UG-192
Evaluation Board User Guide
Rev. 0 | Page 4 of 8
Input Coupling Capacitor Selection—C1 and C2
The input coupling capacitors, C1 and C2, should be large enough
to couple the low frequency signal components in the incoming
signal but small enough to filter out unnecessary low frequency
signals. For music signals, the cutoff frequency chosen is, typically,
between 20 Hz and 30 Hz. The value of the input capacitor is
calculated by
C
= 1/(2
π
Rf
c
)
where:
R
= 40 kΩ +
Rext
(the external resistor used to fine-tune the
desired gain; on the schematics (see Figure 3), this is the 0 Ω
resistor at the input pins).
f
c
is the cutoff frequency.
Output Ferrite Beads—L1 and L2
The output beads, L1 and L2, are necessary components for
filtering out the EMI caused at the switching output nodes when
the length of the speaker wire is greater than 10 cm. The penalty
for using ferrite beads for EMI filtering is slightly worse noise and
distortion performance at the system level due to the nonlinearity
of the beads.
Ensure that these beads have enough current-conducting capability
while providing sufficient EMI attenuation. The current rating
needed for an 8 Ω load is approximately 420 mA, and impedance
at 100 MHz should be ≥120 Ω. In addition, the lower the dc
resistance (DCR) of these beads is, the better for minimizing their
power consumption. Table 1 describes the recommended beads.
Output Shunting Capacitors
There are two output shunting capacitors, C6 and C7, that work
with the ferrite beads, L1 and L2. Use small size (0603 or 0402),
multilayer ceramic capacitors that are made of X7R or COG (NPO)
materials. Note that the capacitors can be used in pairs: a capacitor
with small capacitance (up to 100 pF) plus a capacitor with a
bigger capacitance (less than 1 nF). This configuration provides
thorough EMI reduction for the entire frequency spectrum. For
BOM cost reduction and capable performance, a single capacitor
of approximately 470 pF can be used.
Output Inductors
If using inductors for the purpose of EMI filtering at the output
nodes, choose inductance that is <2.2 μH for these inductors. The
higher the inductance is, the lower the EMI becomes at the output.
However, the cost and power consumption by the inductors are
higher. Using 0.47 μH to 2.2 μH inductors is recommended, and
the current rating needs >600 mA (saturation current) for an 8 Ω
load. Table 2 shows the recommended inductors. Note that these
inductors are not populated on the evaluation board.
GETTING STARTED
To ensure proper operation, carefully follow Step 1 through Step 7.
1.
If a jumper is across Pin 1 and Pin 2 of J2, remove the jumper
to enable the amplifier.
2.
Remove the jumper across Pin 3 and Pin 4 of J2 to ensure that
the device is in ALC mode. To put in standard 12 dB
configuration and disable ALC, insert a jumper across Pin 3
and Pin 4.
3.
For most audio quality testing, the EMI filtering (L1/L2 and
C6/C7) must be removed. Short across the L1 and L2
terminals to make a direct connection from the device output
to the J4 speaker header.
4.
Connect the load to the audio output connector, J4.
5.
Connect the audio input to the board in either differential
mode or single-ended mode, depending on the application.
6.
Connect the power supply with the proper polarity and
voltage.
7.
Turn R5 potentiometer to the desired V
TH
setting.
WHAT TO TEST
•
Electromagnetic interference (EMI)—connect wires for the
speakers, making sure that they are the same length as the
wires required for the actual application environment; then
complete the EMI test.
•
Signal-to-noise ratio.
•
Output noise—make sure to use an A-weighted filter to filter
the output before reading the measurement meter.
•
Maximum output power.
•
Distortion.
•
Efficiency.
Table 1. Recommended Output Beads
Part No.
Manufacturer
Z (Ω)
I
MAX
(mA)
DCR (Ω)
Size (mm)
BLM18PG121SN1D
Murata
120
2000
0.05
1.6 × 0.8 × 0.8
MPZ1608S101A TDK
100
3000
0.03
1.6 × 0.8 × 0.8
MPZ1608S221A TDK
220
2000
0.05
1.6 × 0.8 × 0.8
BLM18EG221SN1D
Murata
220
2000
0.05
1.6 × 0.8 × 0.8
Table 2. Recommended Output Inductors
Part No.
Manufacturer
L (μH)
I
MAX
(mA)
DCR (Ω)
Size (mm)
LQM31PNR47M00
Murata
0.47
1400
0.07
3.2 × 1.6 × 0.85
LQM31PN1R0M00
Murata
1.0
1200
0.12
3.2 × 1.6 × 0.85
LQM21PNR47MC0
Murata
0.47
1100
0.12
2.0 × 1.25 × 0.5
LQM21PN1R0MC0
Murata
1.0
800
0.19
2.0 × 1.25 × 0.5
LQH32CN2R2M53
Murata
2.2
790
0.1
3.2 × 2.5 × 1.55
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