EVAL-SSM2317-MINI
Rev. 0 | Page 3 of 8
EVALUATION BOARD HARDWARE
INPUT AND OUTPUT CONFIGURATION
On the bottom side of the board, there are two pad terminals,
IN+ and IN−, as shown in Figure 5. They are used to feed the
audio signal into the board. The two output terminals, marked
OUT+ and OUT− in Figure 5, drive a loudspeaker whose
impedance should not be less than 4 Ω.
Although the SSM2317 does not require external LC output
filters to operate because it has a low noise modulation scheme,
if the speaker length is >10 cm, it is recommended to put a
ferrite bead (L1 and L2) near each output pin of the SSM2317 to
reduce electromagnetic interference (EMI), as shown in the
schematic in Figure 3. For optimal performance, as specified
in the SSM2317 data sheet (in particular, for THD and SNR),
remove the entire EMI filter, short across the ferrite bead
terminals, and open the capacitor terminals.
COMPONENT SELECTION
Selecting the right components is the key to achieving the
performance required at the budgeted cost.
ALC Threshold Setting Resistor—R3
The maximum output amplitude threshold (V
TH
) during the
limiting operation can be changed from 90% to 45% of V
DD
by
inserting an external resistor, R
TH
, between the VTH pin and
GND. Shorting the VTH pin to GND sets V
TH
to 90% of V
DD
.
Leaving the VTH pin unconnected sets V
TH
to 45% of V
DD
. The
relation of R
TH
to V
TH
is shown by the following equation:
DD
TH
TH
TH
V
R
R
V
×
×
+
+
×
=
2
kΩ
50
kΩ
50
9
.
0
Maximum output power is derived from V
TH
by the following
equation:
SP
TH
OUT
R
V
P
2
2
⎟
⎠
⎞
⎜
⎝
⎛
=
where
R
SP
is the speaker impedance.
Input Coupling Capacitor Selection—C1 and C2
The input coupling capacitors, C1 and C2, should be large
enough to couple the low frequency components in the
incoming signal but small enough to filter out unnecessary
lower frequency signals. For music signals, the cutoff frequency
is, typically, between 20 Hz and 30 Hz.
The cutoff frequency is calculated by
C = 1/(2
π
Rf
c
),
where:
R
= 10 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 L1 and L2 output beads 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 non-
linearity of the beads. Make sure that these beads have enough
current conducting capability while providing sufficient EMI
attenuation.
The current rating needed for an 8
Ω
load is about 420 mA.
Impedance for the beads at 100 MHz must be 220
Ω
. In
addition, the lower the dc resistance (DCR) of the beads, the
better for minimizing their power consumption. Table 1 shows
the recommended beads.
Output Shunting Capacitors—C3 and C4
Two capacitors, C3 and C4, work with the L1 and L2 ferrite
beads. Use small size (0603 or 0402), multilayer ceramic
capacitors made from 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 bigger
capacitance (1 nF). This configuration provides better EMI
reduction for the whole frequency spectrum. For BOM cost
reduction and capable performance, a single capacitor of
approximately 470 pF can be used.
Table 1. Recommended 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
