APPLICATION INFORMATION

Capacitor Recommendations for the PTH12040W Power Module

Input Capacitor

Output Capacitors

Ceramic Capacitors

PTH12040W

SLTS237G – DECEMBER 2004 – REVISED MARCH 2009............................................................................................................................................... www.ti.com

The PTH12040W is a state-of-the-art multi-phase power converter topology that uses three parallel switching and

filter inductor paths between the common input and output filter capacitors. The three paths share the load

current, operate at the same frequency, and are evenly displaced in phase.

With multiple switching paths the transient output current capability is significantly increased. This reduces the

amount of external output capacitance required to support a load transient. As a further benefit, the ripple

current, as seen by the input and output capacitors, is reduced in magnitude and effectively tripled in frequency.

The improved transient response of a multi-phase converter places a bigger burden on the transient capability of

the input source. The size and value of the input capacitor is therefore determined by this converter’s transient

performance capability. The minimum amount of input capacitance required is 560 µF, with an RMS ripple

current rating of 300 mA. 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. For high-performance applications, or wherever the transient

performance of the input source is limited, 1000 µF of input capacitance is recommended.

Ripple current, less than 100 m

Ω of equivalent series resistance (ESR), and temperature are the main

considerations when selecting input capacitors. The ripple current reflected from the input of the PTH12040W

module is moderate to low. Therefore any good quality, computer-grade electrolytic capacitor, of either value

suggested, has an adequate ripple current rating.

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. Adding one or two ceramic

capacitors to the input reduces high-frequency reflected ripple current.

The PTH12040W requires a minimum output capacitance of 660 µF (or 2 × 330 µF), with an ESR of 15 m

Ω to

40 m

Ω. This is necessary for the stable operation of the regulator. Additional capacitance can be added to

improve the module's performance to load transients. 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 0°C. For operation below 0°C, tantalum,

ceramic, or OS-CON type capacitors are necessary.

When using a combination of one or more nonceramic capacitors, the calculated equivalent ESR should be no

lower than 2 m

Ω (4 mΩ when calculating using the manufacturer’s maximum ESR values). A list of preferred

low-ESR type capacitors are identified in Table 1.

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 400 µF. Also, to prevent the formation of local resonances,

do not place more than five identical ceramic capacitors in parallel with values of 10 µF or greater.

8

Submit Documentation Feedback

Copyright © 2004–2009, Texas Instruments Incorporated

Product Folder Link(s): PTH12040W