General Description
The LM4985 is a stereo headphone audio amplifier with an
internal digitally controlled volume control. The LM4985 is
optimized to operate over a power supply voltage range of
2.5V to 5.0V. This amplifier is capable of delivering
40mW
RMS
per channel into a 32
Ω
load at 1% THD when
powered by a 3.6V power supply.
Boomer audio power amplifiers were designed specifically to
provide high quality output power with a minimal amount of
external components. To that end, the LM4985 features two
functions that optimize system cost and minimize PCB area:
an integrated, digitally controlled (I
2
C bus) volume control
and, for maximum flexibility, an operational mode that elimi-
nates output signal-coupling capacitors (OCL mode). Since
the LM4985 does not require bootstrap capacitors, snubber
networks, or output coupling capacitors, it is optimally suited
for low-power, battery powered potable systems. For added
design flexibility, the LM4985 can also be configured for
single-ended capacitively coupled outputs.
The LM4985 includes an internal input MUX that allows the
signal on either input to appear on either amplifier output and
separate shutdown controls for each stereo channel.
The LM4985 features a shutdown mode for micropower
dissipation, an internal thermal shutdown protection mecha-
nism, and is unity gain stable.
Operating Conditions
•
Temperature Range
–40˚C
≤
T
A
≤
85˚C
•
Amplifier Power Supply Voltage
2.3V
≤
V
DD
≤
5.0V
Board Features
The LM4985 demonstration board has all of the necessary
connections, using 100mil headers, to apply the power sup-
ply voltage, the audio input signals, and the I
2
C signal inputs.
The amplified audio signal is available on both a stereo
headphone jack and auxiliary output connections. Jumpers
are included to allow the use of either capacitively coupled or
directly coupled amplifier outputs.
Also included with the demonstration board is an I
2
C signal
generation board and software. With this board and the
software, the user can easily control the LM4985’s input
multiplexer, shutdown function, adjustable turn-on time, and
stereo volume control. Figure 3 shows the software’s graphi-
cal user interface.
Schematic
Figure 2 shows the LM4985 Demonstration Board sche-
matic. Refer to Table 3 for a list of the connections and their
functions.
20184918
Figure 2. The LM4985 Demonstration Board Schematic
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