LM48413

SNAS460B – NOVEMBER 2008 – REVISED MARCH 2013

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PROPER SELECTION OF EXTERNAL COMPONENTS

Power Supply Bypassing/Filtering

Proper power supply bypassing is important for low noise performance and high PSRR. Place the 1

μF supply

bypass capacitor as close to the device as possible. Traditionally, a pair of bypass capacitors with typical value

0.1

μF and 10μF are applied to the supply rail for increasing stability. Nevertheless, these capacitors do not

eliminate the need for bypassing of the LM48413 supply pins.

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 LM48413. The input capacitors create a high-pass filter with the

input resistance RIN. The -3dB point of the high-pass filter is found using Equation 1 below.

f = 1 / 2

πRINCIN

(Hz)

(1)

The input capacitors can also be used to remove low frequency content from the audio signal. When the

LM48413 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.

Texas Instruments 3D Enhancement

The LM48413 features Texas Instruments' 3D enhancement effect that widens the perceived soundstage of a

stereo audio signal. The 3D enhancement increases the apparent stereo channel separation, improving audio

reproduction whenever the left and right speakers are too close to one another.

An external RC network shown in Figure 2 is required to enable the 3D effect. Because the LM48413 is a fully

differential amplifier, there are two separate RC networks, one for each stereo input pair (INL+ and INR+, and

INL- and INR-). Set 3DEN high to enable the 3D effect. Set 3DEN low to disable the 3D effect.

The 3D RC network acts as a high-pass filter. The amount of the 3D effect is set by the R3D resistor. Decreasing

the value of R3D increases the 3D effect. The C3D capacitor sets the frequency at which the 3D effect occurs.

Increasing the value of C3D decreases the low frequency cutoff point, extending the 3D effect over a wider

bandwidth. The low frequency cutoff point is given by Equation 2:

f3D(–3dB) = 1 / 2π(R3D)(C3D)

(Hz)

(2)

Enabling the 3D effect increase the gain by a factor of (1+40k

Ω/R3D). Setting R3D to 40kΩ results in a gain

increase of 6dB whenever the 3D effect is enabled. The Equation 2 holds for both differential and single-end

configuration. The recommended tolerance of the resistor value and capacitor value of the two RC networks are

5% and 10% respectively. Tolerance out of this range may affect the 3D gain and low frequency cut-off point too

much. The desired sound quality of the 3D effect may not be obtained consequently.

SINGLE-ENDED AUDIO AMPLIFIER CONFIGURATION

The LM48413 is compatible with single-ended sources. When configured for single-ended inputs, input

capacitors must be used to block and DC component at the input of the device. Figure 17 shows the typical

single-ended applications circuit.

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