LM48821, LM48821TLEVAL

SNAS354 – JUNE 2007

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SELECTING EXTERNAL COMPONENTS

Optimizing the LM48821’s performance requires properly selecting external components. Though the LM48821

operates well when using external components with wide tolerances, best performance is achieved by optimizing

component values.

Charge Pump Capacitor Selection

Use low ESR (equivalent series resistance) (<100m

Ω) ceramic capacitors with an X7R dielectric for best

performance. Low ESR capacitors keep the charge pump output impedance to a minimum, extending the

headroom on the negative supply. Higher ESR capacitors result in reduced output power from the audio

amplifiers.

Charge pump load regulation and output impedance are affected by the value of the flying capacitor (connected

pump output resistance. Beyond 4.7

μF, the switchon-resistance dominates the output impedance.

PGND pins). Larger capacitors reduce output ripple on the negative power supply. Lower ESR capacitors

minimize the output ripple and reduce the output impedance of the charge pump.

The LM48821 charge pump design is optimized for 4.7

μF, low ESR, ceramic, flying, and output capacitors.

Power Supply Bypass Capacitor

For good THD+N and low noise performance and to ensure correct power-on behavior at the maximum allowed

power supply voltage, a local 4.7

μF power supply bypass capacitor should be connected as physically closed as

Input Capacitor Value Selection

Amplifying the lowest audio frequencies requires high value input coupling capacitors (the 0.47

μF capacitors in

Figure 1). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In

many cases, however, the speakers used in portable systems, whether internal or external, have little ability to

reproduce signals below 150Hz. Applications using speakers with this limited frequency response reap little

improvement by using high value input and output capacitors.

Besides affecting system cost and size, the input coupling capacitor value has an effect on the LM48821’s click

and pop performance. The magnitude of the pop is directly proportional to the input capacitor’s size. Thus, pops

can be minimized by selecting an input capacitor value that is no higher than necessary to meet the desired -3dB

frequency.

The LM48821's nominal input resistance at full volume is 10k

Ω and a minimum of 5kΩ. This input resistance and

the input coupling capacitor value produce a -3dB high pass filter cutoff frequency that is found using Equation 3.

(3)

REVISION HISTORY

Rev

Date

Description

1.0

06/06/07

Initial release.

18

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