PTH04040W

SLTS238C – SEPTEMBER 2005 – REVISED JUNE 2010

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APPLICATION INFORMATION

CAPACITOR RECOMMENDATIONS FOR THE PTH04040W POWER MODULE

The PTH04040W uses state-of-the-art multi-phase power converter topology that employs multiple parallel

switching and filter inductor paths between the input and output capacitors. The PTH04040W uses three

switching paths. The three paths share the total 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. The ripple current,

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

INPUT CAPACITOR

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 940 mF (2 × 470 mF or

3 × 330 mF), with an RMS ripple current rating of 400 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 the converter via PCB power and ground planes. For high performance applications, or

wherever the transient capability of the input source is limited, 2,200 mF of input capacitance is recommended.

Ripple current and less than 100 m

Ω equivalent series resistance (ESR) values are the major considerations,

along with temperature, when designing with different types of capacitors. Unlike polymer tantalum, conventional

tantalum capacitors have a recommended minimum voltage rating of 2 × (maximum dc voltage + ac ripple). This

is standard practice to ensure reliability.

For improved ripple reduction on the input bus, ceramic capacitors may be used to complement electrolytic types

and achieve the minimum required capacitance.

OUTPUT CAPACITORS RECOMMENDED

In order to respond with load transients (sudden changes in load current) the regulator requires external output

capacitance. The minimum output capacitance is 660 mF (2 × 330 mF or 1 × 680 mF) with an ESR of at least

2 m

Ω. This output capacitance is required for the module to meet its transient response specification. For most

applications, a high quality computer grade aluminum electrolytic capacitor is suitable. These capacitors provide

adequate decoupling over the frequency range, 2 kHz to 150 kHz, and when ambient temperatures are above

0°C. For operation 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 m

Ω using the manufacturer’s maximum ESR for a single capacitor. A list of preferred low-ESR type capacitors

are identified in Table 1.

CERAMIC CAPACITORS

Above 150 kHz, the performance of aluminum electrolytic capacitors becomes less effective. To further improve,

the reflected input ripple current or the output transient response, multilayer ceramic capacitors can also be

added. Ceramic capacitors have very low ESR and their resonant frequency is higher than the bandwidth of the

regulator. When used on the output their combined ESR is not critical as long as the total value of ceramic

capacitance does not exceed 300 mF. Also, to prevent the formation of local resonances, do not place more than

five identical ceramic capacitors in parallel with values of 10 mF or greater.

TANTALUM CAPACITORS

Tantalum type capacitors can be used at both the input and output, 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 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

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

also less reliable when determining their power dissipation and surge current capability. 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 is encountered before the

maximum capacitance value is reached.

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