MAX9776 Maxim Integrated Products, MAX9776 Datasheet - Page 32

• Manufacturer Part Number: MAX9776
• Description: (MAX9775 / MAX9776) Stereo Class D Audio Subsystem
• Manufacturer: Maxim Integrated Products

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2 x 1.5W, Stereo Class D Audio Subsystem
with DirectDrive Headphone Amplifier
The MAX9775/MAX9776 can be configured as a single-
ended input amplifier by appropriately configuring the
Input Control Register (see Tables 5a and 5b).
The input amplifier can accept DC-coupled inputs that
are biased to the amplifier’s bias voltage. DC-coupling
eliminates the input-coupling capacitors; reducing com-
ponent count to potentially six external components
(see the Typical Application Circuits ). However, the
highpass filtering effect of the capacitors is lost, allow-
ing low-frequency signals to feed through to the load.
Connect any unused input pin directly to VBIAS. This
saves input capacitors on unused inputs and provides
the highest noise immunity on the input.
An input capacitor (C
impedance of the MAX9775/MAX9776 form a highpass
filter that removes the DC bias from the incoming signal.
The AC-coupling capacitor allows the amplifiers to auto-
matically bias the signal to an optimum DC level.
Assuming zero source impedance, the -3dB point of the
highpass filter is given by:
Choose C
quency of interest. Use capacitors whose dielectrics
have low-voltage coefficients, such as tantalum or alu-
minum electrolytic. Capacitors with high-voltage coeffi-
cients, such as ceramics, may result in increased
distortion at low frequencies.
Other considerations when designing the input filter
include the constraints of the overall system and the
actual frequency band of interest. Although high-fidelity
audio calls for a flat-gain response between 20Hz and
20kHz, portable voice-reproduction devices such as cell
phones and two-way radios need only concentrate on
the frequency range of the spoken human voice (typi-
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IN
so that f
f
−
3
dB
-3dB
IN
) in conjunction with the input
=
is well below the lowest fre-
2π
Component Selection
R C
IN IN
1
Single-Ended Input
DC-Coupled Input
Unused Inputs
Input Filter
cally 300Hz to 3.5kHz). In addition, speakers used in
portable devices typically have a poor response below
300Hz. Taking these two factors into consideration, the
input filter may not need to be designed for a 20Hz to
20kHz response, saving both board space and cost
due to the use of smaller capacitors.
The MAX9775/MAX9776 do not require a Class D out-
put filter. The devices pass EN55022B emission stan-
dards with 152mm of unshielded speaker cables.
However, output filtering can be used if a design is fail-
ing radiated emissions due to board layout or cable
length, or the circuit is near EMI-sensitive devices. Use
a ferrite bead filter when radiated frequencies above
10MHz are of concern. Use an LC filter when radiated
frequencies below 10MHz are of concern, or when long
leads (> 152mm) connect the amplifier to the speaker.
Figure 12 shows optional speaker amplifier output filters.
V
voltage. The V
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
V
Figure 12. Speaker Amplifier Output Filter
BIAS
BIAS
OUT_+
OUT_-
with a 1µF capacitor to GND.
is the output of the internally generated DC bias
33µH
33µH
BIAS
External Component Selection
bypass capacitor, C
0.47µF
0.1µF
0.1µF
22Ω
22Ω
Class D Output Filter
0.033µF
0.033µF
BIAS Capacitor
VBIAS
improves