Datasheet

www.ti.com
CAPACITOR RECOMMENDATIONS for the PTH04070W WIDE-OUTPUT
Input Capacitor
Output Capacitors
Ceramic Capacitors
Tantalum Capacitors
Capacitor Table
PTH04070W
SLTS227B – SEPTEMBER 2004 – REVISED OCTOBER 2007
ADJUST POWER MODULES
The minimum recommended input capacitor(s) is 47- μ F of ceramic capacitance, in either an X5R or X7R
temperature tolerance. The ceramic capacitors should be located within 0.5 in. (1,27 cm) of the regulator's input
pins. Electrolytic capacitors can also be used at the input, but only in addition to the required ceramic
capacitance. The minimum ripple current rating for nonceramic capacitors should be at least 200 mA rms. The
ripple current rating of electrolytic capacitors is a major consideration when they are used at the input.
When specifying regular tantalum capacitors for use at the input, a minimum voltage rating of 2 × (maximum dc
voltage + ac ripple) is highly recommended. This is standard practice to ensure reliability. Polymer-tantalum
capacitors are not affected by this requirement.
For improved ripple reduction on the input bus, additional ceramic capacitors can be used to complement the
minimum requirement.
For most applications only one (1) 47- μ F ceramic capacitor is required. The ceramic capacitor should be located
within 0.5 in. (1,27 cm) of the output pin. Adding a second 47- μ F nonceramic capacitor allows the module to
meet its transient response specification. For applications with load transients (sudden changes in load current),
the regulator response benefits from additional external output capacitance. A high-quality computer-grade
electrolytic capacitor should be adequate.
Electrolytic capacitors should be located close to the load circuit. These capacitors provide decoupling over the
frequency range, 2 kHz to 150 kHz. Aluminum electrolytic capacitors are suitable for ambient temperatures
above 0 ° C. For operation below 0 ° C, tantalum or OS-CON type capacitors are recommended. When using one
or more nonceramic capacitors, the calculated equivalent ESR should be no lower than 4 m Ω (7 m Ω using the
manufacturer's maximum ESR for a single capacitor). A list of preferred low-ESR type capacitors are identified in
Table 3 , the recommended capacitor table.
Above 150 kHz the performance of aluminum electrolytic capacitors becomes less effective. To further improve
the reflected input ripple current, or the output transient response, multilayer ceramic capacitors must be added.
Ceramic capacitors have very low ESR and their resonant frequency is higher than the bandwidth of the
regulator. When placed at the output their combined ESR is not critical as long as the total value of ceramic
capacitance does not exceed 200 μ F. Also, to prevent the formation of local resonances, do not place more than
three identical ceramic capacitors with values of 10 μ F or greater in parallel.
Additional tantalum type capacitors can be used at both the input and output, and are recommended for
applications where the ambient operating temperature can be less than 0 ° C. The AVX TPS, Sprague
593D/594/595 and Kemet T495/T510/T520 capacitors series are suggested over many other tantalum types due
to their rated surge, power dissipation, and ripple current capability. As a caution, many general-purpose
tantalum capacitors have considerably higher ESR, reduced power dissipation, and lower ripple current
capability. These capacitors are also less reliable as they have lower power dissipation and surge current ratings.
Tantalum capacitors that do not have a stated ESR or surge current rating are not recommended for power
applications. When specifying OS-CON and polymer tantalum capacitors for the output, the minimum ESR limit is
encountered well before the maximum capacitance value is reached.
The capacitor table, Table 3 , identifies the characteristics of capacitors from a number of vendors with
acceptable ESR and ripple current (rms) ratings. The recommended 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 rating
and ESR (at 100 kHz) are critical parameters necessary to insure both optimum regulator performance and long
capacitor life.
Copyright © 2004 – 2007, Texas Instruments Incorporated Submit Documentation Feedback 9
Product Folder Link(s): PTH04070W