LM4673 Datasheet, PDF (11/18 Pages) National Semiconductor (TI) – Filterless, 2.65W, Mono, Class D Audio Power Amplifier

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.
(V_SD - GND) / 300kΩ
(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).
A_V = 2 * 150 kΩ / 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.
f_C = 1 / (2πR_i C_i ) (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 R_i.
Equation (2) above is used to determine the value of the R_i
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 R_i3
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 R_i3, are found by calculating
the parallel combination of all C_ivalues and then all R_i val-
ues. Equations (4) and (5) below are for any number of
single-ended sources.
C_i3 = C_i1 + C_i2 + C_in ... (F)
(4)
R_i3 = 1 / (1/R_i1 + 1/R_i2 + 1/R_in ...) (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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