MAX9706/MAX9707
3-Channel, 2.3W, Filterless Class D Amplifiers
with Active Crossover
20
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Power Supplies
The MAX9706/MAX9707 have different supplies for
each portion of the devices, allowing for the optimum
combination of headroom power dissipation and noise
immunity. The speaker amplifiers are powered from
PVDD. PVDD can range from 4.5V to 5.5V and must be
connected to the same potential as VDD. The head-
phone amplifiers are powered from HPVDD and VSS.
HPVDD is the positive supply of the headphone ampli-
fiers and can range from 3V to 5.5V. VSS is the negative
supply of the headphone amplifiers. Connect VSS to
CPVSS. The charge pump is powered by CPVDD.
Connect CPVDD to VDD for normal operation. The
charge pump inverts the voltage at CPVDD, and the
resulting voltage appears at CPVSS. The remainder of
the device is powered by VDD.
Component Selection
Input Filter
An input capacitor, CIN, in conjunction with the input
impedance of the MAX9706/MAX9707 forms a high-
pass filter that removes the DC bias from an incoming
signal. The AC-coupling capacitor allows the amplifier
to automatically bias the signal to an optimum DC level.
Assuming zero-source impedance, the -3dB point of
the highpass filter is given by:
Choose CIN so f-3dB is well below the lowest frequency of
interest. Use capacitors whose dielectrics have low-volt-
age coefficients, such as tantalum or aluminum electrolyt-
ic. Capacitors with high-voltage coefficients, such as
ceramics, may result in increased distortion at low fre-
quencies.
Crossover Selection
Select the crossover filter to suit the chosen speaker.
Many small diameter speakers (as used in notebooks
and smaller displays) are self resonant (fO) at 800Hz to
1000Hz. Often these speakers have a slight peaking at
resonance, so choosing a crossover frequency at 2 x fO
can be effective. Ensure the mono channel speaker has
its fO much lower than crossover frequency (fC).
Supply Bypassing, Layout, and Grounding
Proper layout and grounding are essential for optimum
performance. Use large traces for the power-supply
inputs and amplifier outputs to minimize losses due to
parasitic trace resistance. Large traces also aid in mov-
ing heat away from the package. Proper grounding
improves audio performance, minimizes crosstalk
between channels, and prevents any switching noise
from coupling into the audio signal. Connect PGND and
GND together at a single point on the PC board (star
configuration). Route all traces that carry switching
transients away from GND and the traces/components
in the audio signal path.
Connect the power-supply inputs VDD and PVDD
together and connect CPVDD and HPVDD together.
Bypass HPVDD and CPVDD with a 1F capacitor in par-
allel with a 0.1F capacitor to PGND. Bypass VDD and
PVDD with a 1F capacitor to GND. Place the bypass
capacitors as close to the device as possible. Place a
bulk capacitor between PVDD and PGND if needed.
Use large, low-resistance output traces. Current drawn
from the outputs increase as load impedance decreas-
es. High-output trace resistance decreases the power
delivered to the load. Large output, supply, and GND
traces allow more heat to move from the device to the
air, decreasing the thermal impedance of the circuit if
possible or connect to VSS.
The MAX9706/MAX9707 thin QFN-EP package fea-
tures an exposed thermal pad on its underside. This
pad lowers the package’s thermal impedance by pro-
viding a direct heat conduction path from the die to
the PC board. The exposed thermal pad is not inter-
nally connected. Connect the exposed pad to GND.
BIAS Capacitor
BIAS is the output of the internally generated DC 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/shutdown
DC bias waveforms for the speaker amplifiers. Bypass
BIAS with a 1F capacitor to GND.
f
RC
dB
IN
=
××
3
1
2
π