STEREO-INPUT MULTIPLEXER (STEREO MUX)

The LM4867 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 must be tied to

GND.

Typical LM4867 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 LM4867’s internal pull-up

that applies V

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

present on the internal amplifier’s input may change. This

change can slew at a rate that may produce audible voltage

transients or clicks in the amplifier’s output signal. Using the

MUX to select between two vastly dissimilar gains is a typical

transient-producing situation. As the MUX is switched, an

audible click may occur as the gain suddenly changes.

PIN OUT COMPATIBILITY WITH THE LM4863

The LM4867 pin out was designed to simplify replacing the

LM4863: except for the four Pins(-IN A

and NC) that implement the LM4867’s extra functionality, the

LM4867MT/MTE and LM4863MT/MTE pin outs match.

(Note 22)

Note 22: If the LM4867 replaces an LM4863 and the input MUX circuitry is

the unused MUX inputs must be connected to GND.

EXPOSED-DAP MOUNTING CONSIDERATIONS

The LM4867’s exposed-DAP (die attach paddle) packages

(MTE and LQ) provide a low thermal resistance between the

die and the PCB to which the part is mounted and soldered.

This allows rapid heat transfer from the die to the surround-

ing PCB copper area heatsink, copper traces, ground plane,

and finally, surrounding air. The result is a low voltage audio

power amplifier that produces 2.4W dissipation in a 4

Ω load

at

≤ 1% THD+N and over 3W in a 3Ω load at 10% THD+N.

This high power is achieved through careful consideration of

necessary thermal design. Failing to optimize thermal design

may compromise the LM4867’s high power performance and

activate unwanted, though necessary, thermal shutdown

protection.

DS200013-70

FIGURE 5. Input MUX Example

DS200013-39

FIGURE 6. As configured, connecting headphones to this jack automatically selects the stereo headphone amplifier

and, with the additional NC switch, changes MUX channels (Network 2 in Figure 2 )

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