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Application Notes

Table 2-1: Input/Output Capacitors

Capacitor Recommendations for the PT7670

Series of 30-A Switching Regulators

Input Capacitor:

The recommended input capacitor is determined by a

minimum of 1,500 µF of capacitance with a ripple cur-

rent rating of 1.1 Arms.

Ripple current and <100 m

Ω equivalent series resistance

(ESR) values are the major considerations, along with

temperature, when designing with different types of

capacitors. Tantalum capacitors have a recommended

minimum voltage rating of twice 2

× (the maximum

DC voltage + AC ripple). This is necessary to insure

reliability for input voltage bus applications.

Output Capacitors:

The ESR of the required capacitors is less than 100 m

Ω.

Electrolytic capacitors have marginal ripple performance

at frequencies greater than 400 kHz but excellent low

frequency transient response. Above the ripple frequency,

ceramic capacitors are necessary to improve the transient

response and reduce any high frequency noise components

apparent during higher current excursions. Preferred

low ESR type capacitor part numbers are identified in

Table 2-1.

PT7670 Series

Tantalum Capacitors( Optional Output Capacitors)

Tantalum type capacitors can be used for the output but

only the AVX TPS, Sprague 593D/594/595, or Kemet

T495/T510 series. These capacitors are recommended

over many other tantalum types due to their higher rated

surge, power dissipation, and ripple current capability.

As a caution the TAJ series by AVX is not recommended.

This series has considerably higher ESR, reduced power

dissipation, and lower ripple current capability. The TAJ

series is also less reliable than the AVX TPS series when

determining power dissipation capability. Tantalum or

Oscon® types are recommended for applications where

ambient temperatures fall below 0 °C.

Capacitor Table

Table 2-1 identifies the characteristics of capacitors from a

number of vendors with acceptable ESR and ripple cur-

rent (rms) ratings. The 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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