|Home Analog Devices Feedback Subscribe Archives 简体中文 日本語|
Low Dropout RegulatorsWhy the Choice of Bypass Capacitor Matters
Widely seen as a panacea for solving noise-related issues, capacitors deserve more respect. Designers often think that adding a few capacitors will solve most noise problems, but give little thought to parameters other than capacitance and voltage rating. Like all electronic components, however, capacitors are not perfect. Instead, they possess parasitic effective series resistance (ESR) and inductance (ESL); their capacitance varies with temperature and voltage; they are sensitive to mechanical effects.
Designers must consider these factors when selecting bypass capacitorsas well as for use in filters, integrators, timing circuits, and other applications where the actual capacitance value is important. An inappropriate choice can lead to circuit instability, excessive noise and power dissipation, shortened product life, and unpredictable circuit behavior.
In voltage regulators, three major classes of capacitors are commonly used as voltage input- and output bypass capacitors: multilayer ceramic, solid-tantalum electrolytic, and aluminum electrolytic. The Appendix provides a comparison.
Voltage-controlled oscillators (VCOs), phase-locked loops (PLLs), RF power amplifiers (PAs), and other analog circuits are sensitive to noise on their power-supply rails. This noise manifests itself as phase noise in VCOs and PLLs, amplitude modulation in RF PAs, and display artifacts in ultrasound, CT scans, and other applications that process low-level analog signals. Despite these imperfections, virtually every electronic device uses ceramic capacitors due to their small footprint and low cost. For regulators used in noise-sensitive applications, however, designers must carefully evaluate their side effects.
Conductive polymer tantalum capacitors with low ESR cost more and are somewhat larger than ceramic capacitors, but may be the only choice for applications that cannot tolerate noise due to piezoelectric effects. The leakage current of tantalum capacitors is much larger than for equal-value ceramic capacitors, however, rendering them unsuitable for some low-current applications.
A drawback of the solid polymer electrolyte technology is that this type of tantalum capacitor is more sensitive to the high temperatures encountered in the lead (Pb)-free soldering process, with manufacturers typically specifying that the capacitors not be exposed to more than three soldering cycles. Ignoring this requirement in the assembly process can cause long-term reliability issues.
Although the performance of the OS-CON type capacitor is better than that of conventional aluminum electrolytic capacitors, they tend to be larger and have higher ESR than ceramic or solid polymer tantalum capacitors. Like solid polymer tantalum capacitors, they do not suffer from the piezoelectric effect, so they are suitable for use in low-noise applications.
Capacitors for LDO Circuits
The output capacitance also affects the regulator's response to changes in load current. The control loop has finite large-signal bandwidth, so the output capacitor must supply most of the load current for very fast transients. When the load current switches from 1 mA to 200 mA at 500 mA/µs, a 1-µF capacitor, unable to supply enough current, produces a load transient of about 80 mV, as shown in Figure 1. Increasing the capacitance to 10 µF reduces the load transient to about 70 mV, as shown in Figure 2. Increasing the output capacitance further, to 20 µF, allows the regulator control loop to track, actively reducing the load transient as shown in Figure 3. These examples use the ADP151 linear regulator with a 5-V input and a 3.3-V output.
Figure 1. Transient response with COUT = 1 µF.
Figure 2. Transient response with COUT = 10 µF.
Figure 3. Transient response with COUT = 20 µF.
and Output Capacitor Properties
Figure 4 shows the capacitance vs. bias voltage characteristic of a 1-µF, 10-V X5R capacitor in a 0402 package. The capacitor's package size and voltage rating strongly influence its voltage stability. In general, a larger package or higher voltage rating will provide better voltage stability. The temperature variation of the X5R dielectric is ±15% over the 40°C to +85°C temperature range and is not a function of package or voltage rating.
Figure 4. Capacitance vs. voltage characteristic.
To determine the worst-case capacitance over temperature, component tolerance, and voltage, scale the nominal capacitance by the temperature variation and tolerance, as shown in Equation 1:
Where CBIAS is the nominal capacitance at the operating voltage; TVAR is the worst-case capacitance variation over temperature (as a fraction of 1); TOL is the worst-case component tolerance (as a fraction of 1).
In this example, TVAR is 15% from 40°C to +85°C for an X5R dielectric; TOL is 10%; CBIAS is 0.94 µF at 1.8 V, as shown in Figure 4. Using these values in Equation 1 yields:
CEFF = 0.94 µF × (1 0.15) × (1 0.1) = 0.719 µF
The ADP151 specifies a minimum output bypass capacitance of 0.70 µF over the operating voltage and temperature range, so this capacitor meets this requirement.
Figure A. Capacitors commonly used for bypassing power supply rails.
from top, scale in millimeters:
Comparison of Critical Parameters of Various Capacitor Technologies
Copyright 1995- Analog Devices, Inc. All rights reserved.