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MAX98309/MAX98310
Mono 1.4W Class AB Audio Amplifiers
The increase in power delivered by the BTL configuration
directly results in an increase in internal power dissipa-
tion over the single-ended configuration. The maximum
internal power dissipation for a given VDD and load is
given by the following equation:
2
DD
D(MAX) 2L
2V
P
R
=
π
If the internal power dissipation for a given application
exceeds the maximum allowed for a given package,
reduce power dissipation by increasing the ground plane
heatsinking capability and the size of the traces to the
device. See the Layout and Grounding section. Other
methods for reducing power dissipation are to reduce
VDD, increase load impedance, decrease ambient
temperature, reduce gain, or reduce input signal.
Thermal-overload protection limits total power dissipation
in the MAX98309/MAX98310. When the junction temper-
ature exceeds +160NC, the thermal protection circuitry
disables the amplifier. Operation returns to normal once
the die cools by 15NC.
Amplifier Gain
Fixed Differential Gain (MAX98310)
The MAX98310 features four internally fixed differential
gain options selectable by GAIN (Table 1). This simplifies
design, decreases required application footprint size, and
eliminates external gain-setting resistors.
External Differential Gain (MAX98309)
The MAX98309 features an external gain option. Resistors
RF and RIN. See the Simplified Block Diagram and set the
gain of the amplifier as follows:
F
VIN
R
AR
=
where AV is the desired voltage gain. Hence, an RIN of
10kI and an RF of 20kI yields a gain of 2V/V or 6dB.
RF can be either fixed or variable, allowing the use of a
digitally controlled potentiometer to alter the gain under
software control.
Input Filter
The fully differential amplifier inputs can be biased at
voltages other than midsupply. The common-mode
feedback circuit adjusts for input bias, ensuring the out-
puts are still biased at midsupply. Input capacitors are
not required as long as the input voltage is within the
specified common-mode range listed in the Electrical
Characteristics table.
If input capacitors are used, input capacitor CIN, in con-
junction with the input resistor RIN, 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 imped-
ance, the -3dB point of the highpass filter is given by:
3dB IN IN
1
f2 R C
−=
π
Setting f-3dB too high affects the low-frequency response
of the amplifier. Use capacitors with adequately low volt-
age coefficients, such as X7R ceramic capacitors with a
high voltage rating. Capacitors with higher voltage coef-
ficients result in increased distortion at low frequencies.
BIAS Capacitor
BIAS is the output of the internally generated VDD/2 bias
voltage. The BIAS bypass capacitor, CBIAS, improves
PSRR and THD+N by reducing power supply and other
noise sources at the common-mode bias node, and also
generates the clickless/popless startup DC bias wave-
form for the speaker amplifiers. Bypass BIAS with a 0.1FF
capacitor to GND. Larger values of CBIAS (up to 1FF)
improve PSRR.
Supply Bypassing
Proper power-supply bypassing ensures low-noise, low-
distortion performance. Connect a 1FF ceramic capacitor
from VDD to GND. Add additional bulk capacitance as
required by the application. Locate the bypass capacitor
as close as possible to the device.
Layout and Grounding
Good PCB layout is essential for optimizing performance.
Use large traces for the power-supply inputs and amplifier
outputs to minimize losses due to parasitic trace resis-
tance and route heat away from the device. Good ground-
ing improves audio performance, and prevents any digital
switching noise from coupling into the audio signal.
Table 1. Fixed Differential Gain
GAIN CONNECTION GAIN (dB)
VDD 0
GND 3
Unconnected 6
BIAS 9