LM4867MT National Semiconductor, LM4867MT Datasheet - Page 11

• Manufacturer Part Number: LM4867MT
• 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 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
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
FIGURE 6. As configured, connecting headphones to this jack automatically selects the stereo headphone amplifier
FIGURE 5. Input MUX Example
and, with the additional NC switch, changes MUX channels (Network 2 in Figure 2 )
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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
not being used, the LM4867 MUX CTRL pin must be tied to V
the unused MUX inputs must be connected to GND.
power amplifier that produces 2.4W dissipation in a 4
at
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.
1% THD+N and over 3W in a 3
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to MUX CTRL pin. Simultaneously applying
to the HP-IN and the external 100k
2
load at 10% THD+N.
, MUX CTRL, -IN B
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or GND and
resis-
load
2
,