BM28723AMUV
Selection of Components Externally Connected
1 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 LSI uses sampling frequency 384kHz (fS=48kHz) in the output PWM signals, the high-frequency
components must be appropriately removed.
This section takes an example of an LC type LPF shown below, in which coil L and capacitor Cg compose a differential filter
with an attenuation property of -12dB/oct.
L
OUT1P
OUT2P
Cg
RL
Cg
OUT1N
OUT2N
L
Figure 73. Output LC Filter
Following presents output LC filter constants with typical load impedances.
RL
L
Cg
Note
4Ω
10μH
10μH
15μH
10μH
15μH
1μF
VCCP1, VCCP2≤14V
VCCP1, VCCP2≤22V
VCCP1, VCCP2>22V
VCCP1, VCCP2≤22V
VCCP1, VCCP2>22V
0.68μF
0.47μF
0.47μF
0.47μF
6Ω
8Ω
Use coils with a low direct-current resistance and a sufficient margin of allowable currents. In addition, select a closed
magnetic circuit type product in normal cases to prevent unwanted emission. A high direct-current resistance causes power
losses.
When the short-circuit of the speaker output (After the LC filter) to VCC or GND occurs when the coil with small rated DC
current is used, LSI destruction might be caused. Because thecoil causes the magnetic saturation behavior, it instantaneously
passes the heavy-current to LSI.
When using at VCCP1, VCCP2>22V, the coil of the rated DC current: 7.2A or more will be recommended.
And, fCL (LC resonance frequency) of the LC filter should be lowered and decrease the influence of LC resonance.
Use capacitors with low equivalent series resistance and good impedance characteristics at high frequency ranges.
Also, select the parts with the margin of the ratings enough.
2 The Value of the LC Filter Circuit Computed Equation
The output LC filter circuit of BM28723AMUV is as it is shown in Figure 74. The LC filter circuit of Figure 74 is thought to
substitute it like Figure 75 on the occasion of the computation of the value of the LC filter circuit.
L
OUT1P
OUT2P
L
OUT1P
Cg
Cg
OUT1N
OUT2P
OUT2N
R=RL/2
RL
Cg
OUT1N
OUT2N
L
Figure 74. Output LC Filter 1
Figure 75. Output LC Filter 2
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