max9706etxt Maxim Integrated Products, Inc., max9706etxt Datasheet - Page 16

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

3-Channel, 2.3W, Filterless Class D Amplifiers
with Active Crossover
The SYNC_IN input allows the MAX9706/MAX9707 to
be synchronized to an external clock, or another Maxim
Class D amplifier. This creates a fully synchronous sys-
tem, minimizing clock intermodulation, and allocating
spectral components of the switching harmonics to
insensitive frequency bands. Applying a TTL clock sig-
nal between 1MHz and 1.5MHz to SYNC_IN synchro-
nizes the MAX9706/MAX9707. The period of the
SYNC_IN clock can be randomized, allowing the
MAX9706/MAX9707 to be synchronized to another
Maxim Class D amplifier operating in SSM mode.
SYNC_OUT allows several MAX9706/MAX9707s to be
cascaded. The synchronized output minimizes any
interference due to clock intermodulation caused by
the switching spread between single devices. The
modulation scheme remains the same when using
SYNC_OUT, and audio reproduction is not affected.
Leave SYNC_OUT floating if not used.
The MAX9706/MAX9707 use Maxim’s unique, patented
modulation scheme that eliminates the LC filter
required by traditional Class D amplifiers, improving
efficiency, reducing component count, conserving
board space and system cost. Conventional Class D
amplifiers output a 50% duty-cycle square wave when
no signal is present. With no filter, the square wave
appears across the load as a DC voltage, resulting in
finite load current, increasing power consumption,
especially when idling. When no signal is present at the
input of the MAX9706/MAX9707, the outputs switch as
shown in Figure 4. Because the MAX9706/MAX9707
drive the speaker differentially, the two outputs cancel
each other, resulting in no net idle-mode voltage across
the speaker, minimizing power consumption.
Figure 4. Outputs with No Input Signal
16
Filterless Modulation/Common-Mode Idle
V OUT_+ - V OUT_- = 0V
______________________________________________________________________________________
V IN_ = 0V
OUT_+
OUT_-
External Clock Mode
Efficiency loss of a Class D amplifier is due to the
switching operation of the output stage transistors. In a
Class D amplifier, the output transistors act as current-
steering switches and consume negligible additional
power. Any power loss associated with the Class D out-
put stage is mostly due to the I
on-resistance, and quiescent current overhead.
The theoretical best efficiency of a Class AB linear
amplifier is 78%, however, that efficiency is only exhibit-
ed at peak output powers. Under normal operating lev-
els (typical music reproduction levels), efficiency falls
below 30%, whereas the MAX9706/MAX9707 still
exhibit >90% efficiencies under the same conditions
(Figure 5).
The MAX9706/MAX9707 feature four selectable speak-
er gain and two headphone gain settings controlled by
two gain-control inputs GAIN1 and GAIN2 (see Table 2).
Note that the stereo headphone output is full band-
width, but the stereo speaker outputs are highpass fil-
tered by the crossover circuitry.
Table 2. Speaker Gain
Figure 5. Efficiency vs. Class AB Efficiency
GAIN2
0
0
1
1
GAIN1
100
90
80
70
60
50
40
30
20
10
0
1
0
1
0
0
0.2
SPEAKER
GAIN (dB)
MAX9706
P
+10.5
+13.5
OUT
+12
OUTPUT POWER (W)
CLASS AB
TOTAL P
+9
0.4
PER CHANNEL
OUT
0.6
2
R loss of the MOSFET
MAX9706 HEADPHONE
Signal Path Gain
0.8
V
f
R
IN
DD
L
= 1kHz
= 8Ω
= 5V
1.0
GAIN (dB)
Efficiency
+3
+3
0
0
1.2