pth12040 Astec Powe, pth12040 Datasheet - Page 3

• Manufacturer Part Number: pth12040
• Description: Pth12040 12vin Single
• Manufacturer: Astec Powe

2. System Interface Information
2.1 Input Capacitor
The improved transient response of a multi-phase converter places a
bigger burden on the transient capability of the input source. The
size and value of the input capacitor is therefore determined by this
converter’s transient performance capability. The minimum amount of
input capacitance required is 560µF
rating of 300mA. This minimum value assumes that the converter is
supplied with a responsive, low inductance input source. This source
should have ample capacitive decoupling and be distributed to the
converter via PCB power and ground planes. For high-performance
applications, or wherever the transient performance of the input
source is limited, 1,000µF of input capacitance is recommended.
Ripple current, less than 100mΩ equivalent series resistance (ESR),
and temperature are major considerations when selecting input
capacitors. The ripple current reflected from the input of the
PTH12040W module is moderate to low. Therefore any good quality,
computer-grade electrolytic capacitor, of either value suggested, will
have adequate ripple current rating.
Regular tantalum capacitors are not recommended for the input bus.
These capacitors require a recommended minimum voltage rating of
2 x (max. DC voltage + AC ripple). This is standard practice to
ensure reliability. No tantalum capacitors were found with a sufficient
voltage rating to meet this requirement
temperature is below 0°C, the ESR of aluminum electrolytic
capacitors increases. For these applications Os-Con, polyaluminum,
and polymer-tantalum types should be considered. Adding one or
two ceramic capacitors to the input will reduce high-frequency
reflected ripple current
2.2 Output Capacitance
For applications with load transients (sudden changes in load
current), regulator response will benefit from an external output
capacitance. The recommended output capacitance of 680µF will
allow the module to meet its transient response specification (refer to
the shortform datasheet). For most applications, a high quality
computer-grade aluminum electrolytic capacitor is most suitable.
These capacitors provide adequate decoupling over the frequency
range, 2kHz to 150kHz, and are suitable when ambient temperatures
are above 0ºC. For operation below 0ºC, 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 2mΩ (7mΩ using the manufacturer’s maximum ESR for a
single capacitor). A list of preferred low-ESR type capacitors are
identified in Table 2.
2.2.1 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 0ºC. 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 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 encountered
well before the maximum capacitance value is reached.
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, with an RMS ripple current
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. When the operating
2.2.2 Ceramic Capacitors
Above 150kHz 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 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, with values
between 10µF and 100µF, does not exceed 400µF. Also, to prevent
the formation of local resonances, do not place more than five
identical ceramic capacitors in parallel with values of 10µF or greater.
2.2.3 Capacitor Table
Table 2 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 100kHz) are
critical parameters necessary to insure both optimum regulator per-
formance and long capacitor life.
2.2.4 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 1A/µs. The typical
voltage deviation for this load transient is given in the datasheet
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
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 datasheet, or the total amount of load capacitance is above
3,000µF the selection of output capacitors becomes more important.
Application Note 193
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