Application Notes

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Capacitor Recommendations for

the PTH03020 &

PTH05020 Series of Power Modules

Input Capacitor

The recommended input capacitor(s) is determined by

minimum ripple current rating.

Ripple current and <100 m

Ω equivalent series resistance

(ESR) values are the major considerations, along with

temperature, when designing with different types of

capacitors. Unlike polymer tantalum, conventional tan-

talum capacitors have a recommended minimum voltage

rating of 2

× (maximum DC voltage + AC ripple). This

is standard practice to insure reliability.

For improved ripple reduction on the input bus, ceramic

capacitors may be substituted for electrolytic types using

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 330 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 most suitable. These

capacitors provide adequate decoupling over the frequency

range, 2 kHz to 150 kHz, and are suitable when ambient

temperatures are above 0 °C. For operation below 0 °C,

tantalum, ceramic or Os-Con type capacitors are recom-

mended. 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.

Ceramic Capacitors

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 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 consider-

ably higher ESR, reduced power dissipation and lower

ripple current capability. These capacitors are also less

reliable when determining their power dissipation and

surge current capability. 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 2-1 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 100kHz) 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, 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.”

PTH03020W & PTH05020W