Datasheet
Application Notes
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Capacitor Recommendations for the PTH12000
Wide-Output Adjust Power Modules
Input Capacitor
The recommended input capacitance is determined by
the 100 μF minimum capacitance and 750 mArms mini-
mum ripple current rating. A 10-μF X5R/X7R ceramic
capacitor may also be added to reduce the reflected input
ripple current
[3]
. The ceramic capacitor should be located
between the input electrolytic and the module.
Ripple current, less than 150 mΩ equivalent series resis-
tance (ESR) and temperature are major considerations
when selecting input capacitors. Unlike polymer-tantalum
capacitors, regular tantalum capacitors have a recom-
mended minimum voltage rating of 2 × (max. DC voltage
+ AC ripple). This is standard practice to ensure reliability.
No tantalum capacitors were found with sufficient volt-
age rating to meet this requirement. At temperatures
below 0 °C, the ESR of aluminum electrolytic capacitors
increases. For these applications Os-Con, polymer-tan-
talum, and polymer-aluminum types should be considered.
Output Capacitors (Optional)
For applications with load transients (sudden changes in
load current), regulator response will benefit from external
output capacitance. The value of 100 μF is used to define
the transient response specification (see data sheet). For
most applications, a high quality computer-grade alumi-
num electrolytic capacitor is adequate. These capacitors
provide decoupling over the frequency range, 2 kHz to
150 kHz, and are suitable for ambient temperatures above
0 °C. Below 0 °C, tantalum, ceramic or Os-Con type
capacitors are recommended. When using one or more
non-ceramic capacitors, the calculated equivalent ESR
should be no lower than 4 mΩ (7 mΩ using the manufacturer’s
maximum ESR for a single capacitor). A list of preferred
low-ESR type capacitors are identified in Table 1-1.
In addition to electrolytic capacitance, adding a 10-μF
X5R/X7R ceramic capacitor to the output will reduce
the output ripple voltage and improve the regulator’s
transient response. The measurement of both the output
ripple and transient response is also best achieved across
a 10-μF ceramic capacitor.
Ceramic Capacitors
Above 150 kHz the performance of aluminum electrolytic
capacitors is less effective. Multilayer ceramic capacitors
have very low ESR and a resonant frequency higher than
the bandwidth of the regulator. They can be used to reduce
the reflected ripple current at the input
[3]
and improve
the transient response of the output. When used on the
output their combined ESR is not critical as long as the
total value of ceramic capacitance does not exceed 300 μF.
Also, to prevent the formation of local resonances, do not
place more than five identical ceramic capacitors in par-
allel with values of 10 μF or greater.
Tantalum Capacitors
Tantalum type capacitors are most suited for use on the
output bus, 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 capacitor series are suggested over other tantalum
types due to their higher 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 will be encoun-
tered well before the maximum capacitance value is
reached.
Capacitor Table
Table 1-1 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 ripple current rating and
ESR (at 100 kHz) are critical parameters necessary to insure
both optimum regulator performance and long capacitor life.
Designing for Very Fast Load Transients
The transient response of the DC/DC converter has been
characterized using a load transient with a di/dt of 1 A/μs.
The typical voltage deviation for this load transient is
given in the data sheet specification table using the
optional value of output capacitance. As the di/dt of a
transient is increased, the response of a converter’s
regulation circuit ultimately depends on its output
capacitor decoupling network. This is an inherent
limitation with any DC/DC converter once the speed of
the transient exceeds its bandwidth capability. If the
target application specifies a higher di/dt or lower volt-
age deviation, the requirement can only be met with
additional output capacitor decoupling. In these cases
special attention must be paid to the type, value and ESR
of the capacitors selected.
If the transient performance requirements exceed that
specified in the data sheet, the selection of output ca-
pacitors becomes more important.
PTH12000 Series