Application Notes

For technical support and more information, see inside back cover or visit www.ti.com

PT6880 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

PT6880 Regulator Series

Input Capacitor:

The required input capacitor is determined by a 1.2Arms

minimum ripple current rating and 560µF capacitance

value. The ripple current rating and <120m

Ω equivalent

series resistance (ESR) are the major considerations, along

with temperature, when designing with different types of

capacitors.

Tantalum/Os-Con® capacitors are not recommended

due to a minimum voltage rating of 2

× (the maximum

DC voltage + AC ripple). This is necessary to improve

the reliability of these capacitors in high bus applications.

Output Capacitors:

The ESR specification of the output capacitor should be

at least 50m

Ω. Electrolytic capacitors have marginal

ripple performance at frequencies greater than 400kHz

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 5.1V. 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

protecting the capacitors in event of an over-voltage condi-

tion. The OOVC can be a simple zener high power di-

ode, 3-5W, located on the load side of the output bus. The

zener diode should be rated to 1.3 times the normal out-

put 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 100kHz) are critical parameters

necessary to insure both optimum regulator performance and

long capacitor life.

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