MAX9708 datasheet(12/22 Pages) AD

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MAX9708

20W/40W, Filterless, Spread-Spectrum,

Mono/Stereo, Class D Amplifier

Maxim Integrated

Detailed Description

The MAX9708 filterless, Class D audio power amplifier

features several improvements to switch-mode amplifi-

er technology. The MAX9708 is a two-channel, stereo

amplifier with 21W output power on each channel. The

amplifier can be configured to output 42W 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 MAX9708 features a mono mode that allows the

right and left channels to operate in parallel, achieving

up to 42W of output power. The mono mode is enabled

by applying logic-high to MONO. In this mode, an

audio signal applied to the right channel (INR+/INR-) is

routed to the H-bridge of both channels, while a 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 I2R 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 MAX9708 still exhibits 87% efficiency under the

same conditions.

Shutdown

The MAX9708 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 MAX9708 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

MAX9708, 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 MAX9708 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 330k

Ω 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 un-mutes 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 MAX9708 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.

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