SETTING UP THE EVALUATION BOARD. Analog Devices SSM2375 Filterless Class-D Audio Amplifier

Below you will find brief information for Audio Amplifier SSM2375 Filterless Class-D Audio Amplifier. The SSM2375 is a high efficiency, Class-D audio amplifier designed to maximize performance for mobile phone applications. It 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 Ω load from a 5.0 V supply. 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.

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Evaluation Board User Guide

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 kΩ resistor (R3) between the gain pin and H3. Gain settings between 0 dB and 12 dB are

available; see Table 1 for configuration details.

Table 1. Gain Configuration

Gain J3 H3 Configuration

0 dB Short LO Short gain pin to GND

3 dB

6 dB

Short

Open

Gain pin unconnected

Short gain pin to VDD

9 dB Open LO

12 dB Open HI

Gain pin through 47 kΩ to GND

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 Ω.

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 Table 2. Some users may want to use inductors for

applications with specific EMI vs. audio performance con-

straints; see Table 3 for recommendations.

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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 kΩ 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 Ω 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

f c

) where:

= 80 kΩ + (R1 or R2). f c

is the desired cutoff frequency.

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 Ω. In addition, the lower the dc resistance (DCR) of these beads, the better for minimizing their

power consumption. Table 2 describes the recommended beads.

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Output Shunting Capacitors

There are two output-shunting capacitors, C6 and C7, that work with the ferrite beads, B1 and B2. Use small size (0603 or 0402), multilayer ceramic capacitors made of X7R or C0G (NP0) materials. Note that the capacitors can be used in pairs: a capacitor with small capacitance (up to 100 pF) plus a capacitor with a larger capacitance (less than 1 nF). This configuration provides thorough EMI reduction for the entire frequency spectrum. If the bill of materials must be minimized, a single capacitor of approximately 470 pF can be used with acceptable performance in many cases.

Table 2. Recommended Output Beads

Part No. Manufacturer

BLM18PG121SN1D Murata

MPZ1608S101A

MPZ1608S221A

BLM18EG221SN1D

TDK

Murata

Z (Ω)

120

100

220

Table 3. Recommended Output Inductors

Part No. Manufacturer

LQM31PNR47M00 Murata

LQM31PN1R0M00

LQM21PNR47MC0

LQM21PN1R0MC0

LQH32CN2R2M53

Murata

L (μH)

0.47

1.0

0.47

1.0

2.2

Evaluation Board User Guide

Output Inductors

If inductors are preferred for EMI filtering at the output nodes, choose components with an inductance of less than 2.2 μH.

The higher the inductance, the lower the EMI is at the output; however, cost and power consumption are higher. Using 0.47 μH to 2.2 μH inductors is recommended, and the current rating (and saturation current) should exceed 600 mA for an 8 Ω load.

Table 3 shows the recommended inductors.

MAX

(mA)

2000

3000

2000

MAX

(mA)

1400

1200

1100

800

790

DCR (Ω)

0.05

0.03

0.05

DCR (Ω)

0.07

0.12

0.19

0.1

Size (mm)

1.6 × 0.8 × 0.8

Size (mm)

3.2 × 1.6 × 0.85

2.0 × 1.25 × 0.5

3.2 × 2.5 × 1.55

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Key features

High efficiency
Class-D audio amplifier
Mobile phone applications
Low EMI
Filterless
Spread spectrum PDM
Flexible gain select
Low power
Single supply operation
High performance

Frequently asked questions

The SSM2375 evaluation board provides a complete application circuit for driving a loudspeaker, enabling testing and evaluation of the SSM2375 audio amplifier.

The SSM2375 can deliver up to 3W of continuous output power with less than 1% THD + N driving a 3 Ω load from a 5.0 V supply.

The SSM2375 uses Spread spectrum pulse density modulation (PDM) 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.

The SSM2375 evaluation board offers gain settings of 0 dB, 3 dB, 6 dB, 9 dB, and 12 dB, which can be selected by using jumpers on the board.

The recommended loudspeaker impedance for the SSM2375 evaluation board is 3 Ω or higher. Using a lower impedance may lead to excessive current draw and potential damage to the amplifier.