LM48823, LM48823TLEVAL

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SNAS464E – JUNE 2008 – REVISED OCTOBER 2010

PROPER SELECTION OF EXTERNAL COMPONENTS

Power Supply Bypassing/Filtering

Proper power supply bypassing is critical for low noise performance and high PSRR. Place the supply bypass

capacitors as close to the device as possible. Place a 1µF ceramic capacitor from VDD to GND. Additional bulk

capacitance may be added as required.

Bypass Capacitor Selection

The BYPASS capacitor, CBYPASS, improves PSRR, noise rejection and output offset. For best results, use a

capacitor of identical value to the input coupling capacitors

Charge Pump Capacitor Selection

Use low ESR ceramic capacitors (less than 100m

Ω) for optimum performance.

Charge Pump Flying Capacitor (C1)

The flying capacitor (C1) affects the load regulation and output impedance of the charge pump. A C1 value that

is too low results in a loss of current drive, leading to a loss of amplifier headroom. A higher valued C1 improves

load regulation and lowers charge pump output impedance to an extent. Above 2.2µF, the RDS(ON) of the charge

pump switches and the ESR of C1 and C2 dominate the output impedance. A lower value capacitor can be used

in systems with low maximum output power requirements.

Charge Pump Hold Capacitor (C2)

The value and ESR of the hold capacitor (C2) directly affects the ripple on CPVSS. Increasing the value of C2

reduces output ripple. Decreasing the ESR of C2 reduces both output ripple and charge pump output impedance.

A lower value capacitor can be used in systems with low maximum output power requirements.

Input Capacitor Selection

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 LM48823. 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 1.

f = 1 / 2

πRINCIN (Hz)

where

•

the value of RIN is given in the Electrical Characteristics table.

(1)

High pass filtering the audio signal helps protect the speakers. When the LM48823 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.

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 LM48823 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 switching 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 © 2008–2010, Texas Instruments Incorporated

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