LM4674

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SNAS344E – DECEMBER 2005 – REVISED APRIL 2013

AUDIO AMPLIFIER INPUT CAPACITOR SELECTION

Input capacitors may be required for some applications, or when the audio source is single-ended. Input

capacitors block the DC component of the audio signal, eliminating any conflict between the DC component of

the audio source and the bias voltage of the LM4674. The input capacitors create a high-pass filter with the input

resistance Ri. The -3dB point of the high pass filter is found using Equation (1) below.

f = 1 / 2

πRiCi

(1)

The values for Ri can be found in the EC table for each gain setting.

The input capacitors can also be used to remove low frequency content from the audio signal. Small speakers

cannot reproduce, and may even be damaged by low frequencies. High pass filtering the audio signal helps

protect the speakers. When the LM4674 is using a single-ended source, power supply noise on the ground is

seen as an input signal. Setting the high-pass filter point above the power supply noise frequencies, 217 Hz in a

GSM phone, for example, filters out the noise such that it is not amplified and heard on the output. Capacitors

with a tolerance of 10% or better are recommended for impedance matching and improved CMRR and PSRR.

AUDIO AMPLIFIER GAIN SETTING

The LM4674 features four internally configured gain settings. The device gain is selected through the two logic

inputs, G0 and G1. The gain settings are as shown in the following table.

LOGIC INPUT

GAIN

G1

G0

V/V

dB

0

0

2

6

0

1

4

12

1

0

8

18

1

1

16

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PCB LAYOUT GUIDELINES

As output power increases, interconnect resistance (PCB traces and wires) between the amplifier, load and

power supply create a voltage drop. The voltage loss due to the traces between the LM4674 and the load results

in lower output power and decreased efficiency. Higher trace resistance between the supply and the LM4674 has

the same effect as a poorly regulated supply, increasing ripple on the supply line, and reducing peak output

power. The effects of residual trace resistance increases as output current increases due to higher output power,

decreased load impedance or both. To maintain the highest output voltage swing and corresponding peak output

power, the PCB traces that connect the output pins to the load and the supply pins to the power supply should

be as wide as possible to minimize trace resistance.

The use of power and ground planes will give the best THD+N performance. In addition to reducing trace

resistance, the use of power planes creates parasitic capacitors that help to filter the power supply line.

The inductive nature of the transducer load can also result in overshoot on one or both edges, clamped by the

parasitic diodes to GND and VDD in each case. From an EMI standpoint, this is an aggressive waveform that

can radiate or conduct to other components in the system and cause interference. In is essential to keep the

power and output traces short and well shielded if possible. Use of ground planes beads and micros-strip layout

techniques are all useful in preventing unwanted interference.

As the distance from the LM4674 and the speaker increases, the amount of EMI radiation increases due to the

output wires or traces acting as antennas become more efficient with length. Ferrite chip inductors places close

to the LM4674 outputs may be needed to reduce EMI radiation.

Copyright © 2005–2013, Texas Instruments Incorporated

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