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Application Notes
PT6721/6722, & PT6725 Series
Capacitor Recommendations for the PT6721,
PT6722, and PT6725 Series of Regulators
Input Capacitors
The recommended input capacitance is determined by 1.6
ampere minimum ripple current rating and 1000F mini-
mum capacitance. Tantalum capacitors listed below cannot
be used on the input bus since they are not rated for 12V
operation. Ripple current and Equivalent Series Resistance
(ESR) values are the major considerations along with tem-
perature when selecting the proper capacitor.
Output Capacitors
The minimum required output capacitance is 330F with a
maximum ESR less than or equal to 50mW. Failure to ob-
serve this requirement may lead to regulator instability or
oscillation. Electrolytic capacitors have poor ripple per-
formance at frequencies greater than 400kHz, but excellentlow
frequency transient response. Above the ripple frequency ce-
ramic decoupling capacitors are necessary to improve the
transient response and reduce any microprocessor high
frequency noise components apparent during higher cur-
rent excursions. Preferred low ESR type capacitor part
numbers are identified in the Table 1 below.
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Note: (N/R)10Vtantalumsarenotrecommendfortheinputbus.
Table 1 Capacitors Characteristic Data
Tantalum Characteristics
Tantalum capacitors may be used on the output bus but
only the AVX TPS series, Sprague 593D/594/595 series,
or Kemet T495/T510 series. These capacitors are recom-
mended over many other tantalum types due to their high
surge current, excellent power dissipation and ripple
current ratings. 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 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 necessary to insure both optimum regulator
performance and long capacitor life.