+V

IN

DCH01xxxxDN7

-V

IN

+V

OUT

COM

2.2

m

F

Ceramic

1

m

H to 5

m

H

4.7

m

F

Ceramic

+V

IN

-V

IN

4.7

m

F

1

m

H to 5

m

H

1

m

F

-V

OUT

4.7

m

F

1

m

F

1

m

H to 5

m

H

+V

OUT

-V

OUT

UDG-08076

π FILTERS

DCH01 START-UP

I

V

V

DCH01 Series

Figure 21. DCH01 Series

π Filter (5 V at 1 W)

If a further reduction in ripple/noise level is required for an application, higher order filters must be used. A

π (pi)

filter, employing a ferrite bead inductor in series with the input or output terminals of the regulator reduces the

ripple/noise by at least 20 db (see Figure 20 and Figure 21). Ceramic capacitors are required for the inductor to

be effective in reduction of ripple and noise.

These inductors plus ceramic capacitors form an excellent filter because of the rejection at the switching

frequency. The placement of this filter is critical. It must be located as close as possible to the input or output

pins to be efffective. The ferrite bead is small (12,5 mm x 3 mm), easy to use, low cost, and has low dc

resistance. Fair-Rite manufactures a surface-mount bead (part number 2773021447) or through hole (part

number 2673000701) rated to 5 A. Inductors with a value between 1

µH and 5 µH can be used in place of the

ferrite bead inductor.

Start-up waveforms.

Figure 22. Startup Waveforms

10

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