Datasheet

LM4675, LM4675SDBD, LM4675TLBD
SNAS353C –AUGUST 2006–REVISED MAY 2013 www.ti.com
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 R
i
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 C
i
capacitors for a desired frequency response 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 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 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 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 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
i
values and then all R
i
values. Equation 4 and Equation 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 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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