MAX9704ETJ Maxim, MAX9704ETJ Datasheet - Page 10

• Manufacturer Part Number: MAX9704ETJ
• Description: 15W / Filterless / Spread-Spectrum Mono/Stereo Class D Amplifiers
• Manufacturer: Maxim

The MAX9703/MAX9704 feature a differential input struc-
ture, making them compatible with many CODECs, and
offering improved noise immunity over a single-ended
input amplifier. In devices such as PCs, noisy digital sig-
nals 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, any signal common to both
inputs is canceled.
The MAX9703/MAX9704 can be configured as single-
ended input amplifiers by capacitively coupling either
input to GND and driving the other input (Figure 3).
An input capacitor, C
impedance of the MAX9703/MAX9704, forms a high-
pass filter that removes the DC bias from an incoming
signal. The AC-coupling capacitor allows the amplifier
to bias the signal to an optimum DC level. Assuming
zero-source impedance, the -3dB point of the highpass
filter is given by:
Choose C
of interest. Setting f
quency response of the amplifier. Use capacitors with
dielectrics that 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.
Use capacitors with an ESR less than 100mΩ for opti-
mum performance. Low-ESR ceramic capacitors mini-
mize the output resistance of the charge pump. For
best performance over the extended temperature
range, select capacitors with an X7R dielectric.
The value of the flying capacitor (C1) affects the load
regulation and output resistance of the charge pump. A
C1 value that is too small degrades the device’s ability to
provide sufficient current drive. Increasing the value of
15W, Filterless, Spread-Spectrum
Mono/Stereo Class D Amplifiers
10
______________________________________________________________________________________
IN
Charge-Pump Capacitor Selection
so f
-3dB
f
- 3dB
-3dB
is well below the lowest frequency
IN
, in conjunction with the input
=
too high affects the low-fre-
2
Component Selection
π
R C
IN IN
1
Flying Capacitor (C1)
Single-Ended Input
Input Amplifier
Differential Input
Input Filter
C1 improves load regulation and reduces the charge-
pump output resistance to an extent. Above 1µF, the on-
resistance of the switches and the ESR of C1 and C2
dominate.
The output capacitor value and ESR directly affect the rip-
ple at CHOLD. Increasing C2 reduces output ripple.
Likewise, decreasing the ESR of C2 reduces both ripple
and output resistance. Lower capacitance values can be
used in systems with low maximum output power levels.
The MAX9703/MAX9704 do not require an output filter
and can pass FCC emissions standards with unshield-
ed 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 a ferrite bead filter when radiat-
ed frequencies above 10MHz are of concern. Use an
LC filter when radiated frequencies below 10MHz are of
concern, or when long leads connect the amplifier to
the speaker. Refer to the MAX9704 Evaluation Kit
schematic for details of this filter.
In certain systems, a single audio source can be shared
by multiple devices (speaker and headphone ampli-
fiers). When sharing inputs, it is common to mute the
unused device, rather than completely shutting it down,
preventing the unused device inputs from distorting the
input signal. Mute the MAX9703/MAX9704 by driving SS
low through an open-drain output or MOSFET (see the
System Diagram). Driving SS low turns off the class D
output stage, but does not affect the input bias levels of
the MAX9703/MAX9704. Be aware that during normal
operation, the voltage at SS can be up to 7V, depending
on the MAX9703/MAX9704 supply.
Figure 3. Single-Ended Input
SINGLE-ENDED
AUDIO INPUT
0.47µF
0.47µF
Sharing Input Sources
IN+
IN-
MAX9703/
MAX9704
Hold Capacitor (C2)
Output Filter