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Notes
Table 1: Input/Output Capacitors
N/R –Not recommended. The voltage rating does not meet the minimum operating limits.
Capacitor Recommendations for
the
Dual-Output PT6940 Regulator Series
Input Capacitor:
The recommended input capacitance is determined by 1.0
ampere minimum ripple current rating and 330μF minimum
capacitance (300μF for Oscon or low ESR tantalum).
Ripple current and <100m
equivalent series resistance
(ESR) values are the major considerations, along with tem-
perature, when designing with different types of capacitors.
Tantalum capacitors have a recommended minimum voltage
rating of twice
the maximum DC voltage + AC ripple. This
is necessary to insure reliability for input voltage bus appli-
cations
Output Capacitors: Co
1
/Co
2
The ESR of the required capacitors, Co
1
& Co
2
must not be
greater than 150m
. Electrolytic capacitors have poor
ripple performance at frequencies greater than 400kHz
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 1.
PT6940 Series
Tantalum Capacitors
Tantalum type capacitors can be used for the output but only
the AVX TPS series, Sprague 593D/594/595 series 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 dissipa-
tion, and lower ripple current capability. The TAJ series is
less reliable than the AVX TPS series when determining
power dissipation capability. Tantalum or Oscon types
are recommended for applications where ambient tem-
peratures fall below 0°C.
Capacitor Table
Table 1 identifies the characteristics of capacitors from a
number of vendors with acceptable ESR and ripple current
(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
(Equivalent Series Resistance at 100kHz) are critical parameters
necessary to insure both optimum regulator performance and
long capacitor life.
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