Application Notes

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PT6650 Series

Table 1-1: Input/Output Capacitors

Notes:

[1]

N/R –Not recommended. The voltage rating does not meet the minimum operating limits.

[2]

A fused input bus is recommended when tantalum capacitors are used on the output bus.

Capacitor Recommendations for the

PT6650 Regulator Series

Input Capacitor:

The required input capacitor is determined by 560 µF

capacitance value, and a minimum ripple current rating of

1.2 Arms. The ripple current rating and less than 120 m

Ω

equivalent series resistance (ESR) are the major consid-

erations, along with operating temperature, when selecting

input capacitors.

Tantalum/Os-Con capacitors are not recommended due

to a minimum voltage rating of 2

× (the max. DC voltage

+ AC ripple). This is standard practice to ensure reliability.

Output Capacitors:

The ESR specification of the output capacitor should be

a minimum of 50 m

marginal ripple performance at frequencies greater than

400 kHz but excellent low frequency transient response.

Above the ripple frequency ceramic capacitors are

necessary to improve the transient response and reduce

any high-frequency noise components apparent during

higher current excursions. Electrolytic capacitors with

appropriate ESR values are identified in Table 1-1. In low-

temperature applications (<0 °C), a higher capacitance

with lower ESR will improve performance.

* Os-Con and ultra low ESR type capacitors are not recom-

mended on the output bus as they degrade regulator stability.

Tantalum type capacitors can be used on the output bus for

output voltages less than 5.1 V. Voltages higher than this

will exceed the capacitor’s published surge voltage limits.

If tantalum capacitors are located on the output bus, an

along with an external clamp component. An Output

Over-voltage Clamp (OOVC) will fault the output fuse

to protect the capacitors in event of an over-voltage con-

dition. The OOVC can be a simple zener high power

diode, 3 W to 5 W, located on the load side of the output

bus. The zener diode should be rated to 1.3 times the

normal output voltage.

Capacitor Table

Table 1-1 identifies the characteristics of capacitors from a

number of vendors with acceptable ESR and ripple

current (rms) ratings. The 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

(Equivalent Series Resistance at 100 kHz) are critical param-

eters necessary to insure both optimum regulator performance

and long capacitor life.

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