
MAX9756/MAX9757/MAX9758
2.3W Stereo Speaker Amplifiers and DirectDrive
Headphone Amplifiers with Automatic Level Control
______________________________________________________________________________________
13
Detailed Description
The MAX9756/MAX9757/MAX9758 combine dual, 2W
BTL stereo audio power amplifiers with a DirectDrive
headphone amplifier in a single device. The stereo
power amplifiers deliver up to 2.3W per channel into a
3Ω speaker from a 5V supply and the stereo head-
phone amplifiers deliver up to 130mW per channel into
a 16Ω headphone from a 5V supply.
The MAX9756/MAX9757 feature ALC that automatically
controls output power to the speaker, preventing loud-
speaker, overload and provides optimized dynamic
range.
The MAX9756/MAX9757/MAX9758 feature 31-step ana-
log volume control and a BEEP input. The amplifier
gain is pin programmable. These devices feature click-
and-pop suppression, eliminating the need for discrete
muting circuitry. Speaker and headphone outputs have
short-circuit and thermal protection.
The MAX9756/MAX9758s’ internal LDO features
Maxim’s Dual Mode feedback. The LDO output volt-
age is either fixed at 4.65V (SET = GND), or adjusted
between 1.23V and 5V using a resistive divider at SET.
The LDO delivers up to 150mA of continuous current,
and can be enabled independently from the audio
amplifiers. Short-circuit and thermal-overload protec-
tion are provided for the LDO.
All devices feature a single-supply voltage, a shut-
down mode, logic-selectable gain, and a headphone
sense input. Industry-leading click-and-pop suppres-
sion eliminates audible transients during power and
shutdown cycles.
Each signal path consists of an input amplifier that sets
the signal-path gain and feeds both the speaker and
headphone amplifiers (Figure 1). The speaker amplifier
uses a BTL architecture, doubling the voltage drive to
the speakers and eliminating the need for DC-blocking
capacitors. The output consists of two signals, identical
in magnitude, but 180° out of phase.
The headphone amplifiers use Maxim’s DirectDrive
architecture that eliminates the bulky output DC-blocking
capacitors required by traditional headphone amplifiers.
A charge pump inverts the positive supply (CPVDD), cre-
ating a negative supply (CPVSS). The headphone ampli-
fiers operate from these bipolar supplies with their
outputs biased about GND (Figure 2).
The amplifiers have almost twice the supply range
compared to other single-supply amplifiers, nearly qua-
drupling the available output power. The benefit of the
GND bias is that the amplifier outputs do not have a DC
component (typically VDD/2). This eliminates the large
DC-blocking capacitors required with conventional
headphone amplifiers, conserving board space and
system cost while improving frequency response.
OUT_+
OUT_-
VOLUME
CONTROL
BIAS
IN_
VOL
BIAS
HP_
GND
ALC
Figure 1. MAX9756/MAX9757 Signal Path
+VDD
-VDD
GND
CONVENTIONAL DRIVER-BIASING SCHEME
DirectDrive BIASING SCHEME
VDD/2
VDD
GND
VOUT
Figure 2. Traditional Headphone Amplifier Output Waveform
vs. DirectDrive Headphone Amplifier Output Waveform
Dual Mode is a trademark of Maxim Integrated Products, Inc.