Application Notes

For technical support and more information, see inside back cover or visit www.ti.com/powertrends

PT7708/PT7709, PT7742/PT7743

Capacitor Recommendations for

the PT7708/09

Regulators and PT7742/43 Current Boosters

Input Capacitors

The recommended input capacitance is determined by

1.4 ampere minimum ripple current rating and 1500µF

minimum capacitance. Capacitors listed below must be

rated for a minimum of 2x the input voltage with +5V

operation. Ripple current and

≤100mΩ Equivalent Series

Resistance (ESR) values are the major considerations along

with temperature when selecting the proper capacitor.

Output Capacitors

The minimum required output capacitance is 330µF with

a maximum ESR less than or equal to 100m

Ω. Failure to

observe this requirement may lead to regulator instability

or oscillation. Electrolytic capacitors have poor ripple per-

formance at frequencies greater than 400kHz, but excellent

low frequency transient response. Above the ripple fre-

quency ceramic decoupling capacitors are necessary to

improve the transient response and reduce any microproces-

sor high frequency noise components apparent during

higher current excursions. Preferred low ESR type capaci-

tor part numbers are identified in the Table 1 below.

Note: (N/R) is not recommended for this application, due to extremely low Equivalent Series Resistance (ESR)

Table 1 Capacitors Characteristic Data

Tantalum Characteristics

Tantalum capacitors with a minimum 10V rating are

recommended on the output bus, but only the AVX TPS

Series, Sprague 594/595 Series, or Kemet T495/T510 Series.

The AVX TPS Series, Sprague Series or Kemet Series

capacitors are specified over other types due to their

higher surge current, excellent power dissipation and ripple

current ratings. As an example, the TAJ Series by AVX is

not recommended. This series exhibits considerably higher

ESR, reduced power dissipation and lower ripple current

capability. The TAJ Series is a less reliable compared to

the TPS series when determining power dissipation capability.

Capacitor Table

Table 1 identifies the characteristics of capacitors from a

number of vendors with acceptable ESR and ripple current

(rms) ratings. The suggested minimum quantities per regu-

lator for both the input and output buses are identified.

This is not an extensive capacitor list. The table below is a

selection guide for input and output capacitors. Other capacitor

vendors are available with comparable RMS ripple current rating

and ESR (Equivalent Series Resistance at 100kHz). These

critical parameters are necessary to insure both optimum regulator

performance and long capacitor life.

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