MAX9700
1.2W, Low-EMI, Filterless,
Class D Audio Amplifier
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the power conversion of the class D amplifier. For exam-
ple, an 8mV DC offset across an 8Ω load results in 1mA
extra current consumption in a class AB device. In the
class D case, an 8mV offset into 8Ω equates to an addi-
tional power drain of 8µW. Due to the high efficiency of
the class D amplifier, this represents an additional quies-
cent-current draw of 8µW/(V
DD
/100η), which is on the
order of a few microamps.
Input Amplifier
Differential Input
The MAX9700 features a differential input structure,
making it compatible with many CODECs, and offering
improved noise immunity over a single-ended input
amplifier. In devices such as cellular phones, high-fre-
quency signals from the RF transmitter can be picked
up by the amplifier’s input traces. The signals appear at
the amplifier’s inputs as common-mode noise. A differ-
ential input amplifier amplifies the difference of the two
inputs; any signal common to both inputs is canceled.
Single-Ended Input
The MAX9700 can be configured as a single-ended
input amplifier by capacitively coupling either input to
GND and driving the other input (Figure 6).
DC-Coupled Input
The input amplifier can accept DC-coupled inputs that
are biased within the amplifier’s common-mode range
(see the
Typical Operating Characteristics
). DC cou-
pling eliminates the input-coupling capacitors, reducing
component count to potentially one external component
(see the
System Diagram
). However, the low-frequency
rejection of the capacitors is lost, allowing low-frequen-
cy signals to feedthrough to the load.
Component Selection
Input Filter
An input capacitor, C
IN
, in conjunction with the input
impedance of the MAX9700 forms a highpass filter that
removes the DC bias from an incoming signal. The AC-
coupling capacitor allows the amplifier to bias the sig-
nal to an optimum DC level. Assuming zero source
impedance, the -3dB point of the highpass filter is
given by:
Choose C
IN
so f
-3dB
is well below the lowest frequency
of interest. Setting f
-3dB
too high affects the low-fre-
quency response of the amplifier. Use capacitors
whose dielectrics have low-voltage coefficients, such
as tantalum or aluminum electrolytic. Capacitors with
high-voltage coefficients, such as ceramics, may result
in increased distortion at low frequencies.
Other considerations when designing the input filter
include the constraints of the overall system and the
actual frequency band of interest. Although high-fidelity
audio calls for a flat gain response between 20Hz and
20kHz, portable voice-reproduction devices such as
cellular phones and two-way radios need only concen-
trate on the frequency range of the spoken human
voice (typically 300Hz to 3.5kHz). In addition, speakers
used in portable devices typically have a poor response
below 150Hz. Taking these two factors into considera-
tion, the input filter may not need to be designed for a
20Hz to 20kHz response, saving both board space and
cost due to the use of smaller capacitors.
Output Filter
The MAX9700 does not require an output filter. The
device passes FCC emissions standards with 100mm
of unshielded speaker cables. However, output filtering
can be used if a design is failing radiated emissions
due to board layout or cable length, or the circuit is
near EMI-sensitive devices. Use an LC filter when radi-
ated emissions are a concern, or when long leads are
used to connect the amplifier to the speaker.
Supply Bypassing/Layout
Proper power-supply bypassing ensures low-distortion
operation. For optimum performance, bypass V
DD
to
GND and PV
DD
to PGND with separate 0.1µF capaci-
tors as close to each pin as possible. A low-imped-
ance, high-current power-supply connection to PV
DD
is
assumed. Additional bulk capacitance should be
added as required depending on the application and
power-supply characteristics. GND and PGND should
be star connected to system ground. Refer to the
MAX9700 evaluation kit for layout guidance.
