www.ti.com

Tantalum Capacitors

Capacitor Table

Designing for Fast Load Transients

PTV08040W

SLTS257A – SEPTEMBER 2005 – REVISED JANUARY 2007

APPLICATION INFORMATION (continued)

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, Sprague 593D/594/595, and Kemet T495/T510

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.

When specifying Os-con and polymer-tantalum capacitors for the output, the minimum ESR limit is encountered

before the maximum capacitance value is reached.

Table 1 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

1 A/µs. The typical voltage deviation for this load transient is given in the data sheet specification 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

output capacitor decoupling. In these cases special attention must be paid to the type, value and ESR of the

capacitors selected.

8

Submit Documentation Feedback