characteristics.
2. Reasons why MCU needs decoupling
2.1 Introduction to decoupling circuit
The connection between the power supply and the MCU uses various types of capacitors, magnetic bead inductors and other filter devices. The decoupling circuit formed has three main functions:
One is to suppress the EMI radiation generated inside the MCU or divert external interference noise from entering the MCU; the
other is to provide transient current for MCU operation and voltage maintenance;
the third is to serve as a channel for signal return to improve signal integrity.
When the decoupling circuit on the MCU system level board does not work, the following problems will occur:
∎ Interference noise is introduced from the outside, and the MCU receives noise interference from other ICs, causing operation failures;
∎ There is noise leakage, and the EMI radiation in the MCU exceeds the standard;
∎ Power supply voltage fluctuations interfere with the MCU operation, reduce signal integrity, and cause noise on the signal lines Superposition; The return path on the signal line is longer and the signal integrity is reduced.
2.2 Causes of power supply noise
Most digital ICs such as MCUs use circuit CMOS technology, and the signal can be set to a high "1" or a low "0" by switching to the power supply VDD or the ground GND. As shown in the figure, the simplified model of the CMOS inverter circuit, taking a single CMOS inverter as an example, when Vin switches to low "0", the upper-side PMOS is turned on, the gate capacitor charges, and Vout outputs high "1"; when Vin switches high " 1", the low-side NMOS is turned on, the gate capacitor is discharged, and Vout outputs low "0". When the CMOS inverter switches between high and low levels, the parasitic current will flow through the power supply VDD and the ground GND, as shown in Figure 2-2. CMOS inverter parasitic current. When there are many CMOS inverters inside the MCU, the parasitic current will jump very violently. Many frequency devices such as inductors will radiate energy outwards, causing noise failures, or cause fluctuations in the external power supply to affect other ICs.
Usually in order to control the current flowing through the MCU power supply, a decoupling capacitor needs to be installed between the MCU's power pin and the GND pin. In order to form an effective decoupling circuit, the following main points need to be added:
∎ Use a smaller ESR capacitor to describe a bypass that can operate in the high frequency range;
∎ Strictly limit the range of parasitic current flow and install the capacitor near the MCU;
∎ Keep the parasitic inductance of the layout small, especially the IC and capacitors between.
2.3 Insertion loss
Usually, the insertion loss IL of the filter is used to represent the noise filtering performance. Since the decoupling circuit of the power supply is also a type of filter, its noise suppression performance can be represented by the insertion loss.
Insertion loss IL is described by the effect of a filter installed in a circuit with an impedance of 50Ω. It is the difference in output voltage before and after the filter is installed, in dB. The greater the insertion loss, the better the noise suppression effect. Insertion loss may be replaced by the absolute value of the S-parameter transmission coefficient S21 for a 50Ω system. As shown in the picture
2.4 Introduction to decoupling circuit
Bypass capacitors are widely used as C-type filters (decoupling capacitors). As the impedance of the C-type filter decreases, the insertion loss IL will increase. The capacitor impedance is inversely proportional to frequency and becomes a low-pass filter. Ideally, the higher the frequency, the greater the insertion loss.
In addition to C type, there are also LC type and PI type filters. Based on the C type filter and then string an inductor/magnetic bead on the remote power line of the MCU to become an LC filter. Based on the LC filter, the remote Combining a capacitor is a PI type filter. As shown in the picture:
When capacitors and inductors are combined, as shown in Figure 2-5 MCU power filter configuration (b) and ©, the slope of the insertion loss characteristic curve will be steeper compared to using only capacitors. Because in the attenuation region, the insertion loss will increase at the same time, this method is more useful when the noise needs to be greatly attenuated. Figure 2-6. An example of filter insertion loss after adding magnetic beads shows an example of the change in insertion loss when an inductor is added.
Since the GND pin of the MCU and the nearest capacitor GND become the noise return path, the distance between them should be shortened as much as possible to reduce the impedance. When the CLC-PI filter is laid out, it is best to zigzag the capacitors on both sides so that the capacitor GND is separated by VIA. . Because PI type and LC filtering have large insertion loss for interference noise, when anti-interference EMS protects the MCU, the combined decoupling circuit is more effective in dealing with strong interference.