Application Notes
For technical support and more information, see inside back cover or visit www.ti.com
PT7708/09, PT7615, PT7742/PT7743
Capacitor Recommendations for
20-A
Programmable Switching Regulators
Input Capacitors
The recommended input capacitance is determined by the
1.4 ampere minimum ripple current rating and 1500F
minimum capacitance. Capacitors listed below must be
rated for a minimum of two times (2
×) the input voltage
with +5V operation. Ripple current and
≤100m ESR
(equivalent series resistance) values are the major consider-
ations along with temperature when selecting the proper
capacitor.
Output Capacitors
The minimum required output capacitance is 330F 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
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.
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 10-V 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 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 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 comparible specifications.
The RMS ripple current rating and ESR (Equivalent Series
Resistance at 100kHz) are the critical parameters necessary
to insure both optimum regulator performance and long
capacitor life.
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