Application Notes

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PT4840 Input/ Output Filter Capacitance Selection

for Excalibur™ Triple-Ouput DC/DC Converters

General Requirements

The capacitors on the input bus are optional but may be

required to insure dynamic response to load transients.

The suggested capacitors on the input bus include

ceramic noise attenuation components. The PT4840

series has an internal 1µH input inductor. This inductor

provides an effective input differential noise filter when

1µF ceramic capacitors are connected across the input

terminals. This low impedance filter has an attenuation

factor of typically 15dB.

The output capacitors are all optional and may be used

to optimize dynamic and transient load performance. The

maximum capacitance allowed for each bus is given in the

electrical specification table on p.3.

Input Capacitors

The input capacitors are all optional. The 33µF/100V

electrolytic capacitor should have a minimum ripple

current 400mA. Ripple current and <350m

Ω equivalent

series resistance (ESR) are the major considerations along

with temperature when selecting electrolytic capacitors.

The ceramic capacitors, each 1µF (3

×0.33µF), X7R type,

have a wide temperature range and are suggested for

noise reduction. The module’s internal inductor and the

external ceramic capacitors form an excellent differential

will reject common-mode input noise (See Figure 1). A

common-mode choke can also be used to reduce common-

mode noise levels. These optional filter components may

be required to meet FCC Class A limits for conducted

emissions.

PT4840 Series

Figure 1; Optional Capacitor and FIlter Components

IC Chip Set

+V

IN

– V

IN

+15VDC

+3.3VDC

+1.5VDC

PT4841

1

2

11

15 , 16

22 , 23

18 – 20

12 , 13

3

Vo

1

COM

COM

Vo

2

Vo

3

+V

IN

– V

IN

EN 1

Vo

Vo

17

21

C

OUT 1

C

OUT 3

C

OUT 2

4

EN 2

L1

1 µ H

Optional

L1

1 µ H

Optional

C

Filter

0.33 µ Fx3

Ceramic

C

IN

C

Filter

0.33 µ Fx3

Ceramic

must be

≥40mΩ. Electrolytic capacitors have minimal

effect on ripple at frequencies greater than 200kHz but

excellent low-frequency transient response. At the ripple

frequency, ceramic decoupling capacitors improve the re-

sponse to fast transients and reduce any high-frequency

noise components during higher current excursions.

The preferred capacitor part numbers are identified in the

Table 1. The table identifies vendors with acceptable

ESR and ripple current (rms) ratings. The suggested

capacitors, rated 30V or greater with an equivalent ESR

of

The combined ESR of the output capacitors selected for

ESR electrolytic capacitors have minimal effect on ripple

at frequencies greater than 200kHz but excellent low-

frequency transient response. Above the ripple frequency,

ceramic decoupling capacitors improve the response to

fast transients and reduce any high-frequency noise

components during higher current excursions. The

preferred lower ESR type capacitor part numbers for

vendors with acceptable ESR and ripple current (rms)

ratings. The tantalum and Oscon® type capacitors have

both low ESR and stable characteristics. These are

temperature range extends below 0°C.

Capacitors from other vendors may also be suitable. The speci-

fications in both tables provide guidance for the selection of

alternative parts. The ripple current (rms) rating and ESR

(Equivalent Series Resistance at 100kHz) are the critical

parameters necessary to insure both optimum regulator and

long term capacitor life cycles.