MAX9709
25W/50W, Filterless, Spread-Spectrum,
Stereo/Mono, Class D Amplifier
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Detailed Description
The MAX9709 filterless, Class D audio power amplifier
features several improvements to switch mode amplifi-
er technology. The MAX9709 is a two-channel, stereo
amplifier with 25W output power on each channel. The
amplifier can be configured to output 50W output
power in mono mode. The device offers Class AB per-
formance with Class D efficiency, while occupying min-
imal board space. A unique filterless modulation
scheme and spread-spectrum switching mode create a
compact, flexible, low-noise, efficient audio power
amplifier. The differential input architecture reduces
common-mode noise pickup, and can be used without
input-coupling capacitors. The device can also be con-
figured as a single-ended input amplifier.
Mono/Stereo Configuration
The MAX9709 features a mono mode that allows the
right and left channels to operate in parallel, achieving
up to 50W of output power. The mono mode is enabled
by applying logic high to MONO. In this mode, audio
signal applied to the right channel (INR+/INR-) is rout-
ed to the H-bridge of both channels, while signal
applied to the left channel (INL+/INL-) is ignored.
OUTL+ must be connected to OUTR+ and OUTL- must
be connected to OUTR- using heavy PC board traces
as close to the device as possible (see Figure 2).
When the device is placed in mono mode on a PC
board with outputs wired together, ensure that the
MONO pin can never be driven low when the device is
enabled. Driving the MONO pin low (stereo mode)
while the outputs are wired together in mono mode may
trigger the short-circuit or thermal protection or both,
and may even damage the device.
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
2
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%, where-
as the MAX9709 still exhibits 87% efficiency under the
same conditions.
Shutdown
The MAX9709 features a shutdown mode that reduces
power consumption and extends battery life. Driving
SHDN low places the device in low-power (0.1µA) shut-
down mode. Connect SHDN to digital high for normal
operation.
Mute Function
The MAX9709 features a clickless/popless mute mode.
When the device is muted, the outputs stop switching,
muting the speaker. Mute only affects the output stage
and does not shut down the device. To mute the
MAX9709, drive MUTE to logic low. Driving MUTE low
during the power-up/down or shutdown/turn-on cycle
optimizes click-and-pop suppression.
Click-and-Pop Suppression
The MAX9709 features comprehensive click-and-pop
suppression that eliminates audible transients on start-
up and shutdown. While in shutdown, the H-bridge is
pulled to GND through a 330kΩ resistor. During startup
or power-up, the input amplifiers are muted and an
internal loop sets the modulator bias voltages to the
correct levels, preventing clicks and pops when the H-
bridge is subsequently enabled. Following startup, a
soft-start function gradually unmutes the input ampli-
fiers. The value of the soft-start capacitor has an impact
on the click-and-pop levels, as well as startup time.
Thermal Sensor
The MAX9709 features an on-chip temperature sensor
that monitors the die temperature. When the junction
temperature exceeds a programmed level, TEMP is
pulled low. This flags the user to reduce power or shut
down the device. TEMP may be connected to SS or
MUTE for automatic shutdown during overheating. If
TEMP is connected to MUTE, during thermal protection
mode, the audio is muted and the device is in mute
mode. If TEMP is connected to SS, during thermal pro-
tection mode, the device is shut down but the thermal
sensor is still active.
