Application Notes

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PTH12050 Series

Capacitor Recommendations for

the PTH12050

Series of Power Modules

Input Capacitor

The recommended input capacitor(s) 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 in-

put ripple current. This is recommended for output

voltage set points of 3.3 V and higher.

Ripple current, less than 100 m

Ω 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.

Only a few tantalum capacitors have sufficient voltage

rating to meet this requirement. At temperatures below

0 °C, the ESR of aluminum electrolytic capacitors in-

creases. For these applications Os-Con, polymer-tantalum,

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 330 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 m

Ω (7 mΩ using the manufacturer’s

maximum ESR for a single capacitor). A list of preferred

low-ESR type capacitors are identified in Table 2-1.

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 as well as 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 no surge current rating. 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, or the total amount of load

capacitance is above 3,000 µF, the selection of output

capacitors becomes more important. For further guidance

consult the separate application note, “Selecting Output

Capacitors for PTH Products in High-Performance Applica-

tions.”