Datasheet

Technical Note
28/31
BD5446EFV
www.rohm.com
2011.06 - Rev.C
© 2011 ROHM Co., Ltd. All rights reserved.
●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
6Ω 22µH 0.68µF 0.15µF 6Ω 15µH 0.47µF 0.1µF
8Ω 33µH 0.47µF 0.1µF 8Ω 22µH 0.33µF 0.068µF
16Ω 68µH 0.22µF 0.047µF 16Ω 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
Cg
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