
MAX9738
Shutdown Mode
The MAX9738 has a shutdown mode that reduces power
consumption and extends battery life. Driving SHDN low
places the MAX9738 in a low-power (0.6μA) shutdown
mode. Connect SHDN to VDD for normal operation.
Click-and-Pop Suppression
The MAX9738 Class G amplifier features Maxim’s com-
prehensive, industry-leading click-and-pop suppres-
sion. During startup, the click-and-pop suppression
circuitry eliminates any audible transient sources inter-
nal to the device.
Applications Information
Differential Input Amplifier
The MAX9738 features a differential input configuration,
making the device compatible with many CODECs, and
offering improved noise immunity over a single-ended
input amplifier. In devices such as PCs, noisy digital
signals can be picked up by the amplifier’s input
traces. The signals appear at the amplifiers’ inputs as
common-mode noise. A differential input amplifier
amplifies the difference of the two inputs, and signals
common to both inputs are canceled out. When config-
ured for differential inputs, the voltage gain of the
MAX9738 is set by:
where AV is the desired voltage gain in dB. RIN+ should
be equal to RIN-, and RFB+ should be equal to RFB-.
The Class G output stage has a fixed gain of 4V/V
(12dB). Any gain or attenuation set by the external
input stage resistors add to or subtract from this fixed
gain. See Figure 2.
In differential input configurations, the common-mode
rejection ratio (CMRR) is primarily limited by the exter-
nal resistor and capacitor matching. Ideally, to achieve
the highest possible CMRR the following external com-
ponents should be selected where:
and
Driving a Ceramic Speaker
Applications that require a thin profile, such as today’s
mobile phones, demand that components have a small
form factor. Dynamic loudspeakers that use a cone and
voice coil typically cannot conform to the height
requirements. The option for these applications is to
use a ceramic/piezoelectric loudspeaker.
Ceramic speakers are much more capacitive than a
conventional loudspeaker. Typical capacitance values
for such a speaker can be greater than 1μF. High peak-
to-peak voltage drive is required to achieve acceptable
sound pressure levels. The high output voltage require-
ment coupled with the capacitive nature of the speaker
demand that the amplifier supply much more current at
high frequencies than at lower frequencies. Above 5kHz
the typical speaker impedance can be less than 20
Ω.
The MAX9738 is ideal for driving a capacitive ceramic
speaker. The high boost converter current limit allows
for a flat frequency response out to 20kHz while main-
taining high output voltage swings. Figure 3 shows a
typical circuit for driving a ceramic speaker.
A 20
Ω series resistance is required between the ampli-
fier output and the ceramic speaker load to ensure the
output of the amplifier sees some fixed resistance at
high frequencies when the speaker is essentially an
electrical short.
CC
IN+
IN-
=
R
FB+
IN+
FB-
IN-
=
A
R
dB
V
FB_
IN_
24 (
=×
()
0log[
)]
16VP-P Class G Amplifier with
Inverting Boost Converter
10
______________________________________________________________________________________
MAX9738
+
IN+
FB+
RIN+
RIN-
CIN+
CIN-
IN-
FB-
-
CLASS G
OUTPUT
STAGE
RFB+
RFB-
Figure 2. Gain Setting