LM4868MT National Semiconductor, LM4868MT Datasheet - Page 13

• Manufacturer Part Number: LM4868MT
• Description: Output-Transient-Free Dual 2.1W Audio Amplifier Plus No Coupling Capacitor Stereo Headphone Function
• Manufacturer: National Semiconductor

Application Information
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.
(Pin out shown for the 24-pin Exposed-DAP LLP package. Numbers in ( ) are for the 20-pin MTE and MT packages.)
FIGURE 5. Input MUX Example
FIGURE 4. Typical Audio Amplifier Application Circuit
(Continued)
DD
to the MUX
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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
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
DD
DD
to MUX CTRL pin. Simultaneously applying
to the HP-IN and the external 100kΩ resis-
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