Why capacitor selection is critical

Capacitors are often overlooked. Capacitors are neither billions of transistors nor do they use the latest sub-micron manufacturing processes. In the minds of many engineers, capacitors are nothing more than two conductors with an isolated electrolyte in between. In short, they are among the lowest-level electronic components.

Engineers usually solve the noise problem by adding some capacitors. This is because they generally see capacitors as a good helper to solve noise-related problems and rarely consider parameters other than capacitance and voltage rating. However, like other electronic components, capacitors have their drawbacks, such as parasitic capacitance, inductance, capacitance temperature drift and voltage offset, and other non-ideal characteristics.

These factors must be considered when selecting capacitors for many bypass applications or applications where the actual capacitance of the capacitor is very important. Improper capacitor selection may result in circuit instability, excessive noise or power consumption, shortened product life, and unpredictable circuit behavior.

Capacitor Technology

Capacitors are available in various sizes, voltage ratings, and other characteristics to meet the specific requirements of different applications. Dielectric materials commonly include oil, paper, glass, air, mica, polymer films, and metal oxides. Each of these electrolytes has a specific set of properties to meet the unique needs of each application.

Three major capacitors are commonly used as voltage input and output bypass capacitors in voltage regulators: multilayer ceramic capacitors, solid-state tantalum electrolytic capacitors, and aluminum electrolytic capacitors.

Multilayer Ceramic Capacitors

Multilayer ceramic capacitors (MLCCs) are often preferred as bypass capacitors because they combine small size, low effective series resistance and inductance (ESR and ESL), and wide operating temperature range.

It is not infallible. Depending on the dielectric material, the capacitance can shift significantly with temperature variations and AC/DC bias. In addition, because in many ceramic capacitors, the dielectric material is piezoelectric, vibration or mechanical shock may translate into an AC noise voltage on the capacitor. In most cases, this noise is generally in the microvolt range. However, in extreme cases, millivolt levels of noise may be generated.

Applications such as VCOs, PLLs, RF PAs, and low-level analog signal chains are very sensitive to noise on the power supply rails. This noise manifests as phase noise in VCOs and PLLs and carrier amplitude modulation in RFPAs. In low-level signal chain applications such as EEG, ultrasonic, and CAT scan preamplifiers, noise can cause spurious noise to appear in the output of these instruments. Multilayer ceramic capacitors must be carefully evaluated in all of these noise-sensitive applications.

It is important to consider temperature and voltage effects when selecting ceramic capacitors. The multilayer ceramic capacitor selection section discusses determining the minimum capacitance value for a particular capacitor based on tolerances and DC bias characteristics.

Although ceramic capacitors still have drawbacks, they are the smallest and most cost-effective solution for many applications and are therefore found in almost every type of electronic device today.

Solid State Tantalum Electrolytic Capacitors

These capacitors have the highest capacitance per volume (CV product). Only double layer or supercapacitors have a higher CV product.

Ceramic is still smaller and has a lower ESR than tantalum in the 1μF range, but solid tantalum capacitors are less susceptible to temperature, bias voltage, or vibration effects. Tantalum is several times more expensive than ceramic capacitors, but in low noise applications where piezoelectric effects cannot be tolerated, tantalum is often the only viable option.

Commercially available conventional low-value solid tantalum capacitors use cases that are generally small, so the equivalent series resistance (ESR) is high. Large capacitance ("68 μF) tantalum capacitors can have an ESR of less than 1 Ω but are generally larger.

Recently, a new tantalum capacitor has appeared on the market, which uses a conductive polymer electrolyte instead of the common manganese dioxide solid-state electrolyte. In the past, solid-state tantalum capacitors had limited inrush current capability and required a series resistor to limit the inrush current to a safe value. Conductive polymer tantalum capacitors are not limited by inrush current. Another benefit of this technology is the much lower capacitance ESR.

Any tantalum capacitor has a leakage current several times greater than an equivalent ceramic capacitor and may not be suitable for ultra-low current applications.

For example, a 1μF/25V tantalum capacitor has a maximum leakage current of 2.5μA at the rated voltage at 85°C operating temperature.

