Rainbow Electronics MAX9770 User Manual
Page 13
External Clock Mode
The SYNC input allows the MAX9770 to be synchro-
nized to a system clock (allowing a fully synchronous
system), or allocating the spectral components of the
switching harmonics to insensitive frequency bands.
Applying an external clock of 800kHz to 2MHz to SYNC
synchronizes the switching frequency of both the Class
D and charge pump. The period of the SYNC clock can
be randomized, enabling the MAX9770 to be synchro-
nized to another spread-spectrum Class D amplifier
operating in SSM mode.
Filterless Modulation/Common-Mode Idle
The MAX9770 uses Maxim’s unique, patented modulation
scheme that eliminates the LC filter required by traditional
Class D amplifiers, improving efficiency, reducing com-
ponent 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 con-
sumption. When no signal is present at the device input,
the outputs switch as shown in Figure 4. Because the
MAX9770 drives the speaker differentially, the two out-
puts cancel each other, resulting in no net idle mode volt-
age across the speaker, minimizing power consumption.
Efficiency
Efficiency of a Class D amplifier is attributed to the
region of 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*R loss of the MOSFET
on-resistance, and quiescent current overhead.
MAX9770
1.2W Low-EMI, Filterless, Mono Class D Amplifier
with Stereo DirectDrive Headphone Amplifiers
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13
30.0
60.0
80.0
100.0 120.0 140.0 160.0 180.0
280.0 300.0
220.0
200.0
240.0 260.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
AMPLITUDE (dB
µ
V/m)
FREQUENCY (MHz)
Figure 3. MAX9770 EMI with 75mm of Speaker Cable
VIN = 0V
OUT-
OUT+
VOUT+ - VOUT- = 0V
Figure 4. MAX9770 Output with No Signal Applied