LM48556

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SNAS452B – JUNE 2008 – REVISED MAY 2013

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

Gain Setting Resistor Selection

The amplifier gain is given by Equation 1:

(1)

mismatch between the resistors results in a differential gain error that leads to an increase in THD+N, decrease

in PSRR and CMRR, as well as an increase in output offset voltage. Resistors with a tolerance of 1% or better

are recommended.

The gain setting resistors should be placed as close to the device as possible. Keeping the input traces close

together and of the same length increases noise rejection in noisy environments. Noise coupled onto the input

traces which are physically close to each other will be common mode and easily rejected.

Feedback Capacitor Selection

Due to their capacitive nature, ceramic speakers poorly reproduce high frequency audio content. At high

frequencies, a ceramic speaker presents a low impedance load to the amplifier, increasing the required drive

current. The higher output current can drive the device into clipping, increasing THD+N. Low-pass filtering the

audio signal improves audio quality by decreasing the signal amplitude at high frequencies, reducing the speaker

drive current. Adding a capacitor in parallel with each feedback resistor creates a simple low-pass filter with the -

3dB point determined by Equation 2:

Where

•

section

•

(2)

The feedback capacitor is optional and not required for normal operation.

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 LM48556. The input capacitors create a high-pass filter with the

f = 1 / 2

Where

•

(3)

When the LM48556 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 1% or

better are recommended for impedance matching and improved CMRR and PSRR.

SINGLE-ENDED AUDIO AMPLIFIER CONFIGURATION

The LM48556 is compatible with single-ended sources. Figure 4 shows the typical single-ended applications

circuit.

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