MAX8678 Maxim Integrated Products, MAX8678 Datasheet - Page 10

• Manufacturer Part Number: MAX8678
• Description: White LED Charge Pump
• Manufacturer: Maxim Integrated Products

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The audio amplifier has an internal gain adjustment,
controlled by a serial-pulse interface. The gain setting
of the input amplifier is adjustable from -9dB to +18dB
in 3dB steps. This allows the amplifier to be used for
both hands-free and for receiver mode, without any
external components.
Driving ENAMP high turns on the amplifier and sets the
internal register to -9dB. Adjust the gain by pulsing
ENAMP low (500ns to 500µs pulse width). Each rising
edge sets the gain as shown in Table 3. Once the
desired setting is reached, hold ENAMP high for at
least 3.2ms to set the internal register and change the
gain. To set a new current level, repeat the previous
sequence from the beginning. To shut down the ampli-
fier, hold ENAMP low for at least 3.2ms. Figure 3 is a
timing diagram for ENAMP.
The MAX8678 is designed to drive a load differentially,
a configuration referred to as bridge-tied load (BTL).
The BTL configuration offers advantages over the sin-
gle-ended configuration, where one side of the load is
connected to ground. Driving the load differentially
doubles the output voltage, thus quadrupling output
power compared to a single-ended amplifier under
similar configurations.
White LED Charge Pump with
1.1W Audio Amplifier
Figure 4. Differential Audio Input with Optional Filter Capacitors
Figure 5. Single-Ended Audio Input
10
______________________________________________________________________________________
DIFFERENTIAL
AUDIO INPUT
SINGLE-ENDED
AUDIO INPUT
Serial-Pulse Gain Adjustment (ENAMP)
OPTIONAL FILTER
CAPACITORS
Applications Information
A+
A+
A-
A-
NOTE: FILTER CAPACITORS REQUIRED FOR SINGLE-ENDED AUDIO OUTPUT.
R
R
R
R
IN
IN
IN
IN
BTL Amplifier
MAX8678
MAX8678
Since the differential outputs are biased at midsupply,
there is no net DC voltage across the load. This elimi-
nates the need for DC-blocking capacitors required for
single-ended amplifiers. These capacitors can be
large, expensive, consume board space, and degrade
low-frequency performance.
Table 3. Audio Amplifier-Input Impedance
ENAMP
EDGES
RISING
10
1
2
3
4
5
6
7
8
9
AUDIO AMP
AUDIO AMP
CLASS AB
CLASS AB
MONO
MONO
SETTING
GAIN
(dB)
12
15
18
-9
-6
-3
0
3
6
9
OUT+
OUT-
OUT+
OUT-
69.1
62.4
54.9
46.8
38.8
31.3
24.6
18.8
14.1
10.5
MIN
EARPIECE /
LOUDSPEAKER
EARPIECE /
LOUDSPEAKER
R
IN
TYP
99.0
89.2
78.4
66.9
55.5
44.7
35.1
26.9
20.2
15.0
(kΩ)
140.0
116.0
102.0
MAX
87.0
72.1
58.1
45.6
35.0
26.2
19.5