Application Notes
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Capacitor Recommendations for the PTH03000W,
Wide-Output Adjust Power Modules
Input Capacitor
The recommended input capacitor(s) is determined by
the 100 µF minimum capacitance and 300 mArms mini-
mum ripple current rating.
Ripple current, less than 300 mΩ equivalent series resis-
tance (ESR), and temperature are the major considerations
when selecting input capacitors. Unlike polymer tantalum,
regular tantalum capacitors have a recommended mini-
mum voltage rating of 2 × (maximum DC voltage + AC
ripple). This is standard practice to ensure reliability.
For improved ripple reduction on the input bus, ceramic
capacitors
[2]
may used to complement electrolytic types
and achieve the minimum required capacitance.
Output Capacitors (Optional)
For applications with load transients (sudden changes in
load current), regulator response will benefit from an
external output capacitance. The recommended output
capacitance of 100 µF will allow the module to meet
its transient response specification (see product data sheet).
For most applications, a high quality computer-grade
aluminum electrolytic capacitor is adequate. These capaci-
tors provide decoupling over the frequency range, 2 kHz
to 150 kHz, and are suitable for ambient temperatures
above 0 °C. For operation below 0 °C tantalum, ceramic
or Os-Con type capacitors are recommended. When using
one or more non-ceramic capacitors, the calculated equiva-
lent 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.
Ceramic Capacitors
Above 150 kHz the performance of aluminum electrolytic
capacitors becomes less effective. To further improve the
reflected input ripple current
[2]
or the output transient
response, multilayer ceramic capacitors can also be added.
Ceramic capacitors have very low ESR and their resonant
frequency is higher than the bandwidth of the regulator.
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 ce-
ramic capacitors in parallel with values of 10 µF or greater.
Tantalum Capacitors
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 capacitor series are suggested over many other
tantalum types due to their higher rated surge, power
dissipation, and ripple current capability. As a caution
many general purpose tantalum capacitors have consid-
erably 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 regu-
lation 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 voltage 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. For further guidance
consult the separate application note, “Selecting Output
Capacitors for PTH Products in High-Performance Applica-
tions.”
PTH03000W
