In designing electronic circuits, it is fair to say that a "bypass capacitor" almost always appears.A bypass capacitor is formally known as a decoupling capacitor, it acts to "separate" as the name suggests, and is an essential part for stable operation of the circuit.
We will explain in detail the role and importance of this small part, that is inserted between the IC power line and the GND, in preventing IC malfunction.
The important role of the bypass capacitor and the reason for its importance
The bypass capacitor mainly performs the following two roles to maximize IC performance.
1. Fast current supply
The digital IC and switched-mode power supply IC are synchronized to the clock to repeatedly turn on and off at an extremely high speed. This instantaneous switching operation requires a large, instantaneous current.However, the inductance (L component) of the power supply line interferes with the response to this sudden current demand from a power source that is separated from the IC, so that the current is not supplied in time.
As a result of this, the IC power supply voltage momentarily falls, and in the worst case, this causes malfunction or a reset operation.
The bypass capacitor is placed right next to the IC power supply pin so that it performs the role of immediately supplying the accumulated charge for the instantaneous current demand.Due to this, it is possible to suppress sudden changes in the large current flowing through the power supply line, and to suppress momentary fluctuations in the IC power supply voltage.
2. Noise elimination
The power supply line is susceptible to the mixing in of electromagnetic noise from outside and high frequency noise (switching noise, etc.) caused by the operation of the IC itself.
This noise impacts the analog circuits and digital signals inside the IC, and can cause malfunction of the whole circuit.The bypass capacitor functions so that the noise, that is an alternating current component, bypasses (flows) the GND.Due to this action, the bypass capacitor eliminates the noise from outside and suppresses mixing in of the noise inside the IC, so preventing negative effects.
In addition, the high frequency noise caused by the operation of the IC itself can be eliminated using the bypass capacitor.
Due to this elimination, it is also possible to suppress the leaking of noise outside the IC.
Points for correct usage
To maximize the effectiveness of the bypass capacitor, particular attention must be paid to the type used and its placement.
- Placement principles
It is a golden rule that the bypass capacitor be as close as possible to the IC power supply pin and the connection to the GND be as short as possible.
If the wire is long, the inductance component of the wire increases, leading to a loss in performance of the bypass capacitor. - Choosing the capacitor
For high-frequency noise elimination and fast current supply,low ESR (equivalent series resistance) / low ESL(equivalent series inductance) are suitable.
In general, the lower the ESR/ESL, the greater the noise elimination performance of the capacitor. - Separate uses of multiple capacitances
With a single bypass capacitor, it is difficult to eliminate the noise in all frequency ranges.
Generally, it is recommended to connect in parallel and use a bypass capacitor with a low capacitance like 0.1 μF or 0.01 μF for high-frequency noise, and a bypass capacitor with a large capacitance like 10 μF or 100 μF for low frequency noise.
Doing this forms a "composite filter" that can handle a large range of frequencies.
When using multiple bypass capacitors in parallel, the low capacitance (small) capacitor for high-frequency noise is placed closest to the IC, and the large capacitance (large) capacitor for low-frequency noise is placed slightly separated from the IC.
Final remarks
The bypass capacitor is inconspicuous, but it is the power behind the scenes in supporting the stable operation of the circuit from the foundations.
Focusing on these points will enable highly reliable circuit design.
Joined Torex Semiconductor in 2005.
After six years engaged in the development of power ICs such as DC/DC converters, worked for three years providing technical support for offline LED drivers.
Later took charge of technical support for Torex’s power IC customers.
With expertise gained through product development and extensive know-how accumulated from years of customer support — including solutions for power-related issues and troubleshooting — currently engaged in providing technical support and product proposals for Torex products.