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CAPACITOR RECOMMENDATIONS FOR THE PTH12050 SERIES OF POWER MODULES

Input Capacitor

Output Capacitors (Optional)

Ceramic Capacitors

Tantalum Capacitors

Capacitor Table

PTH12050W/L

SLTS214H – MAY 2003 – REVISED OCTOBER 2007

The recommended input capacitor(s) is determined by the 100 µF minimum capacitance and 750 mArms

minimum ripple current rating. A 10-µF X5R/X7R ceramic capacitor may also be added to reduce the reflected

input ripple current. This is recommended for output voltage set points of 3.3 V and higher.

Ripple current, less than 100 m

Ω equivalent series resistance (ESR) and temperature are major considerations

when selecting input capacitors. Unlike polymer-tantalum capacitors, regular tantalum capacitors have a

recommended minimum voltage rating of 2

× (maximum dc voltage + ac ripple). This is standard practice to

ensure reliability. Only a few tantalum capacitors have sufficient voltage rating to meet this requirement. At

temperatures below 0

°C, the ESR of aluminum electrolytic capacitors increases. For these applications, Os-Con

types , polymer-tantalum, and polymer-aluminum types should be considered.

For applications with load transients (sudden changes in load current), regulator response benefits from external

output capacitance. The value of 100 µF is used to define the transient response specification. For most

applications, a high quality computer-grade aluminum electrolytic capacitor is adequate. These capacitors

provide decoupling over the frequency range, 2 kHz to 150 kHz, and are suitable for ambient temperatures

above 0

°C. Below 0°C, tantalum, ceramic or Os-Con type capacitors are recommended. When using one or

more nonceramic capacitors, the calculated equivalent ESR should be no lower than 4 m

Ω (7 mΩ using the

manufacturer'fs maximum ESR for a single capacitor). A list of preferred low-ESR type capacitors are identified in

Table 4.

In addition to electrolytic capacitance, adding a 10-µF X5R/X7R ceramic capacitor to the output reduces the

output ripple voltage and improve the regulator's transient response. The measurement of both the output ripple

and transient response is also best achieved across a 10-µF ceramic capacitor.

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 capacitance

does not exceed 300 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.

Tantalum type capacitors are most suited for use on the output bus, and are recommended for applications

where the ambient operating temperature can be less than 0

°C. The AVX TPS, Sprague 593D/594/595 and

Kemet T495/ T510 capacitor series are suggested over other tantalum types due to their higher rated surge,

power dissipation, and ripple current capability. As a caution many general purpose tantalum capacitors have

considerably higher ESR, reduced power dissipation and lower ripple current capability. These capacitors are

also less reliable as they have no surge current rating. Tantalum capacitors that do not have a 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 are encountered

well before the maximum capacitance value is reached.

Table 1 identifies the characteristics of capacitors from 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 insure both optimum regulator performance and long capacitor life.

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