
Application Notes
For technical support and more information, see inside back cover or visit www.ti.com
PT6721/22 & PT6725 Series
Capacitor Recommendations for the 12V-Input
PT6721/22 and PT6725 Series of ISRs
Input Capacitors
The recommended input capacitor(s) is determined by
1.6Arms minimum ripple current rating and 1,000F mini-
mum capacitance. Ripple current and Equivalent Series
Resistance (ESR) values are the major considerations along
with temperature when selecting the proper capacitor.
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 330F with a
maximum ESR less than or equal to 50m
. Failure to
observe this requirement may lead to regulator instability
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 Series. These capacitors are recom-
mended 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 recommended. This
series exhibits considerably higher ESR, reduced power
dissipation and lower ripple current capability. 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 cur-
rent (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.