max9706etxt Maxim Integrated Products, Inc., max9706etxt Datasheet - Page 17

• Manufacturer Part Number: max9706etxt
• Description: 3-channel, 2.3w, Filterless Class D Amplifiers With Active Crossover
• Manufacturer: Maxim Integrated Products, Inc.

The left and right channels are summed and passed
through a lowpass filter to generate the mono output.
The mono speaker gain offset is an attenuation of the
selected speaker gain. The MAX9706/MAX9707 offer
three options for this summing gain. Select mono out-
put gain by setting MGAIN high, low, or leave floating
(see Table 3).
The left- and right-speaker impedance should be twice
that of the MONO channel (8Ω L/R, 4Ω MONO), then
from the same voltage swing, the mono speaker will
have 2 times the power. Over the left and right mono
channels, a 1.5dB increase improves matching
between the high- and low-frequency drivers.
The MAX9706/MAX9707 feature an internal active filter
with adjustable crossover frequency (f
low-frequency transducer. The crossover filter consists of
a complementary 2nd-order lowpass and 2nd-order
highpass Butterworth filter (Figure 6). Crossover fre-
quency is variable over the 800Hz to 2133.3Hz range to
accommodate different speaker types. There are four
selectable crossover frequencies selected by FS0 and
FS1 (Table 4).
The BTL outputs provide the option of phase-inverting
the mono (LF) output with respect to the main (L/R) out-
puts. Depending on the speaker placement and dis-
tance from the listener, this can smooth the crossover
transition between low and high frequencies.
Table 3. Mono Speaker Gain
Table 4. Crossover Frequency Selection
FS0
3-Channel, 2.3W, Filterless Class D Amplifiers
0
0
1
1
FLOATING
MGAIN
GND
V
DD
______________________________________________________________________________________
FS1
0
1
0
1
Crossover Frequency
MONO SPEAKER GAIN
FREQUENCY (f
OFFSET (dB)
CROSSOVER
XO
Mono Output
1066.7
2133.3
-4.5
-6.0
-7.5
1600
) for use with a
800
XO
) (Hz)
In conventional single-supply headphone amplifiers,
the output-coupling capacitor is a major contributor of
audible clicks and pops. Upon startup, the amplifier
charges the coupling capacitor to its bias voltage, typi-
cally half the supply. Likewise, during shutdown, the
capacitor is discharged to GND. This results in a DC
shift across the capacitor, which in turn appears as an
audible transient at the speaker. Since the MAX9706
headphone amplifier does not require output-coupling
capacitors, no audible transients appear.
The MAX9706 offers 0dB and 3dB headphone amplifier
gain settings controlled through the GAIN2 gain-select
input (see Table 2).
Traditional single-supply headphone amplifiers have
outputs biased at a nominal DC voltage (typically half
the supply) for maximum dynamic range. Large cou-
pling capacitors are needed to block this DC bias from
the headphone. Without these capacitors, a significant
amount of DC current flows to the headphone, resulting
in unnecessary power dissipation and possible dam-
age to both headphone and headphone amplifier.
Maxim’s patented DirectDrive architecture uses a
charge pump to create an internal negative supply volt-
age. This allows the headphone outputs of the
MAX9706 to be biased at GND, almost doubling
dynamic range while operating from a single supply
(Figure 7). With no DC component, there is no need for
the large DC-blocking capacitors. Instead of two large
Figure 6. Crossover Frequency
with Active Crossover
2nd-ORDER SLOPE
Headphone Amplifier (MAX9706)
HP FUNCTION
f
X
LP FUNCTION
2nd-ORDER SLOPE
FREQUENCY (Hz)
DirectDrive
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