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APPLICATION INFORMATION

CAPACITOR RECOMMENDATIONS FOR THE PTH08T240W POWER MODULE

Input Capacitor (Required)

TurboTrans Output Capacitor

Non-TurboTrans Output Capacitor

PTH08T240W

SLTS264A–NOVEMBER 2005–REVISED MARCH 2006

mArms. The size, type and value of 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.

For high-performance applications, or wherever the input source performance is degraded, 470 µF of input

capacitance is recommended. The additional input capacitance above the minimum level insures an optimized

performance.

Ripple current (rms) rating, less than 100 mΩ of equivalent series resistance (ESR), and temperature are the

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

PTH08T240W module is moderate to low. Therefore any good quality, computer-grade electrolytic capacitor will

have 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 attenuates high-frequency reflected ripple current.

The PTH08T240W requires a minimum output capacitance of 220 µF. The required capacitance above 220 µF

will be determined by actual transient deviation requirements.

TurboTrans allows the designer to optimize the capacitance load according to the system transient design

requirement. High quality, ultra-low ESR capacitors are required to maximize TurboTrans effectiveness.

Capacitors with a capacitance (µF) × ESR (mΩ) product of ≤ 10,000 mΩ×µF are required.

Working Example:

A bank of 6 identical capacitors, each with a capacitance of 330 µF and 5 mΩ ESR, has a C × ESR product of

1650 µFxmΩ (330 µF × 5 mΩ).

Using TurboTrans in conjunction with the high quality capacitors (capacitance (µF) × ESR (mΩ)) reduces the

overall capacitance requirement while meeting the minimum transient amplitude level.

Note: See the TurboTrans Technology Application Notes within this document for selection of specific

capacitance.

The PTH08T240W requires a minimum output capacitance of 220 µF. Non-TurboTrans applications must

observe minimum output capacitance ESR limits.

A combination of 200 µF of ceramic capacitors plus low ESR (15 mΩ to 30 mΩ) Os-Con electrolytic/tantalum

type capacitors can be used. When using Polymer tantalum types, tantalum type, or Oscon types only, the

capacitor ESR bank limit is 3 mΩ to 5 mΩ. (Note: no ceramic capacitors are required). 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 -20°C. For operation below -20°C, tantalum, ceramic, or Os-Con type

capacitors are necessary.

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