LM49200, LM49200TLEVAL

SNAS459A – MAY 2009 – REVISED APRIL 2013

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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 LM49200. 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 below.

f = 1 / 2

πRINCIN (Hz)

where

•

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

(2)

High-pass filtering the audio signal helps protect the speakers. When the LM49200 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.

CHARGE PUMP FLYING CAPACITOR (C1)

The flying capacitor (C1), see Figure 1, 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 2.2

μF, the RDS(ON)

of the charge pump switches and the ESR of C1 and Cs5 dominate the output impedance. A lower value

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

CHARGE PUMP HOLD CAPACITOR (CS3)

The value and ESR of the hold capacitor (Cs5) directly affects the ripple on CPVSS. Increasing the value of Cs5

reduces output ripple. Decreasing the ESR of Cs5 reduces both output ripple and charge pump output

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

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