Several manufacturers offer conductive polymer tantalum capacitors in 0805 cases, 1μF/25V, and 500mΩ ESR. Although somewhat larger than typical 1μF ceramic capacitors in 0402 or 0603 cases, 0805 capacitors are significantly smaller in size for applications where low noise is the primary design goal, such as RF and PLLs.

Because the capacitance value of solid tantalum capacitors can maintain stable capacitance characteristics relative to temperature and bias voltage, selection criteria include only tolerance, dropout over the operating temperature range, and maximum ESR.

A major drawback of solid-state polymer electrolyte technology is that these tantalum capacitors are more susceptible to high temperatures in lead-free soldering processes. Typically, the manufacturer will detail that the capacitor must not be exposed to more than three solder cycles. If this requirement is ignored in the assembly process, it can lead to long-term reliability issues.

Aluminum Electrolytic Capacitors

Conventional aluminum electrolytic capacitors tend to be large, have high ESR and ESL, relatively high leakage current, and limited lifetime (in the thousands of hours).

OS-CON type capacitors are a technology related to solid-state polymer tantalum capacitors and were introduced 10 years earlier than tantalum capacitors. They use an organic semiconductor electrolyte and an aluminum foil cathode to achieve a low ESR. Because there is no problem of the liquid electrolyte drying out, OS-CON type capacitors have a much longer life than conventional aluminum electrolytic capacitors.

The OS-CON type capacitors on the market can withstand high temperatures of 125°C, but most stay at 105°C.

Although the performance of OS-CON type capacitors is significantly better than conventional aluminum electrolytic capacitors, they tend to be larger and have higher ESR than ceramic or solid-state polymer tantalum capacitors. Like solid-state polymer tantalum capacitors, they are unaffected by piezoelectric effects and are suitable for applications requiring low noise.

Multilayer Ceramic Capacitor Selection

Output Capacitors

ADI's LDO designs work with small, space-saving ceramic capacitors, but most common capacitors can be used as long as ESR values are considered. The ESR of the output capacitor can affect the stability of the LDO control loop. To ensure stable LDO operation, it is recommended to use capacitors with at least 1μF and an ESR of 1Ω or less.

Output capacitance also affects the transient response to load current changes. Using a larger output capacitance value can improve the LDO's transient response to large load current variations.

Because the LDO control loop has a limited bandwidth, the output capacitor must supply most of the load current needed for fast transients. 1μF capacitors cannot supply current for very long and produce a load transient of about 80mV. 10μF capacitors reduce the load transient to about 70mV. By increasing the output capacitance to 20μF, the LDO control loop can capture and actively reduce the load transient.

Input Bypass Capacitor

Connecting a 1μF capacitor between VIN and GND can reduce the circuit's sensitivity to PCB layout, especially with long input alignments or high source impedance. If an output capacitance greater than 1μF is required, a higher input capacitance should be selected.

Input and Output Capacitance Characteristics

LDOs can use any good quality capacitor as long as it meets the minimum capacitance and maximum ESR requirements. Ceramic capacitors can be manufactured with various dielectrics and have different characteristics depending on the temperature, and the voltage applied. The capacitor must have a dielectric sufficient to ensure minimum capacitance over the operating temperature range and DC bias conditions. X5R or X7R dielectrics with voltage ratings of 6.3V or 10V are recommended for 5V applications. Y5V and Z5U dielectrics have poor temperature and DC bias characteristics and are not recommended for use.

The voltage stability of the capacitors is greatly affected by the capacitor package size and voltage rating. Capacity with larger packages or higher voltage ratings generally has better voltage stability. x5R dielectrics have a temperature variation of ±15% over the -40°C to +85°C temperature range, not as a package or voltage rating function.

Finally

To ensure LDO performance, it is important to understand and evaluate the impact of DC bias, temperature variation, and tolerance of the bypass capacitor on the selected capacitor. In addition, capacitance techniques must also be carefully considered in applications requiring low noise, low drift, or high signal integrity. All capacitors are subject to non-ideal behavior, but some capacitance techniques are better suited to specific applications than others.