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Application Notes
For technical support and more information, see inside back cover or visit www.ti.com
PT7761, PT7769
Capacitor Recommendations for
the
PT7761 Integrated Switching Regulator
Input Capacitors
The recommended input capacitance is determined by
1.4 ampere minimum ripple current rating and 1500μF
minimum capacitance. Capacitors listed below must be
rated for a minimum of 2x the input voltage with +5V
operation. Ripple current and
≤
100m
Equivalent Series
Resistance (ESR) values are the major considerations along
with temperature when selecting the proper capacitor.
Output Capacitors
The minimum required output capacitance is 330μF with
a maximum ESR less than or equal to 100m
. Failure to
observe this requirement may lead to regulator instability or
oscillation. Electrolytic capacitors have poor ripple perfor-
mance at frequencies greater than 400kHz, but excellent low
frequency transient response. Above the ripple frequency
ceramic decoupling capacitors are necessary 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.
Note:
(N/R) is not recommended for this application, due to extremely low Equivalent Series Resistance (ESR)
Table 1 Capacitors Characteristic Data
Tantalum Characteristics
Tantalum capacitors with a minimum 10V rating are
recommended on the output bus, but only the AVX TPS
series, Sprague 594/595 series, or Kemet T495/T510
series. These AVX, Sprague, and Kemet capacitors are
specified 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 than the TPS series when deter-
mining 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 regu-
lator for both the input and output buses are identified.
This is not an extensive capacitor list. The table below is a
selection guide for input and output capacitors. Other capacitor
vendors are available with comparable RMS ripple current rating
and ESR (Equivalent Series Resistance at 100kHz). These
critical parameters are necessary to insure both optimum regulator
performance and long capacitor life.
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