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Optional Input/Output Filters
Input/Output Capacitors
PTN78060A
2
1,7
+
4
Vo
GND
Adjust
Vo
6
GND
+
C1
4.7 mF
Ceramic
C2
100
F
Electrolytic
(Required)
C3(1)
3 y 4.7 mF
Ceramic
(Required)
RSET
C4 (2)
100
F
(Required)
C5
4.7
F
Ceramic
VI
UDG05092
C6
4.7
F
Ceramic
π Filters
SLTS245B – APRIL 2005 – REVISED AUGUST 2006
Power modules include internal input and output ceramic capacitors in all of their designs. However, some
applications require much lower levels of either input reflected or output ripple/noise. This application describes
various filters and design techniques found to be successful in reducing both input and output ripple/noise.
The easiest way to reduce output ripple and noise is to add one or more 1-F ceramic capacitors, such as C5
shown in
Figure 20. Ceramic capacitors should be placed close to the output power terminals. A single 4.7-F
capacitor reduces the output ripple/noise by 10% to 30% for modules with a rated output current of less than 3
A. (Note: C4 is required to improve the regulators transient response, and does not reduce output ripple and
noise.)
Switching regulators draw current from the input line in pulses at their operating frequency. The amount of
reflected (input) ripple/noise generated is directly proportional to the equivalent source impedance of the power
source including the impedance of any input lines. The addition of C1,
≥4.7-F (2 ×2.2 F) ceramic capacitor,
near the input power pins, reduces reflected conducted ripple/noise by up to 20%.
(1)
See the specifications for required value and type.
(2)
See the Application Information section for suggested value and type.
Figure 20. Adding High-Frequency Bypass Capacitors to the Input and Output
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 (Fair-Rite Pt. No. 2673000701 or equivalent) in series with the input or output
terminals of the regulator reduces the ripple/noise by at least 20 db (see
Figure 21 and
Figure 22). In order for
the inductor to be effective ceramic capacitors are also required. (See the Capacitor Recommendations for
additional information on vendors and component suggestions.)
These inductors plus ceramic capacitors form an excellent filter because of the rejection at the switching
frequency (650 kHz - 1 MHz). The placement of this filter is critical. It must be located as close as possible to the
input or output pins to be effective. The ferrite bead is small (12,5 mm × 3 mm), easy to use, low cost, and has
low dc resistance. Fair-Rite also manufactures a surface-mount bead (part number 2773021447). It is rated to 5
A, and can be used on the output bus. As an alternative, suitably rated 1-H to 5-H wound inductors can be
used in place of the ferrite inductor bead.
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