Evaluation Board User Guide
UG-192
Rev. 0 | Page 3 of 8
SETTING UP THE EVALUATION BOARD
INPUT CONFIGURATION
A 4-pin header (J1) on the middle left side of the board feeds the
audio signal into the board (see Figure 1). If the input audio signal
is differential (IN+ and IN−), three pins of J1 are used for IN+,
IN−, and signal ground. For a single-ended audio input, only two
pins of J1 are used. One is for the signal ground and the other is
for either IN+ or IN−. If IN+ is used, place a jumper between
Pin 3 and Pin 4 of J1, shorting IN− to ground. If IN− is used,
place the jumper between Pin 1 and Pin 2 of J1, connecting IN+
to ground.
OPERATION MODE CONFIGURATION
The 6-pin header, J2, is used to turn on/off the SSM2317 amp-
lifier and configure the ALC operation modes. Placing a jumper
across Pin 1 and Pin 2 of J2 shuts down the
so that only
a minimum current (about 20 nA) is drawn from the power supply
(when R3 is shorted). Removing the jumper puts the SSM2317 in
normal operation. Placing a jumper across Pin 3 and Pin 4 of J2
disables ALC mode, while removing the jumper activates ALC
mode. When ALC is disabled, the SSM2317 behaves like a tradi-
tional amplifier with a fixed 18 dB. Placing a jumper across Pin 5
and Pin 6 of J2 shorts across the ALC threshold resistor (on
board) and sets a maximum limiter level of 90% of V
DD
(when
ALC is activated).
OUTPUT CONFIGURATION
The output connector, J4, is located on the right side of the board
(see Figure 1). J4 drives a loudspeaker whose impedance should
be no less than 3 Ω.
Although the SSM2317 does not require any external LC output
filters due to 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 interfer-
ence (EMI), as shown in the schematic in Figure 3. Some users may
want to replace the ferrite beads with these inductors to evaluate
applications with specific EMI vs. audio performance constraints. As
an aid, a properly tuned ferrite bead-based EMI filter is assembled at
the output terminals of the device.
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.
POWER SUPPLY CONFIGURATION
The evaluation board schematic is shown in Figure 3. The 2-pin
header, J3, must be used to power the board. Care must be taken
to connect the dc power with correct polarity and voltage. The
positive voltage terminal of J3 (VDD) is indicated with an arrow
in Figure 1.
Polarity and Voltage
The wrong power supply polarity or an overvoltage may damage
the board permanently. The maximum peak current is approxi-
mately 0.33 A when driving an 8 Ω load and when the input
voltage is 5 V.
COMPONENT SELECTION
Selecting the proper components is the key to achieving the
performance required at the cost budgeted.
ALC Threshold Setting Resistor—R5
When ALC mode is active, the maximum output amplitude
threshold (V
TH
) during the limiting operation can be adjusted
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.
To tune a variety of V
TH
levels, a potentiometer, R5, is mounted on
the evaluation board. To measure the potentiometer resistance set-
ting, insert an ohmmeter across Pin 5 and Pin 6 of J2.
Note that measuring the resistance across the potentiometer is not
adequate to determine the actual R
TH
value. This is due to the
internal input resistance of 50 kΩ at the VTH pin. The user must
take into account the internal resistance while evaluating actual
R
TH
. For example, after tuning R5 to a desirable level, the user
measures the resistance from Pin 5 and Pin 6 of J2. Then, to infer
the actual R
TH
, use this simple calculation:
kΩ
50
1
1
1
−
=
Measured
Threshold
R
R
where:
R
Threshold
is the desired external resistor value from the VTH
pin
to GND.
R
Measured
is the measured resistance from the VTH pin to GND.
