Application Notes

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

PT8120 Series & PT8139

Capacitor Recommendations for the

PT8120 Series of Programmable ISRs

Input Capacitors

The recommended input capacitor(s) is determined by

1.6 Arms minimum ripple current rating and 1,500µF

minimum capacitance. Ripple current and Equivalent

Series Resistance (ESR) values are the major considerations

along with temperature when selecting the proper capaci-

tor. The tantalum capacitors listed below cannot be used

on the input bus since they are not rated for 12V operation.

Output Capacitors

The minimum required output capacitance is 660µF

(organic/polymer), or 1,200 (aluminum electrolytic) with

a maximum ESR less than or equal to 50m

Ω. Failure to

observe this requirement may lead to regulator instabil-

ity or oscillation. Electrolytic capacitors have poor ripple

performance at frequencies greater than 400kHz, but

excellent low frequency transient response. Above the

ripple frequency ceramic decoupling capacitors are nec-

essary to improve the transient response and reduce any

microprocessor high frequency noise components apparent

during higher current excursions. Preferred low ESR type

capacitor part numbers are identified in the Table 1 below.

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Note: (N/R -Not recommended) The 10V-rated tantalum capacitors cannot be used on the input bus.

Table 1 Capacitors Characteristic Data

Tantalum Characteristics

Tantalum capacitors are recommended on the output bus

but only AVX TPS Series, Sprague 593D/594/595 Series,

or Kemet T495/T510,520 Series. These capacitors are

recommended over other types due to their higher surge

current, excellent power dissipation and ripple current

ratings. As a caution, the TAJ Series by AVX is not rec-

ommended. This series exhibits considerably higher ESR,

reduced power dissipation and lower ripple current capa-

bility. The TAJ Series is also 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

regulator for both the input and output buses are identified.

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 (Equivalent Series Resistance at 100kHz) are the

critical parameters are necessary to insure both optimum

regulator performance and long capacitor life.