MAX9705 Maxim Integrated Products, MAX9705 Datasheet - Page 11

• Manufacturer Part Number: MAX9705
• Description: 2.3W Ultra-Low-EMI Filterless Class D Audio Amplifier
• Manufacturer: Maxim Integrated Products

Figure 2. MAX9705 Output with an Input Signal Applied (SSM Mode)
The MAX9705 uses Maxim’s unique, patented modula-
tion scheme that eliminates the LC filter required by
traditional Class D amplifiers, improving efficiency,
reducing component count, and 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 a finite load cur-
rent, increasing power consumption. When no signal is
present at the input of the MAX9705, the outputs switch
as shown in Figure 3. Because the MAX9705 drives the
speaker differentially, the two outputs cancel each other,
resulting in no net idle-mode voltage across the speak-
er, minimizing power consumption.
Efficiency of a Class D amplifier is attributed to the
region of operation of the output stage transistors. In a
Filterless Modulation/Common-Mode Idle
V
V
V
OUT+
IN+
IN-
- V
OUT-
OUT+
OUT-
______________________________________________________________________________________
t
ON(MIN)
2.3W, Ultra-Low-EMI, Filterless,
t
SW
Efficiency
t
SW
Class D Audio Amplifier
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 supply current.
The theoretical best efficiency of a linear amplifier is
78%; however, that efficiency is only exhibited at peak
output powers. Under normal operating levels (typical
music reproduction levels), efficiency falls below 30%,
whereas the MAX9705 still exhibits >70% efficiencies
under the same conditions (Figure 4).
The MAX9705 has a shutdown mode that reduces power
consumption and extends battery life. Driving SHDN low
places the MAX9705 in a low-power (0.3µA) shutdown
mode. Connect SHDN to V
t
SW
t
SW
DD
for normal operation.
2
R loss of the MOSFET
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