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Application Notes
Table 2-1: Input/Output Capacitors
Capacitor Recommendations for the PT7670
Series of 30-A Switching Regulators
Input Capacitor:
The recommended input capacitor is determined by a
minimum of 1,500 F of capacitance with a ripple cur-
rent rating of 1.1 Arms.
Ripple current and <100 m
equivalent series resistance
(ESR) values are the major considerations, along with
temperature, when designing with different types of
capacitors. Tantalum capacitors have a recommended
minimum voltage rating of twice 2
× (the maximum
DC voltage + AC ripple). This is necessary to insure
reliability for input voltage bus applications.
Output Capacitors:
The ESR of the required capacitors is less than 100 m
.
Electrolytic capacitors have marginal ripple performance
at frequencies greater than 400 kHz 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 2-1.
PT7670 Series
Tantalum Capacitors( Optional Output Capacitors)
Tantalum type capacitors can be used for the output but
only the AVX TPS, Sprague 593D/594/595, 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
dissipation, and lower ripple current capability. The TAJ
series is also less reliable than the AVX TPS series when
determining power dissipation capability. Tantalum or
Oscon types are recommended for applications where
ambient temperatures fall below 0 °C.
Capacitor Table
Table 2-1 identifies the characteristics of capacitors from a
number of vendors with acceptable ESR and ripple cur-
rent (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
(at 100 kHz) are critical parameters necessary to insure both
optimum regulator performance and long capacitor life.
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