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CAPACITOR RECOMMENDATIONS FOR THE PTN78020 WIDE-OUTPUT

Input Capacitor

Output Capacitor

Ceramic Capacitors

Tantalum Capacitors

Capacitor Table

PTN78020A

SLTS244A – APRIL 2005 – REVISED MAY 2005

ADJUST POWER MODULES

The minimum requirement for the input capacitance is 18.8-µF (4× 4.7-µF) ceramic capacitance, in either a X5R

or X7R temperature characteristic, and 100-µF of nonceramic capacitance. Ceramic capacitors should be located

within 0.5 inch (1,27 cm) of the regulator's input pins. Electrolytic capacitors must be used at the input in addition

to the required ceramic capacitance. The minimum ripple current rating for any nonceramic capacitance must be

at least 750 mA rms for output currents of 3 A or less. The ripple current rating of electrolytic capacitors is a

major consideration when they are used at the input. This ripple current requirement can be reduced by placing

more ceramic capacitors at the input, in addition to the minimum required 18.8-µF.

Tantalum capacitors are not recommended for use at the input bus, as none were found to meet the minimum

voltage rating of 2 × (maximum dc voltage + ac ripple). This voltage derating is standard practice for regular

tantalum capacitors to ensure reliability. Polymer-tantalum capacitors are more reliable, and are available with a

maximum rating of typically 20 V. These can be used with input voltages up to 16 V.

The minimum electrolytic capacitance required to ensure stability is a 100-µF capacitor. Either ceramic or

electrolytic-type capacitors can be used. The minimum ripple current rating for the nonceramic capacitance must

be at least 250 mA rms. The stability of the module and voltage tolerances is compromised if the capacitor is not

placed near the output bus pins. A high-quality, computer-grade electrolytic capacitor should be adequate. A

ceramic capacitor can be also be located within 0.5 inch (1,27 cm) of the output pin.

For applications with load transients (sudden changes in load current), the regulator response improves with

additional capacitance. Additional electrolytic capacitors should be located close to the load circuit. These

capacitors provide decoupling over the frequency range, 2 kHz to 150 kHz. Aluminum electrolytic capacitors are

suitable for ambient temperatures above 0°C. For operation below 0°C, tantalum or Os-Con-type capacitors are

recommended. When using one or more nonceramic capacitors, the calculated equivalent ESR should be no

lower than 10 m

Ω (17 mΩ using the manufacturer's maximum ESR for a single capacitor). A list of recommended

capacitors and vendors are identified in Table 3.

Above 150 kHz, the performance of aluminum electrolytic capacitors becomes less effective. To further reduce

the reflected input ripple current or the output transient response, multilayer ceramic capacitors must be added.

Ceramic capacitors have low ESR, and their resonant frequency is higher than the bandwidth of the regulator.

When placed at the output, their combined ESR is not critical as long as the total value of ceramic capacitance

does not exceed 200 µF.

Tantalum-type capacitors may be used at the 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/T520 capacitors series are suggested over many other tantalum types due to their 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 is encountered well before the

maximum capacitance value is reached.

The capacitor table, Table 3, identifies the characteristics of capacitors from 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 rating and ESR (at 100 kHz) are

critical parameters necessary to ensure both optimum regulator performance and long capacitor life.

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