lm4671itlx National Semiconductor Corporation, lm4671itlx Datasheet - Page 12

• Manufacturer Part Number: lm4671itlx
• Description: Filterless High Efficiency 2.5w Switching Audio Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
the LM4671 is 0V. Input capacitors create a high-pass filter
with the input resistors, R
filter is found using Equation (3) below.
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 LM4671
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 LM4671 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
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
sponse due to the high-pass filter created by C
Equation (2) above is used to determine the value of the R
resistors for a desired gain
The LM4671 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-
f
i
C
capacitors for a desired frequency re-
= 1 / (2πR
i
. The –3dB point of the high-pass
i
C
i
) (Hz)
FIGURE 2. Differential input configuration
(Continued)
i
i
resistors
and R
(3)
i
.
i
12
mum design flexibility using the R
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 LM4671 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
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
the parallel combination of all C
ues. Equations (4) and (5) below are for any number of
single-ended sources.
The LM4671 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.
R
i3
= 1 / (1/R
C
i3
= C
i1
i3
i1
+ C
+ 1/R
and R
i2
201073I7
+ C
i2
i
i3
values and then all R
, are found by calculating
+ 1/R
i
in
resistors for each input
... (µF)
in
...) (Ω)
i3
and R
i
val-
(4)
(5)
i3