When and How To Use a Decoupling Capacitor in Your PCB

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Upverter Expert - When and How To Use a Decoupling Capacitor in Your PCB

Do you know how to use a decoupling capacitor? 

You’ve probably read about decoupling capacitors in datasheets or technical notes, but the function of these capacitors is rarely articulated properly. Aside from looking at advice in datasheets, it helps to know what a decoupling capacitor does and how to use a decoupling capacitor correctly.

Demystifying Decoupling Capacitors

Any ripple or noise that is output from your power supply can cause major performance degradation in ICs. A digital IC can suffer a reduction in noise margin, and an IC used to drive a downstream component can see increased clock jitter if there is noise on the power bus. Analog ICs typically specify a power supply rejection ratio (PSRR), which defines degradation in the output due to noise from your power supply. These problems are worse when the supply noise is concentrated at a higher frequency (i.e., a higher PWM switching frequency). This is one reason why voltage regulators are used in a circuit board.

Even if you use a regulator, noise on the supply can still affect digital and analog circuits. One of the most common ways to reduce noise is to place a capacitor very close to the IC. This capacitor is called a decoupling capacitor and it acts like a charge reservoir. The capacitor is meant to cancel out any current fluctuations on your power rail so that they do not affect the voltage seen by an IC. Once completely charged, the decoupling capacitor opposes any change in the voltage across it by providing discharging if the voltage drops, or vice-versa. Understanding the decoupling capacitor, its placement, and how to choose them is the key to getting the best performance from your design.

When working on a schematic design with ideal power supplies, it is easy for new designers to forget that the real-world power supplies are often far from ideal. When there are a large number of gates in an IC switch, a large surge in current can cause the IC’s supply voltage to change significantly. Consider a digital design with thousands of transistors switching simultaneously. This leads to a current spike that can cause the output voltage from the regulator to drop. This also causes a large current to flow through to ground that causes an inductive voltage drop, known as ground bounce.

How To Use a Decoupling Capacitor

The following figure shows one of the most effective decoupling techniques, which uses two parallel capacitors.

decouplingPlacing two decoupling capacitors in parallel

A large electrolytic capacitor (typically 10 to 100 μF) must be placed no more than 2 inches from the chip. The purpose of this capacitor is to act as a large charge reservoir, which keeps the voltage across the IC’s power and ground terminals constant.

The smaller capacitor (typically 0.01 to 0. 1μF ceramic capacitor with low effective series inductance) should be placed as physically close to the power pins of the chip as possible. The purpose of this capacitor (sometimes called a bypass capacitor) is to short high-frequency noise on the power rail to ground. This small capacitor has a smaller time constant, thus it reacts faster than the large capacitor (in other words, it has higher bandwidth), but it has lower charge capacity.

The large capacitor, despite being slow, has a large charge capacity. Taken together, these capacitors help provide a smooth supply for the chip. They also provide a low impedance path to ground for any higher-frequency noise, hence any noise generated within one IC does not propagate to other ICs.

Here are some tips for connecting decoupling capacitors to an IC:

  • When multiple capacitors are placed on the same supply pin, make sure that the smallest capacitor is closest to the IC, and place the rest of the capacitors away in ascending order. When placed close to the IC, the smallest capacitor will provide the fastest short for high-frequency noise.
  • Devices with multiple power pins usually need to have at least one capacitor per power pin. Be sure to check the component datasheet for the manufacturer’s recommendations!
  • For this scheme to work well it is important to connect all decoupling capacitors to a low-impedance ground plane with a large area through a via or short trace.
  • Always make sure you are placing the smallest decoupling capacitor on the power pin itself and not on a net tied to logic “HIGH”.

Types of Decoupling Capacitors

Note that a decoupling capacitor is not a specific type of capacitor. In theory, any capacitor could be used for decoupling. Electrolytic capacitors are excellent candidates for use as large decoupling capacitors as they are cheap, available in a wide range, have high capacitance-to-volume ratio, and have a broad range of operating voltages. Electrolytic capacitors are polarized and reversing the polarity can damage the component. Care must be taken to ensure you have placed the capacitor with the correct polarity. They also have high leakage current, but for most basic applications they work very well.

shutterstock_731355235Different types of capacitors

Aluminum electrolytic capacitors are designed to handle high frequency switching pulses. Solid tantalum electrolytic capacitors generally have smaller operating voltage and smaller available capacitance values, but they have a higher capacitance-to-volume ratio. They are more expensive and should be used carefully.

Ceramic or multilayer ceramic (MLCC) are among the best candidates for high frequency filtering because of their small size and low losses. The performance of these capacitors varies widely with the type of dielectric being used. The X7R type is preferred as the capacitance does not vary with the input voltage. Multi-layer ceramic capacitors are also popular because of their low inductance design.

In general film capacitors are not used for decoupling applications because they are generally wound, which increases the capacitor’s parasitic inductance. Here is a good article for calculating the value of a decoupling capacitor.

Placing a Decoupling Capacitor on the Board

Ideally, you would place a decoupling capacitor on the opposite side of the board as the IC as it can be placed right below the SMD pads (see the right side of the figure below). This provides a shorter path to ground and frees up space for fanning out traces. In most situations, you will place all the capacitors on the same side of the board with the decoupling capacitor as closely as possible to the power/ground pins. An alternative option with the IC, capacitor, and other components on the same side of the board is shown in the left side of the figure below.

cap_placementStrategically placing a decoupling capacitor on the board can free up board space

When placing these capacitors on the layout it is important to make sure that the capacitor you are placing is a verified component with a correct footprint. Upverter® provides a vast library of verified components, including ceramic capacitors with low ESL, aluminum electrolytic capacitors, and tantalum capacitors that you can use for decoupling. Upverter’s browser-based platform also provides schematic design and layout capabilities for designing boards from start to finish.

You can sign up for free and get access to the best browser-based PCB editor, schematic editor, and component database. Visit Upverter today to learn more.

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