Rainbow Electronics MAX9712 User Manual
Page 10
MAX9712
500mW, Low EMI, Filterless,
Class D Audio Amplifier
10
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the load as a DC voltage, resulting in finite load current,
increasing power consumption. When no signal is pre-
sent at the input of the MAX9712, the outputs switch as
shown in Figure 4. Because the MAX9712 drives the
speaker differentially, the two outputs cancel each
other, resulting in no net idle mode voltage 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.
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 MAX9712 still exhibits >80% efficiencies
under the same conditions (Figure 5).
Figure 2. MAX9712 Output with an Input Signal Applied (SSM Mode)
V
OUT+
- V
OUT-
t
SW
t
SW
t
SW
t
SW
V
IN-
V
IN+
OUT+
OUT-
t
ON(MIN)