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Application Notes
Capacitor Recommendations for the PT6440
Excalibur 5V/3.3V Bus Step-Down ISRs
Input Capacitors
The recommended input capacitance is determined by
350 milli-amperes (rms) minimum ripple current rating
and 100μF minimum capacitance. Capacitors placed at
the input must be rated for a minimum of twice the input
voltage with +5V operation. Ripple current and
≤
200m
Equivalent Series Resistance (ESR) values are the major
considerations, along with temperature, when selecting
the proper input capacitor.
Output Capacitors
The ESR of the required 330μF output capacitor must
be less than or equal to 100m
. Failure to observe this
requirement may lead to regulator instability or oscillation.
Electrolytic capacitors have poor ripple performance at
frequencies greater than 300kHz but excellent low fre-
quency transient response. Above the ripple frequency,
ceramic decoupling capacitors are necessary to improve the
transient response and reduce any high frequency noise
components apparent during higher current excursions.
The preferred low ESR type capacitor part numbers are
identified in Table 1.
Tanatalum Capacitors
Tantalum capacitors are recommended on the output bus
but only the AVX TPS series, Sprague 593D/594/595
series or Kemet T495/T510 series. These capacitors are
specified over many other types due to their higher surge
current, power dissipation and ripple current capability.
As a caution, the TAJ Series by AVX is not recommended.
This series exhibits considerably higher ESR and lower
ripple current capability. The TAJ series is also less reliable
than the TPS series when determining power dissipation
capability. Tantalum or Oscon types are recommended
in applications where ambient temperatures fall below 0°C.
Capacitor Table
Table 1 identifies vendors with acceptable ESR and
maximum allowable 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 critical
parameters necessary to insure both optimum regulator perfor-
mance and long capacitor life.
PT6440 Series
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Table 1; Input/Output Capacitors