LM4673 datasheet(11/18 Pages) NSC

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Application Information (Continued)

pin is floating. The LM4673 will enter the shutdown state

when the Shutdown pin is left floating or if not floating, when

the shutdown voltage has crossed the threshold. To mini-

mize the supply current while in the shutdown state, the

Shutdown pin should be driven to GND or left floating. If the

Shutdown pin is not driven to GND, the amount of additional

resistor current due to the internal shutdown resistor can be

found by Equation (1) below.

SD - GND) / 300k

Ω

(1)

With only a 0.5V difference, an additional 1.7µA of current

will be drawn while in the shutdown state.

PROPER SELECTION OF EXTERNAL COMPONENTS

The gain of the LM4673 is set by the external resistors, Ri in

Figure 1, The Gain is given by Equation (2) below. Best

THD+N performance is achieved with a gain of 2V/V (6dB).

V = 2 * 150 k

Ω /R

i (V/V)

(2)

It is recommended that resistors with 1% tolerance or better

be used to set the gain of the LM4673. The Ri resistors

should be placed close to the input pins of the LM4673.

Keeping the input traces close to each other and of the same

length in a high noise environment will aid in noise rejection

due to the good CMRR of the LM4673. Noise coupled onto

input traces which are physically close to each other will be

common mode and easily rejected by the LM4673.

Input capacitors may be needed for some applications or

when the source is single-ended (see Figures 3, 5). Input

capacitors are needed to block any DC voltage at the source

so that the DC voltage seen between the input terminals of

the LM4673 is 0V. Input capacitors create a high-pass filter

with the input resistors, R

i. The –3dB point of the high-pass

filter is found using Equation (3) below.

C =1/(2

πR

i Ci ) (Hz)

(3)

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 LM4673

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 Figures 5 – 7). Setting the

high-pass filter point above the power supply noise frequen-

cies, 217Hz in a GSM phone, for example, will filter out this

noise so it is not amplified and heard on the output. Capaci-

tors with a tolerance of 10% or better are recommended for

impedance matching.

DIFFERENTIAL CIRCUIT CONFIGURATIONS

The LM4673 can be used in many different circuit configu-

rations. The simplest and best performing is the DC coupled,

differential input configuration shown in Figure 2. Equation

(2) above is used to determine the value of the R

i resistors

for a desired gain.

Input capacitors can be used in a differential configuration as

shown in Figure 3. Equation (3) above is used to determine

the value of the C

i capacitors for a desired frequency re-

sponse due to the high-pass filter created by C

i and Ri.

Equation (2) above is used to determine the value of the R

resistors for a desired gain.

The LM4673 can be used to amplify more than one audio

source. Figure 4 shows a dual differential input configuration.

The gain for each input can be independently set for maxi-

mum design flexibility using the R

i 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 LM4673 can also be used with single-ended sources but

input capacitors will be needed to block any DC at the input

terminals. Figure 5 shows the typical single-ended applica-

tion configuration. The equations for Gain, Equation (2), and

frequency response, Equation (3), hold for the single-ended

configuration as shown in Figure 5.

When using more than one single-ended source as shown in

Figure 6, the impedance seen from each input terminal

should be equal. To find the correct values for C

i3 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, C

i3 and Ri3, are found by calculating

the parallel combination of all C

ivalues and then all Ri val-

ues. Equations (4) and (5) below are for any number of

single-ended sources.

i3 =Ci1 +Ci2 +Cin ... (F)

(4)

i3 = 1 / (1/Ri1 + 1/Ri2 + 1/Rin ...) (

Ω)

(5)

The LM4673 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

7. Using the principle of superposition, the external compo-

nent values can be determined with the above equations

corresponding to the configuration.

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