ANALOG DEVICES UG-196 Service Manual

Evaluation Board User Guide
One Technology Way P. O . Box 9106 Norwood, MA 02062-9106, U.S.A. Tel : 781.329.4700 Fax : 781.461.3113 www.analog.com
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.
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Figure 2. SSM2375 Evaluation Board Bottom View
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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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