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.

1.2W Low-EMI, Filterless, Mono Class D Amplifier

with Stereo DirectDrive Headphone Amplifiers

______________________________________________________________________________________

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

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