MAX9750/MAX9751/MAX9755
2.6W Stereo Audio Power Amplifiers and
DirectDrive Headphone Amplifiers
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Detailed Description
The MAX9750/MAX9751/MAX9755 combine a 2.6W BTL
speaker amplifier and a 110mW DirectDrive headphone
amplifier with integrated headphone sensing and com-
prehensive click-and-pop suppression. The MAX9750
features an analog volume control, BEEP input, and
four-level gain control. The MAX9751 features a 2:1
input stereo multiplexer and two-level gain control. All
devices feature high 90dB PSRR, low 0.01% THD+N,
industry-leading click-pop performance, and a low-
power shutdown mode.
Each signal path consists of an input amplifier that sets
the gain of the signal path and feeds both the speaker
and headphone amplifier (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-block-
ing capacitors required by traditional headphone ampli-
fiers. A charge pump inverts the positive supply
(CPV
DD
), creating a negative supply (CPV
SS
). The
headphone amplifiers operate from these bipolar sup-
plies 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 no longer have a
DC component (typically V
DD
/ 2). This eliminates the
large DC-blocking capacitors required with convention-
al headphone amplifiers, conserving board space and
system cost, and improving frequency response.
The MAX9750 features an analog volume control that
varies the gain of the amplifiers based on the DC volt-
age applied at VOL. Both devices feature an undervolt-
age lockout that prevents operation from an insufficient
power supply and click-and-pop suppression that elim-
inates audible transients on startup and shutdown. The
amplifiers include thermal-overload and short-circuit
protection, and can withstand ±8kV ESD strikes on the
headphone amplifier outputs (IEC air discharge). An
additional feature of the speaker amplifiers is that there
is no phase inversion from input to output.
DirectDrive
Conventional single-supply headphone amplifiers have
their outputs biased about a nominal DC voltage (typi-
cally half the supply) for maximum dynamic range.
Large coupling capacitors are needed to block this DC
bias from the headphones. Without these capacitors, a
