LM4675, LM4675SDBD, LM4675TLBD

SNAS353C – AUGUST 2006 – REVISED MAY 2013

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The input capacitors may also be used to remove low audio frequencies. Small speakers cannot reproduce low

bass frequencies so filtering may be desired . When the LM4675 is using a single-ended source, power supply

noise on the ground is seen as an input signal by the +IN input pin that is capacitor coupled to ground (See

Figure 28 – Figure 30). Setting the high-pass filter point above the power supply noise frequencies, 217Hz in a

GSM phone, for example, will filter out this noise so it is not amplified and heard on the output. Capacitors with a

tolerance of 10% or better are recommended for impedance matching.

DIFFERENTIAL CIRCUIT CONFIGURATIONS

The LM4675 can be used in many different circuit configurations. The simplest and best performing is the DC

coupled, differential input configuration shown in Figure 25. Equation 2 above is used to determine the value of

the Ri resistors for a desired gain.

Input capacitors can be used in a differential configuration as shown in Figure 26. Equation 3 above is used to

determine the value of the Ci capacitors for a desired frequency response due to the high-pass filter created by

Ci and Ri. Equation 2 above is used to determine the value of the Ri resistors for a desired gain.

The LM4675 can be used to amplify more than one audio source. Figure 27 shows a dual differential input

configuration. The gain for each input can be independently set for maximum design flexibility using the Ri

resistors for each input and Equation 2. Input capacitors can be used with one or more sources as well to have

different frequency responses depending on the source or if a DC voltage needs to be blocked from a source.

SINGLE-ENDED CIRCUIT CONFIGURATIONS

The LM4675 can also be used with single-ended sources but input capacitors will be needed to block any DC at

the input terminals. Figure 28 shows the typical single-ended application configuration. The equations for Gain,

Equation 2, and frequency response, Equation 3, hold for the single-ended configuration as shown in Figure 28.

When using more than one single-ended source as shown in Figure 29, the impedance seen from each input

terminal should be equal. To find the correct values for Ci3 and Ri3 connected to the +IN input pin the equivalent

impedance of all the single-ended sources are calculated. The single-ended sources are in parallel to each other.

The equivalent capacitor and resistor, Ci3 and Ri3, are found by calculating the parallel combination of all

Civalues and then all Ri values. Equation 4 and Equation 5 below are for any number of single-ended sources.

Ci3 = Ci1 + Ci2 + Cin (F)

(4)

Ri3 = 1 / (1/Ri1 + 1/Ri2 + 1/Rin)

(

Ω)

(5)

The LM4675 may also use a combination of single-ended and differential sources. A typical application with one

single-ended source and one differential source is shown in Figure 30. Using the principle of superposition, the

external component values can be determined with the above equations corresponding to the configuration.

Figure 25. Differential Input Configuration

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