LM49120, LM49120TLEVAL

www.ti.com

SNAS431C – JUNE 2008 – REVISED MAY 2013

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 LM49120. The input capacitors create a high-pass filter with the

input resistors RIN. The -3dB point of the high pass filter is found using Equation 10 below.

f = 1 / (2

πRINCIN) (Hz)

(10)

Where the value of RIN is given in the Electrical Characteristics Table.

High pass filtering the audio signal helps protect the speakers. When the LM49120 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, 217Hz 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.

Bias Capacitor Selection

The LM49120 internally generates a VDD/2 common-mode bias voltage. The BIAS capacitor CBIAS, improves

PSRR and THD+N by reducing noise at the BIAS node. Use a 2.2µF ceramic placed as close to the device as

possible.

PCB LAYOUT GUIDELINES

Minimize trace impedance of the power, ground and all output traces for optimum performance. Voltage loss due

to trace resistance between the LM49120 and the load results in decreased output power and efficiency. Trace

resistance between the power supply and ground has the same effect as a poorly regulated supply, increased

ripple and reduced peak output power. Use wide traces for power supply inputs and amplifier outputs to minimize

losses due to trace resistance, as well as route heat away from the device. Proper grounding improves audio

performance, minimizes crosstalk between channels and prevents digital noise from interfering with the audio

signal. Use of power and ground planes is recommended.

Place all digital components and route digital signal traces as far as possible from analog components and

traces. Do not run digital and analog traces in parallel on the same PCB layer. If digital and analog signal lines

must cross either over or under each other, ensure that they cross in a perpendicular fashion.

Copyright © 2008–2013, Texas Instruments Incorporated

Submit Documentation Feedback

23

Product Folder Links: LM49120 LM49120TLEVAL