Datasheet

Detailed Description
The MAX9709 filterless, Class D audio power amplifier
features several improvements to switch mode ampli-
fier 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 performance
with Class D efficiency, while occupying minimal 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 configured as a single-ended input
amplifier.
Mono/Stereo Conguration
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 routed
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 out-
puts are wired together in mono mode may trigger the
short-circuit or thermal protection or both, and may even
damage the device.
Efciency
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 output 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 effi-
ciency is only exhibited at peak output powers. Under nor-
mal operating levels (typical music reproduction levels),
efficiency falls below 30%, whereas 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 switch-
ing, 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 opti-
mizes click-and-pop suppression.
Click-and-Pop Suppression
The MAX9709 features comprehensive click-and-pop
suppression that eliminates audible transients on startup
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 subse-
quently enabled. Following startup, a soft-start function
gradually unmutes the input amplifiers. 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 tem-
perature 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 protection mode, the
device is shut down but the thermal sensor is still active.
MAX9709 25W/50W, Filterless, Spread-Spectrum,
Stereo/Mono, Class D Amplier
www.maximintegrated.com
Maxim Integrated
12