Evaluation Board User Guide
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UG-196
Evaluation Board for SSM2375 Filterless Class-D Audio Amplifier
PACKAGE CONTENTS
EVAL-SSM2375Z evaluation board
OTHER SUPPORTING DOCUMENTATION
SSM2375 data sheet
GENERAL DESCRIPTION
The SSM2375 is a fully integrated, high efficiency, Class-D
audio amplifier. It is designed to maximize performance for
mobile phone applications. The application circuit requires a
minimum of external components and operates from a single
2.5 V to 5.5 V supply. It is capable of delivering 3 W of continuous
output power with less than 1% THD + N driving a 3 load
from a 5.0 V supply.
Spread spectrum pulse density modulation (PDM) is used to
provide lower EMI-radiated emissions compared with other
Class-D architectures. The inherent randomized nature of
spread spectrum PDM eliminates clock intermodulation
(beating effect) of several amplifiers in close proximity. The
SSM2375 includes an optional modulation select pin that
enables a low EMI mode, which significantly reduces the
radiated emissions at the Class-D outputs, particularly above
100 MHz. With this option selected, the SSM2375 can pass
FCC Class B radiated emissions testing with a 50 cm unshielded
speaker cable without any external filtering.
The device also includes a flexible gain select pin that only
requires one series resistor to select among 0 dB, 3 dB, 6 dB,
9 dB, or 12 dB. The benefit of this is to improve gain matching
between multiple SSM2375 devices within a single application
as compared to using external resistors to set gain. This user
guide describes how to configure and use the SSM2375 evaluation
board. It is recommended that this user guide be read in conjunction with the SSM2375 data sheet, which provides specifications,
internal block diagrams, and application guidance for the
amplifier IC.
EVALUATION BOARD OVERVIEW
The SSM2375 evaluation board carries a complete application
circuit for driving a loudspeaker. Figure 1 shows the top view of
the evaluation board, and Figure 2 shows the bottom view.
Figure 1. SSM2375 Evaluation Board Top View
PLEASE SEE THE LAST PAGE FOR AN IMPORTANT
WARNING AND LEGAL TERMS AND CONDITIONS.
09393-001
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Figure 2. SSM2375 Evaluation Board Bottom View
9393-002
UG-196 Evaluation Board User Guide
TABLE OF CONTENTS
Package Contents.............................................................................. 1
Other Supporting Documentation................................................. 1
General Description ......................................................................... 1
Evaluation Board Overview ............................................................1
Revision History ............................................................................... 2
Setting Up the Evaluation Board .................................................... 3
Input Configuration..................................................................... 3
Shutdown Mode............................................................................ 3
Gain Configuration...................................................................... 3
REVISION HISTORY
4/11—Revision 0: Initial Version
Output Configuration...................................................................3
Power Supply Configuration .......................................................3
Component Selection ...................................................................3
Getting Started...................................................................................5
What to Test...................................................................................5
Evaluation Board Schematic and Artwork.....................................6
Ordering Information.......................................................................8
Bill of Materials..............................................................................8
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Evaluation Board User Guide UG-196
SETTING UP THE EVALUATION BOARD
INPUT CONFIGURATION
A 4-pin header (H2) 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 H2 are used
for IN+, IN−, and signal ground. For a single-ended audio
input, only two pins of H2 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 H2, shorting IN−
to ground. If IN− is used, place the jumper between Pin 1 and
Pin 2 of H2, connecting IN+ to ground.
SHUTDOWN MODE
The 2-pin header, J1, is used to turn on and off the SSM2375
amplifier. Placing a jumper across Pin 1 and Pin 2 of H1 puts
the SSM2375 in normal operation. Removing the jumper shuts
down the SSM2375 so that only a minimum current (about
20 nA) is drawn from the power supply.
GAIN CONFIGURATION
Two headers, H3 and J3, control the SSM2375 analog gain. By
placing a jumper across two pins of H3, the amplifier’s gain pin can
be connected to GND or VDD. Three jumper settings are used:
between the center pin and the left pin (HI), between the center pin
and the right pin (LO), and no jumper (open).
Placing a jumper on J3 removes the 47 k resistor (R3) between
the gain pin and H3. Gain settings between 0 dB and 12 dB are
available; see Tab le 1 for configuration details.
Table 1. Gain Configuration
Gain J3 H3 Configuration
0 dB Short LO Short gain pin to GND
3 dB Short Open Gain pin unconnected
6 dB Short HI Short gain pin to VDD
9 dB Open LO Gain pin through 47 kΩ to GND
12 dB Open HI Gain pin through 47 kΩ to VDD
OUTPUT CONFIGURATION
The output connector, H4, is located on the right side of the
board (see Figure 1). H4 can drive a loudspeaker whose
impedance should be no less than 3 .
Because the SSM2375 does not typically require any external LC
output filters due to a low noise modulation scheme, no output
filter is installed on the evaluation board. In this case, thick wire
connects the empty pads of B1 and B2, and the capacitor pads
are left unpopulated.
If the speaker length exceeds 10 cm, place Ferrite Bead B1 and
Ferrite Bead B2 in the output paths, and use Capacitors C6 and
C7 to couple the output terminals to ground, as shown in the
schematic in Figure 3. Some recommended ferrite beads are
listed in Tabl e 2. Some users may want to use inductors for
applications with specific EMI vs. audio performance constraints; see Table 3 for recommendations.
For optimal THD and SNR performance as specified in the
SSM2375 data sheet, remove the entire EMI filter, short across
the ferrite bead terminals, and open the capacitor terminals.
POWER SUPPLY CONFIGURATION
The 2-pin header, H1, is used to power the board. Care must be
taken to connect the dc power with correct polarity and voltage.
Reverse polarity or overvoltage may damage the board permanently. The maximum supply current is approximately 0.33 A
when driving an 8 Ω load and when the input voltage is 5 V. Do
not allow VDD to exceed 5.5 V.
COMPONENT SELECTION
Selecting the proper components is the key to achieving the
performance required at the cost budgeted.
Input Gain Resistor Selection—R1 and R2
If the desired gain must be adjusted beyond the available gain
settings (see the Gain Configuration section), a series resistor
can be placed in the input signal path. This creates a voltage
divider with the 80 k input resistance on each input pin,
allowing an arbitrary reduction of the input signal. Note that
input signal attenuation directly reduces SNR performance;
therefore, large values compared to the built-in input resistance
should be avoided. These components are populated with 0
values on the evaluation board.
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 reject unnecessary
extremely low frequency signals. For music signals, the cutoff
frequency is typically between 20 Hz and 30 Hz. The value of
the input capacitor is calculated by
C = 1/(2πR
where:
R
= 80 kΩ + (R1 or R2).
IN
f
is the desired cutoff frequency.
c
Output Ferrite Beads—B1 and B2
The output beads, B1 and B2, 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 . In addition, the lower the dc
resistance (DCR) of these beads, the better for minimizing their
power consumption. Ta ble 2 describes the recommended beads.
IN fc
)
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