Datasheet

MAX9722A/MAX9722B
5V, Differential Input, DirectDrive, 130mW
Stereo Headphone Amplifiers with Shutdown
8 _______________________________________________________________________________________
Detailed Description
The MAX9722A/MAX9722B stereo headphone amplifiers
feature Maxim’s DirectDrive architecture, eliminating the
large output-coupling capacitors required by conven-
tional single-supply headphone amplifiers. The devices
consist of two class AB headphone amplifiers, undervolt-
age lockout (UVLO)/shutdown control, charge pump,
and comprehensive click-and-pop suppression circuitry
(see Typical Application Circuit). The charge pump
inverts the positive supply (PV
DD
), creating a negative
supply (PV
SS
). The headphone amplifiers operate from
these bipolar supplies with their outputs biased about
GND (Figure 1). The benefit of this GND bias is that the
amplifier outputs do not have a DC component, typically
V
DD
/2. The large DC-blocking capacitors required with
conventional headphone amplifiers are unnecessary,
thus conserving board space, reducing system cost,
and improving frequency response. The device features
an undervoltage lockout that prevents operation from an
insufficient power supply and click-and-pop suppression
that eliminates audible transients on startup and shut-
down. Additionally, the MAX9722A/MAX9722B feature
thermal-overload and short-circuit protection and can
withstand ±8kV ESD strikes at the output pins.
Differential Input
The MAX9722 can be configured as a differential input
amplifier (Figure 2), making it compatible with many
CODECs. A differential input offers improved noise
immunity over a single-ended input. In devices such as
cellular phones, high-frequency signals from the RF
transmitter can couple into the amplifier’s input traces.
The signals appear at the amplifier’s inputs as com-
mon-mode noise. A differential input amplifier amplifies
the difference of the two inputs, and signals common to
both inputs are cancelled. Configured differentially, the
gain of the MAX9722 is set by:
A
V
= R
F1
/R
IN1
R
IN1
must be equal to R
IN2
, and R
F1
must be equal to
R
F2
.
The common-mode rejection ratio (CMRR) is limited by
the external resistor matching. For example, the worst-
case variation of 1% tolerant resistors results in 40dB
CMRR, while 0.1% resistors result in 60dB CMRR. For
best matching, use resistor arrays.
The R
IN1
and R
F1
of the MAX9722B are internal, set
R
IN2
= 15kΩ and R
F2
= 30kΩ. However, for best
results, use the MAX9722A.
+V
DD
OR 3V
-V
DD
OR -3V
GND
V
OUT
CONVENTIONAL DRIVER-BIASING SCHEME
DirectDrive BIASING SCHEME
V
DD
/2
V
DD
GND
V
OUT
Figure 1. Conventional Driver Output Waveform vs. MAX9722A/
MAX9722B Output Waveform
R
IN1
= R
IN2
, R
F1
= R
F2
*R
IN1
AND R
F1
ARE INTERNAL FOR MAX9722B.
IN-
IN+
OUT
R
F1
*
R
IN2
R
IN1
*
R
F2
Figure 2. Differential Input Configuration