MAX9705_09 Datasheet, PDF (11/18 Pages) Maxim Integrated Products – 2.3W, Ultra-Low-EMI, Filterless, Class D Audio Amplifier

2.3W, Ultra-Low-EMI, Filterless,
Class D Audio Amplifier
t_SW
t_SW
t_SW
t_SW
V_IN-
V_IN+
OUT-
OUT+
t_ON(MIN)
V_OUT+ - V_OUT-
Figure 2. MAX9705 Output with an Input Signal Applied (SSM Mode)
Filterless Modulation/Common-Mode Idle
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
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 I²R loss of the MOSFET
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).
Shutdown
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_DD for normal operation.
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