max9702etit Maxim Integrated Products, Inc., max9702etit Datasheet - Page 24

• Manufacturer Part Number: max9702etit
• Description: Max9702 1.8w, Filterless, Stereo, Class D Audio Power Amplifier And Directdrive Stereo Headphone Amplifier
• Manufacturer: Maxim Integrated Products, Inc.

The MAX9702 features a 0.1µA shutdown mode that
reduces power consumption to extend battery life.
Shutdown is controlled by the hardware or software
interface. Drive SHDN low to disable the drive ampli-
fiers, bias circuitry, charge pump, and set the head-
phone amplifier output impedance to 1kΩ. Similarly, the
MAX9702 enters shutdown when bit 7 (B7) in the con-
trol register is set to zero. Connect SHDN to V
set bit 7 = 1 for normal operation (see Table 7). The I
interface is active and the contents of the command
register are not affected when in shutdown. This allows
the master to write to the MAX9702 while in shutdown.
Traditional Class D amplifiers require an output filter to
recover the audio signal from the amplifier’s PWM out-
put. The filters add cost, increase the solution size of
the amplifier, and can decrease efficiency. The tradi-
tional PWM scheme uses large differential output
swings 2 x V
Any parasitic resistance in the filter components results
in a loss of power, lowering the efficiency.
The MAX9702 does not require an output filter. The
device relies on the inherent inductance of the speaker
coil and the natural filtering of both the speaker and the
human ear to recover the audio component of the
square wave output. By eliminating the output filter, this
results in a smaller, less costly, more efficient solution.
Because the frequency of the MAX9702 output is well
beyond the bandwidth of most speakers, voice coil
movement due to the square wave frequency is very
small. Although this movement is small, a speaker not
designed to handle the additional power may be dam-
aged. For optimum results, use a speaker with a series
inductance >10µH. Typical 8Ω speakers, for portable
audio applications, exhibit series inductances in the
range of 20µH to 100µH.
Unlike a Class AB amplifier, the output offset voltage of
Class D amplifiers does not noticeably increase quies-
cent current draw when a load is applied. This is due to
the power conversion of the Class D amplifier. For
1.8W, Filterless, Stereo, Class D Audio Power
Amplifier and DirectDrive Stereo Headphone Amplifier
Table 7. Shutdown Control
24
B7
______________________________________________________________________________________
0
1
Soft shutdown
Normal operation
DD(P-P)
Applications Information
Filterless Class D Operation
and causes large ripple currents.
Class D Output Offset
FUNCTION
(SHDN)
Shutdown
DD
and
2
C
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 repre-
sents an additional quiescent current draw of
8µW/(V
microamps.
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
component count to potentially one external component
(see the System Diagram). However, the highpass fil-
tering effect of the capacitors is lost, allowing low-fre-
quency signals to feed through to the load.
The MAX9702 has different supplies for each portion of
the device, allowing for the optimum combination of
headroom power dissipation and noise immunity. The
speaker amplifiers are powered from PV
range from 2.5V to 5.5V and must be connected to the
same potential as V
powered from V
of the headphone amplifiers and can range from 2.5V
to 5.5V. V
amplifiers. Connect V
powered by CPV
operation. The charge pump inverts the voltage at
CPV
The remainder of the device is powered by V
An input capacitor, C
impedance of the MAX9702 forms a highpass 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 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.
DD
, and the resulting voltage appears at CPV
DD
IN
SS
/100 x η), which is on the order of a few
such that f
is the negative supply of the headphone
DD
DD
f
and V
−
. Connect CPV
3
DD
SS
dB
IN
-3dB
. The headphone amplifiers are
, in conjunction with the input
to CPV
=
SS
2 π
Component Selection
. V
is well below the lowest fre-
R C
DD
IN IN
SS
1
DC-Coupled Input
. The charge pump is
is the positive supply
DD
Power Supplies
to V
DD
DD
Input Filter
. PV
DD
for normal
.
DD
can
SS
.