2.4W, Single-Supply, Class G Power Amplifier
MAX9730
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:
to provide sufficient current drive. Increasing the value
of 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. A 4.7µF capacitor is recommended.
Hold Capacitor (C2)
R
R
R
FB−
FB+
The output capacitor value and ESR directly affect the
=
ripple at PV . Increasing C2 reduces output ripple.
SS
R
IN+
IN−
Likewise, decreasing the ESR of C2 reduces both rip-
ple and output resistance. A 10µF capacitor is recom-
mended.
and
C
= C
IN+
IN−
Charge-Pump Frequency Set Resistor (R
)
FS
The charge pump operates in two modes. When the
charge pump is loaded below 100mA, it operates in a
slow mode where the oscillation frequency is reduced
to 1/4 of its normal operating frequency. Once loaded,
the charge-pump oscillation frequency returns to nor-
mal operation. In applications where the design may be
sensitive to the operating charge-pump oscillation fre-
Component Selection
Input-Coupling Capacitor
The AC-coupling capacitors (C ) and input resistors
IN_
(R ) form highpass filters that remove any DC bias from
IN_
an input signal (see the Typical Application
Circuit/Functional Diagram). C
blocks DC voltages
IN_
from the amplifier. The -3dB point of the highpass filter,
assuming zero source impedance due to the input signal
source, is given by:
quency, the value of the external resistor R can be
FS
changed to adjust the charge-pump oscillation fre-
quency (see Figure 4).
1
f−3dB
=
Hz
(
)
2π ×R
× C
IN_
IN_
CHARGE-PUMP OSCILLATION
Choose C so that f
is well below the lowest fre-
-3dB
IN
FREQUENCY vs. R
FS
quency of interest. Setting f
too high affects the
-3dB
600
550
500
450
400
350
300
250
200
amplifier’s low frequency response. Use capacitors with
low-voltage coefficient dielectrics. Aluminum electrolytic,
tantalum, or film dielectric capacitors are good choices
for AC-coupling capacitors. Capacitors with high-voltage
coefficients, such as ceramics (non-C0G dielectrics),
can result in increased distortion at low frequencies.
I
> 100mA
LOAD
Charge-Pump Capacitor Selection
Use capacitors with an ESR less than 50mΩ 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.
50
75
100
(kΩ)
125
150
R
FS
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
Figure 4. Charge-Pump Oscillation Frequency vs. R
FS
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