STEREO-INPUT MULTIPLEXER (STEREO MUX)

The LM4868 has two stereo inputs. The MUX CTRL Pin

controls which stereo input is active. As shown in the Truth

Table for Logic Inputs, applying 0V to the MUX CTRL input

activates stereo input 1, whereas applying V

CTRL inputs activates stereo input 2. To ensure correct

amplifier operation, unused MUX inputs should be tied to

GND.

Typical LM4868 applications use the MUX to switch between

two stereo input signals. Each stereo channel’s gain can be

tailored to produce the required output signal level by choos-

ing the appropriate input and feedback resistor ratio.

Another configuration uses the MUX to select two different

gains or frequency compensated gains that amplify a single

pair of stereo input signals. Figure 5 shows two different

feedback networks, Network 1 and Network 2. Network 1

produces increasing gain as the input signal’s frequency

decreases. This can be used to compensate a small,

full-range speaker’s low frequency response roll-off. Network

2 sets the gain for an alternate load such as headphones.

The circuit in Figure 6 uses Network 1 when driving external

speakers, switching to Network 2 when headphones are

connected. The normally closed control switch in Figure 6’s

headphone jack connects to the MUX CTRL pin. When

headphones are connected, the LM4868’s internal pull-up

that applies V

Ω resis-

tor applies V

these control voltages automatically selects the amplifier

(headphone or bridge) and switches the gain (MUX channel

selection). Alternatively, leaving the MUX CTRL pin indepen-

dently accessible allows a user to select bass boost as

needed. This alternative user-selectable bass-boost scheme

requires connecting equal ratio resistor feedback networks

to each MUX input channel. The value of the resistor in the

RC network is chosen to give a gain that is necessary to

achieve the desired bass-boost.

Switching between the MUX channels may change the input

signal source or the feedback resistor network. During the

channel switching transition, the average voltage level

20026731

FIGURE 4. Typical Audio Amplifier Application Circuit

(Pin out shown for the 24-pin Exposed-DAP LLP package. Numbers in ( ) are for the 20-pin MTE and MT packages.)

20026728

FIGURE 5. Input MUX Example

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