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Low ESR Tantalum Capacitors Make a Difference
in Circuit Designs
Application Note
www.vishay.com
Vishay Sprague
APPLICATION NOTE
Revision: 06-Sep-2019
2
Document Number: 40256
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BULK CAPACITANCE
In addition to maximum working voltage and voltage derating, an important characteristic of any capacitor is its ability to store
an electrical charge. Some applications require the capacitor to store large amounts of charge.
Solid tantalum devices are well-suited for bulk energy storage due to their high and stable capacitance values and are widely
used to hold up voltage rails during times of peak current demand. Here, two factors must be considered. The first is the total
capacitance required to supply the required energy for the necessary time. In some cases, a single tantalum capacitor is
sufficient, but in more demanding applications, multiple capacitors may be configured in parallel so that their capacitance
values are cumulative and the combined resistance of the array is reduced. The second factor is the ESR of the capacitors.
Lower ESR results in higher deliverable current levels and lower voltage drop during discharge for improved circuit performance.
An example is the bulk capacitance required on a 3.3 V rail that supplies power to a microprocessor. During turn-on, and in the
application when processing demands are high, the microprocessor will have high current demands that must be satisfied. The
ability of the capacitors to effectively deliver this bulk energy is often characterized with a specification called the “slew rate,”
which is defined as “idle current” to “peak current” with a specified slope “A/μs”. During times of peak demand, it may be
necessary to keep the rail voltage within a required specification range (for example, a drop of less than 10% = 0.33 V).
Capacitors with lower ESR can provide higher discharge currents with reduced heat generation to meet these demands more
quickly and efficiently.
Designers may actually place several lower-capacitance multilayer ceramic capacitors (MLCCs) close to the processor for
short-term current supply and then add larger bulk storage capacitors (tantalum, polymer, or aluminum electrolytic devices)
slightly farther away to meet longer-term current demands. The ultra low ESR of the MLCCs allows for very high instantaneous
currents, but their limited capacitance means that they are able to supply the required current for only a short period of time.
After that time, the bulk capacitors can take over and meet required circuit demands. When lower ESR tantalum capacitors are
used in this application, there is less reliance on the MLCCs and fewer of them are required. This saves PCB board space as
well as component costs. They also improve board-level reliability with their more rugged construction.
Low ESR capacitors are available in a number of case sizes
Another advantage of using low ESR tantalum devices as bulk energy capacitors is reduced heat generation during charge /
discharge cycles. This improves circuit power efficiency and results in a lower operational temperature for the circuit. It may
also allow for the use of smaller power supplies for further cost savings.
WAVEFORM FILTERING
When low-ESR capacitors are used for smoothing a signal, they reduce the amount of ripple current that appears on the DC
bus, such as output filter capacitors in switch mode power supplies (SMPS). This is accomplished by allowing for higher
charge/discharge currents to better follow the voltage cycles and supply energy during any peaks and valleys in the waveform.
As the ripple current (peak to peak) is reduced, less heat is dissipated on each charge/discharge cycle. Lower ESR (and lower
inductance) capacitors also allow ripple filter capacitors to be used effectively in circuits with higher frequency AC noise
components.