Datasheet

Gain Selection
The MAX9703/MAX9704 feature an internally set, logic-
selectable gain. The G1 and G2 logic inputs set the gain
of the MAX9703/MAX9704 speaker amplifier (Table 2).
Output Offset
Unlike a Class AB amplifier, the output offset voltage of
Class D amplifiers does not noticeably increase quiescent
current draw when a load is applied. This is due to the
power conversion of the Class D amplifier. For example,
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 additional
power drain of 8μW. Due to the high efficiency of the
Class D amplifier, this represents an additional quiescent
current draw of: 8μW/(V
DD
/100 x η), which is in the order
of a few microamps.
Input Amplier
Differential Input
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 commonmode
noise. A differential input amplifier amplifies the difference
of the two inputs, any signal common to both inputs is
canceled.
Single-Ended Input
The MAX9703/MAX9704 can be configured as singleen-
ded input amplifiers by capacitively coupling either input
to AGND and driving the other input (Figure 4).
Component Selection
Input Filter
An input capacitor, C
IN
, in conjunction with the input
impedance of the MAX9703/MAX9704, forms a highpass
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:
-
3dB
IN IN
1
f
2R C
=
π
Choose C
IN
so f
-3dB
is well below the lowest frequency of
interest. Setting f
-3dB
too high affects the low-frequency
response of the amplifier. Use capacitors with dielectrics
that have low-voltage coefficients, such as tantalum or
aluminum electrolytic. Capacitors with highvoltage coef-
ficients, such as ceramics, may result in increased distor-
tion at low frequencies.
Charge-Pump Capacitor Selection
Use capacitors with an ESR less than 100mΩ for optimum
performance. Low-ESR ceramic capacitors minimize
the output resistance of the charge pump. For best per-
formance over the extended temperature range, select
capacitors with an X7R dielectric.
Flying Capacitor (C1)
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 C1
improves load regulation and reduces the chargepump
output resistance to an extent. Above 1μF, the onresis-
tance of the switches and the ESR of C1 and C2 dominate.
Hold Capacitor (C2)
The output capacitor value and ESR directly affect the
ripple 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.
Output Filter
The MAX9703/MAX9704 do not require an output filter
and can pass FCC emissions standards with unshielded
speaker cables. However, output filtering can be used if a
Figure 4. Single-Ended Input
Table 2. Gain Selection
G1 G2 GAIN (dB)
0 0 29.6
0 1 19.1
1 0 13
1 1 16
IN+
IN-
0.47µF
0.47µF
SINGLE-ENDED
AUDIO INPUT
MAX9703/
MAX9704
MAX9703/MAX9704 10W Stereo/15W Mono, Filterless,
Spread-Spectrum, Class D Ampliers
www.maximintegrated.com
Maxim Integrated │ 11