EVAL-SSM2306
Rev. 0 | Page 4 of 8
COMPONENT SELECTION
Selecting the right components is the key to achieving the
performance required at the budgeted cost. The BOM for the
SSM2306 evaluation board is provided in Table 4.
Input Coupling Capacitors
The input coupling capacitors, C1, C2, C3, and C4, should be
large enough to couple the low frequency signal components in
the incoming signal and small enough to filter out unnecessary
low frequency signals. For music signals, the selected cutoff
frequency is often between 20 Hz and 30 Hz. The value of the
coupling capacitor is calculated as follows:
C
= 1/(2
π
×
R
×
f
c
)
where:
R
= 43 kΩ + R
EXT
.
f
c
is the cutoff frequency
.
Input Series Resistors
The input series resistors, R1, R2, R3, and R4, are not necessary
for amplifier operation and are needed only when special gain
values are required. Using resistors with too high a value increases
the input noise.
Output Beads
The output beads, B1, B2, B3, and B4, 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 600 mA, and impedance
at 100 MHz must be > 220 Ω. In addition, the lower the dc
resistance (DCR) of these beads, the better for minimizing their
power consumption. Table 2 describes the recommended bead.
Table 2. Recommended Output Bead
Part Number
Manu-
facturer
Z (Ω)
I
MAX
(mA)
DCR
(Ω)
Size (mm)
BLM18EG221SN1D Murata
220
2000
0.05
1.6 × 0.8 × 0.8
Output Shunting Capacitors for the Beads
There are two groups of output shunting capacitors for the
beads: C11 to C14 and C23 to C26. Capacitors C11 to C14 filter
out lower frequency EMI ≤250 MHz; Capacitors C23 to C26 filter
out higher frequency EMI >250 MHz. Use small size (0603 or
0402) multilayer ceramic capacitors made of X7R or C0G (NP0)
materials. The higher the value of these capacitors, the lower the
residual EMI level at the output, and the higher the quiescent
current at the power supply. The recommended values for
Capacitors C11 to C14 are 500 pF to 1 nF; the recommended
values for Capacitors C23 to C26 are 100 pF to 200 pF.
Output Inductors
Some users do not allow high frequency EMI in the system and
prefer using inductors to filter out high frequency components
at the output nodes. Choose inductance >2.2
μ
H for these
inductors. The higher the inductance, the lower the EMI at the
output and the lower the quiescent current at the power supply.
However, note that higher inductance increases the power
consumption of the inductors when the output power level is
high. Analog Devices, Inc., recommends using 2.2
μ
H to 10
μ
H
inductors with a current rating of >600 mA (saturation current)
for an 8 Ω load. Table 3 describes the recommended inductors.
Table 3. Recommended Output Inductors
Part Number
Manu-
facturer
L
(
μ
H)
I
MAX
(mA)
DCR
(Ω)
Size (mm)
LQH32CN4R7M53 Murata
4.7
650
0.15
3.2 × 2.5 × 1.55
LQH32CN3R3M53 Murata
3.3
710
0.12
3.2 × 2.5 × 1.55
LQH32CN2R2M53 Murata
2.2
790
0.1
3.2 × 2.5 × 1.55
SD3118-100-R Cooper
Bussmann
10
900
0.3
3.1 × 3.1 × 1.8
ELL4LM100M Panasonic 10
690
0.18
3.8 × 3.8 × 1.8
LBC2518T2R2M Taiyo
Yuden
2.2
630
0.169 2.5 × 1.8 × 1.8
1033AS-4R7M
Toko
4.7
680
0.31
3.8 × 3.8 × 1.0
PCB LAYOUT GUIDELINES
Correct PCB layout is critical in application designs to prevent
EMI from exceeding allowable limits and to ensure that the
amplifier chip operates below the temperature limit. Follow
these guidelines carefully when designing the PCB layout.
1.
Place nine vias onto the thermal pad of the amplifier. The
outer diameter of the vias should be 0.5 mm, and the inner
diameter should be 0.33 mm. If internal layers are available
in the PCB, allocate an area as large as possible to the ground
planes and connect these vias to the ground planes (see
Figure 3). If internal layers are available, allocate a specific
area as a heat sink; make sure to connect the vias
conducting heat to the internal layers.
TOP LAYER
INTERNAL AND/OR
BOTTOM LAYER
06
85
6-
00
3
Figure 3. Heat Sink Layout
2.
Place the EMI filtering beads (B1, B2, B3, and B4) as close to
the amplifier chip as possible. If you use output inductors
(L1, L2, L3, and L4) in the application design, place them
as close to the amplifier chip as possible.
