Capacitors are often overlooked. They lack billions of transistors and do not employ the latest sub-micron manufacturing processes. In the minds of many engineers, a capacitor is merely two conductors with an isolating electrolyte in between. In short, they are considered one of the most basic electronic components.

Why Is Capacitor Selection Critical?

Engineers often address noise issues by adding a few capacitors. This is because they generally view capacitors as a “panacea” for noise-related problems, rarely considering parameters beyond capacitance and rated voltage. However, like other electronic components, capacitors have flaws, such as non-ideal characteristics like parasitic resistance, inductance, capacitance temperature drift, and voltage offset.

When selecting capacitors for many bypass applications or applications where the actual capacitance value is crucial, these factors must be considered. Improper capacitor selection can lead to circuit instability, excessive noise or power consumption, shortened product lifespan, and unpredictable circuit behavior.

Capacitor Technologies

Capacitors come in various sizes, rated voltages, and other characteristics to meet the specific requirements of different applications. Common dielectric materials include oil, paper, glass, air, mica, various polymer films, and metal oxides. Each dielectric offers a unique set of properties to meet the distinct needs of each application.

In voltage regulators, three major types of capacitors are commonly used as voltage input and output bypass capacitors: multilayer ceramic capacitors, solid-state tantalum electrolytic capacitors, and aluminum electrolytic capacitors.

Multilayer Ceramic Capacitors

Multilayer ceramic capacitors (MLCCs) are often the preferred choice for bypass capacitors due to their small size, low effective series resistance (ESR) and inductance (ESL), and wide operating temperature range.

However, they are not without flaws. Depending on the dielectric material used, the capacitance can shift significantly with temperature changes and AC/DC bias. Additionally, because the dielectric material in many ceramic capacitors is piezoelectric, vibrations or mechanical shocks can be converted into AC noise voltage across the capacitor. In most cases, this noise is typically in the microvolt range. However, in extreme cases, it can generate millivolt-level noise.

Applications such as VCOs, PLLs, RF PAs, and low-level analog signal chains are highly sensitive to noise on the power rail. This noise manifests as phase noise in VCOs and PLLs and as carrier amplitude modulation in RF PAs. In low-level signal chain applications like EEG, ultrasound, and CAT scan preamplifiers, noise can result in spurious noise in the output of these instruments. In all these noise-sensitive applications, MLCCs must be carefully evaluated.

When selecting ceramic capacitors, it is crucial to consider temperature and voltage effects. The section on MLCC selection discusses the process of determining the minimum capacitance value of a capacitor based on tolerance and DC bias characteristics.

Despite their drawbacks, ceramic capacitors offer the smallest size and most cost-effective solution for many applications, making them ubiquitous in almost every category of electronic device today.

Solid-State Tantalum Electrolytic Capacitors

Solid-state tantalum electrolytic capacitors offer the highest capacitance per unit volume (CV product). Only double-layer or supercapacitors have a higher CV product.

In the 1 μF range, ceramic capacitors are still smaller and have lower ESR than tantalum capacitors. However, solid-state tantalum capacitors are less susceptible to temperature, bias voltage, or vibration effects. Tantalum capacitors are several times more expensive than ceramic capacitors but are often the only viable option in low-noise applications where the piezoelectric effect is intolerable.

Traditional low-capacitance solid-state tantalum capacitors on the market often come in smaller cases, resulting in higher equivalent series resistance (ESR). High-capacitance (>68 μF) tantalum capacitors can have ESR values below 1 Ω but are generally larger in size.

A new type of tantalum capacitor has recently emerged on the market, using a conductive polymer electrolyte instead of the conventional solid manganese dioxide electrolyte. In the past, solid-state tantalum capacitors had limited surge current capabilities and required a series resistor to limit surge currents to safe levels. Conductive polymer tantalum capacitors are not subject to surge current limitations. Another advantage of this technology is lower capacitor ESR.

The leakage current of any tantalum capacitor is several times higher than that of an equivalent ceramic capacitor and may not be suitable for ultra-low current applications.

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

Several manufacturers offer conductive polymer tantalum capacitors with a 0805 case, 1 μF/25 V, and 500 mΩ ESR. Although larger than typical 1 μF ceramic capacitors in 0402 or 0603 cases, the 0805 size still represents a significant reduction in capacitor size for applications where low noise is the primary design goal, such as RF and PLL applications.

Because solid-state tantalum capacitors maintain stable capacitance characteristics relative to temperature and bias voltage, selection criteria only include tolerance, voltage derating over the operating temperature range, and maximum ESR.

