CAPACITOR RECOMMENDATIONS FOR THE PTH08T255W POWER MODULE

Capacitor Technologies

Input Capacitor (Required)

Input Capacitor Information

PTH08T255W

Electrolytic Capacitors

When using electrolytic capacitors, high quality, computer-grade electrolytic capacitors are recommended.

Aluminum electrolytic capacitors provide adequate decoupling over the frequency range, 2 kHz to 150 kHz,

and are suitable when ambient temperatures are above -20°C. For operation below -20°C, tantalum,

ceramic, or OS-CON type capacitors are required.

Ceramic Capacitors

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.

Tantalum, Polymer-Tantalum Capacitors

Tantalum type capacitors may only be 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 lower ESR, higher rated surge, power dissipation,

and ripple current capability. Tantalum capacitors that have no stated ESR or surge current rating are not

recommended for power applications.

The PTH08T255W requires a minimum input capacitance of 1000µF. The ripple current rating of the input

capacitor must be at least 750 mArms. An optional 22µF X5R/X7R ceramic capacitor is recommended to reduce

RMS ripple current.

The size and value of the input capacitor is determined by the converter’s transient performance capability. This

minimum value assumes that the converter is supplied with a responsive, low inductance input source. This

source should have ample capacitive decoupling, and be distributed to the converter via PCB power and ground

planes.

Ceramic capacitors should be located as close as possible to the module's input pins, within 0.5 inch (1,3 cm).

Adding ceramic capacitance is necessary to reduce the high-frequency ripple voltage at the module's input. This

reduces the magnitude of the ripple current through the electroytic capacitor, as well as the amount of ripple

current reflected back to the input source. Additional ceramic capacitors can be added to further reduce the RMS

ripple current requirement for the electrolytic capacitor.

The main considerations when selecting input capacitors are the RMS ripple current rating, temperature stability,

and less than 100 m

Ω of equivalent series resistance (ESR).

Regular tantalum capacitors are not recommended for the input bus. These capacitors require a recommended

minimum voltage rating of 2 × (maximum dc voltage + ac ripple). This is standard practice to ensure reliability.

No tantalum capacitors were found with a sufficient voltage rating to meet this requirement.

When the operating temperature is below 0°C, the ESR of aluminum electrolytic capacitors increases. For these

applications, OS-CON, poly-aluminum, and polymer-tantalum types should be considered.

Copyright © 2008, Texas Instruments Incorporated

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Product Folder Link(s): PTH08T255W