Ceramic Capacitors

Tantalum, Polymer-Tantalum Capacitors

Capacitor Table

Designing for Fast Load Transients

PTH05T210W

Above 150 kHz the performance of aluminum electrolytic capacitors is less effective. Multilayer ceramic

capacitors have very low ESR and a resonant frequency higher than the bandwidth of the regulator. They can be

used to reduce the reflected ripple current at the input as well as improve the transient response of the output.

When used on the output their combined ESR is not critical as long as the total value of ceramic capacitors, with

values between 10 µF and 100 µF, does not exceed 5000 µF (non-TurboTrans). In TurboTrans applications,

when ceramic capacitors are used on the output bus, total capacitance including bulk and ceramic types is not to

exceed 12,000

µF.

Tantalum type capacitors are only used on the output bus, and are recommended for applications where the

ambient operating temperature is less than 0°C. The AVX TPS series and Kemet capacitor series are suggested

over many other tantalum types due to their higher rated surge, power dissipation, and ripple current capability.

As a caution, many general-purpose tantalum capacitors have higher ESR, reduced power dissipation, and lower

ripple current capability. These capacitors are also less reliable due to their reduced power dissipation and surge

current ratings. Tantalum capacitors that have no stated ESR or surge current rating are not recommended for

power applications.

Table 3 identifies the characteristics of capacitors from a number of vendors with acceptable ESR and ripple

current (rms) ratings. The recommended number of capacitors required at both the input and output buses is

identified for each capacitor type.

This is not an extensive capacitor list. Capacitors from other vendors are available with comparable

specifications. Those listed are for guidance. The RMS ripple current rating and ESR (at 100 kHz) are critical

parameters necessary to ensure both optimum regulator performance and long capacitor life.

The transient response of the dc/dc converter has been characterized using a load transient with a di/dt of

2.5A/µs. The typical voltage deviation for this load transient is given in the Electrical Characteristics table using

the minimum required value of output capacitance. As the di/dt of a transient is increased, the response of a

converter’s regulation circuit ultimately depends on its output capacitor decoupling network. This is an inherent

limitation with any dc/dc converter once the speed of the transient exceeds its bandwidth capability.

If the target application specifies a higher di/dt or lower voltage deviation, the requirement can only be met with

additional low ESR ceramic capacitor decoupling. Generally, with 50% load steps at > 100A/

µs, adding multiple

10

µF ceramic capacitors, 3225 case size, plus 10×1µF, including numerous high frequency ceramics (≤0.1µF)

are all that is required to soften the transient higher frequency edges. Special attention is essential with regards

to location, types, and position of higher frequency ceramic and lower ESR bulk capacitors. DSP, FPGA and

ASIC vendors identify types, location and capacitance required for optimum performance of the high frequency

devices. The details regarding the PCB layout and capacitor/component placement are important at these high

frequencies. Low impedance buses and unbroken PCB copper planes with components located as close to the

high frequency processor are essential for optimizing transient performance. In many instances additional

capacitors may be required to insure and minimize transient aberrations.

Copyright © 2007–2009, Texas Instruments Incorporated

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