A significant drawback of solid polymer electrolyte technology is that these tantalum capacitors are more susceptible to high temperatures during lead-free soldering processes. Generally, manufacturers specify that capacitors should not be exposed to more than three soldering cycles. Neglecting this requirement during assembly processes can lead to long-term reliability issues.

Aluminum Electrolytic Capacitors

Traditional aluminum electrolytic capacitors are often bulky, have high ESR and ESL, relatively high leakage current, and a limited lifespan (measured in thousands of hours).

OS-CON type capacitors represent a technology related to solid polymer tantalum capacitors and actually predate them by 10 years or more. They employ an organic semiconductor electrolyte and aluminum foil cathode to achieve low ESR. Because they do not suffer from the gradual drying out of liquid electrolytes, OS-CON type capacitors have a significantly improved lifespan compared to traditional aluminum electrolytic capacitors.

Currently available OS-CON type capacitors can withstand temperatures up to 125°C, although most remain at 105°C.

Although OS-CON type capacitors offer significantly improved performance over traditional aluminum electrolytic capacitors, they tend to be larger and have higher ESR compared to ceramic capacitors or solid polymer tantalum capacitors. Like solid polymer tantalum capacitors, they are not affected by the piezoelectric effect and are suitable for low-noise applications.

Multilayer Ceramic Capacitor Selection

Output Capacitor

The output capacitor also affects the transient response to changes in load current. Using a larger output capacitance value improves the transient response of an LDO to large changes in load current. Figures 1 to 3 show the transient response of the ADP151 with output capacitance values of 1 μF, 10 μF, and 20 μF, respectively.

Because the bandwidth of the LDO control loop is limited, the output capacitor must supply most of the load current required for rapid transients. A 1 μF capacitor cannot sustain current supply for long and produces a load transient of approximately 80 mV. A 10 μF capacitor reduces the load transient to approximately 70 mV. Increasing the output capacitance to 20 μF allows the LDO control loop to respond quickly and actively reduce the load transient. Test conditions are shown in Table 1.

Table 1. Test Conditions

Figure 1. Output Load Transient Response, COUT = 1 μF

Figure 2. Output Load Transient Response, COUT = 10 μF

Figure 3. Output Load Transient Response, COUT = 20 μF

Input Bypass Capacitor

Connecting a 1 µF capacitor between VIN and GND reduces the circuit’s sensitivity to PCB layout, especially in cases of long input traces or high source impedance. If an output capacitance greater than 1 μF is required, a higher input capacitance should be selected.

Input and Output Capacitor Characteristics

An LDO can use any high-quality capacitor as long as it meets the minimum capacitance and maximum ESR requirements. Ceramic capacitors can be manufactured using a variety of dielectrics, and their characteristics vary with temperature and applied voltage. The capacitor must have a dielectric sufficient to ensure minimum capacitance over the operating temperature range and DC bias conditions. It is recommended to use X5R or X7R dielectrics with voltage ratings of 6.3 V or 10 V in 5V applications. Y5V and Z5U dielectrics have poor temperature and DC bias characteristics and are not recommended.

Figure 4 shows the capacitance versus voltage bias characteristic for a 0402, 1 μF, 10 V, X5R capacitor. The voltage stability of the capacitor is greatly influenced by the capacitor package size and voltage rating. Generally, capacitors with larger packages or higher voltage ratings have better voltage stability. The temperature variation rate of the X5R dielectric is approximately ±15% over the temperature range of −40°C to +85°C and is not a function of package or voltage rating.

Figure 4. Capacitance vs. Voltage Bias Characteristic

When considering the variation of capacitance with temperature, component tolerance, and voltage, Equation 1 can be used to determine the worst-case capacitance.

Where:

CBIAS is the effective capacitance at the operating voltage.

TVAR is the worst-case capacitance variation with temperature (fraction).

TOL is the worst-case component tolerance (fraction).

In this example, assume the worst-case capacitance variation (TVAR) for an X5R dielectric over the range of −40°C to +85°C is 0.15 (15%). Assuming a capacitor tolerance (TOL) of 0.10 (10%), CBIAS is 0.94 μF at 1.8 V, as shown in Figure 4.

Substituting these values into Equation 1 yields:

In this example, the LDO specifies a minimum output bypass capacitance of 0.70 μF over the expected operating voltage and temperature range. Therefore, the capacitor selected for this application meets this requirement.

Conclusion

To ensure LDO performance, it is essential to understand and evaluate the impact of DC bias, temperature variation, and tolerance of bypass capacitors on the selected capacitor.

Additionally, in applications requiring low noise, low drift, or high signal integrity, capacitor technology must also be carefully considered. All capacitors are subject to non-ideal behavior, but some capacitor technologies are better suited for specific applications than others.

 

 

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