Datasheet

MAX9742
Single-/Dual-Supply, Stereo 16W,
Class D Amplifier with Differential Inputs
18 ______________________________________________________________________________________
Summing Configuration (Audio Mixer)
Figure 6 shows the MAX9742 configured as a summing
amplifier, which allows multiple audio sources to be lin-
early mixed together. Using this configuration, the out-
put of the MAX9742 is equal to the weighted sum of the
input signals:
As shown in the above equation, the weighting or
amount of gain applied to each input signal source is
determined by the ratio of R
F
and the respective input
resistor (R
IN1
, R
IN2
, R
IN3
) connected to each signal
source. Select R
F
and R
IN_
so that the dynamic range
of the MAX9742 is not exceeded when the input signals
are at their maximum values and in phase with each
other (see the
Output Dynamic Range section).
To minimize output offset voltages due to input bias
currents, connect a resistor, R
OS
, (see Figure 6)
between IN_+ and MID. Select the value of R
OS
such
that the DC resistances looking out of inputs of the
amplifier (IN_+ and IN_-) are equal. For example, when
using the dual-supply configuration with a DC-coupled
input source, the value of R
OS
should be equal to
R
F
||R
IN1
||R
IN2
|| ||R
INn
.
Mono Bridge-Tied-Load (BTL)
Configuration
The MAX9742 also accommodates a mono bridge-tied-
load (BTL) configuration that can be used in single-
supply and dual-supply applications. In the BTL
configuration, the speaker load is driven differentially
by connecting the half-bridge outputs as a full H-bridge
driver. To drive the speaker differentially, the inputs of
both channels must be driven by the same audio signal
with one channel 180° out-of-phase with the other
channel. Figure 7 shows the connections required for
BTL operation.
The advantages of BTL operation include reduced
component count due to the elimination of the output-
coupling capacitors when using single-supply opera-
tion, a 6dB increase in gain due to the load being
driven differentially, increased output power into a sin-
gle load, and the minimization of the supply-pumping
since each half bridge is driven 180° out-of-phase (see
the Supply Pumping Effects section). For single-supply
applications, the output-coupling capacitors are not
needed for BTL operation since the DC voltage present
at each half-bridge output is equal in value and applies
to each side of the load. This means no DC voltage
appears across the load, and therefore, no DC current
flows into the speaker.
V (V
R
R
V
R
R
V
R
R
)
OUT_ IN1
F
IN1
IN2
F
IN2
IN3
F
IN3
= − ++
OUT_
V
IN2
C
IN
R
IN2
R
OS
MAX9742
V
IN1
C
IN
R
IN1
V
IN3
C
IN
R
IN3
MID
TO CLASS D
MODULATOR
R
F
IN_-
IN_+
C
FB_
FB_
V
OUT_
(V
IN1
R
F
× V
IN2
R
F
× V
IN3
R
F
)
R
IN1
R
IN2
R
IN3
Figure 6. Summing Amplifier Configuration