Technical Note
27/30
BD5446EFV
www.rohm.com
2010.05 - Rev.B
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●
Output LC Filter Circuit
An output filter is required to eliminate radio-frequency components exceeding the audio-frequency region supplied to a
load (speaker). Because this IC uses sampling clock frequencies from 200kHz to 400kHz in the output PWM signals, the
high-frequency components must be appropriately removed.
This section takes an example of an LC type LPF shown in Fig.29, in which coil L and capacitor C compose a differential
filter with an attenuation property of -12dB/oct. A large part of switching currents flow to capacitor C, and only a small part
of the currents flow to speaker
R
L
. This filter reduces unwanted emission this way. In addition, coil L and capacitor Cg
compose a filter against in-phase components, reducing unwanted emission further.
Filter constants depend on load impedances. The following are formulas to calculate values of
L, C,
and
Cg
when Q=0.707
is specified.
Fig. 29
R
L
: Load impedance (
Ω
)
f
C
: LPF cut off frequency (Hz)
Following presents output LC filter constants with typical load impedances.
f
C
= 30kHz
f
C
= 40kHz
R
L
L C Cg
R
L
L C Cg
6
Ω
22µH
0.68µF
0.15µF
6
Ω
15µH
0.47µF
0.1µF
8
Ω
33µH
0.47µF
0.1µF
8
Ω
22µH
0.33µF
0.068µF
16
Ω
68µH
0.22µF
0.047µF
16
Ω
47µH
0.15µF
0.033µF
Use coils with a low direct-current resistance and with a sufficient margin of allowable currents. A high direct-current
resistance causes power losses. In addition, select a closed magnetic circuit type product in normal cases to prevent
unwanted emission.
Use capacitors with a low equivalent series resistance, and good impedance characteristics at high frequency ranges
(100kHz or higher). Also, select an item with sufficient withstand voltage because flowing massive amount of
high-frequency currents is expected.
L
L
C
C
g
C
g
R
L
4 8 , 4 9
3 8 , 3 9
4 5 , 4 6
3 3 , 3 4
o r
o r
L
＝
R
L
2
4
π
f
C
H
C
＝
2
π
f
C
R
L
2
1
F
Cg
＝
0 . 2 C
F
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