Maxim Integrated Products 44
MAX97003
High-Efficiency, Low-Noise Audio Subsystem
Applications Information
Filterless Class D Operation
Traditional Class D amplifiers require an output filter
to recover the audio signal from the amplifier’s output.
The filters add cost, increase the solution size of the
amplifier, and can decrease efficiency and THD+N
performance. The traditional PWM scheme uses large
differential output swings (2 x VPVDD peak-to-peak) and
causes large ripple currents. Any parasitic resistance in
the filter components results in a loss of power, lowering
the efficiency.
The IC 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 out-
put. Eliminating the output filter results in a smaller, less
costly, more efficient solution.
Because the frequency of the IC 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 can be damaged. For optimum
results, use a speaker with a series inductance > 10FH.
Typical 8I speakers exhibit series inductances in the
20FH to 100FH range.
RF Susceptibility
GSM radios transmit using time-division multiple access
(TDMA) with 217Hz intervals. The result is an RF signal
with strong amplitude modulation at 217Hz and its har-
monics that is easily demodulated by audio amplifiers.
The IC is designed specifically to reject RF signals. PCB
layout, however, has a large impact on the susceptibility
of the end product.
In RF applications, improvements to both layout and
component selection decreases the IC’s susceptibility to
RF noise and prevent RF signals from being demodulated
into audible noise. Trace lengths should be kept below
1/4 of the wavelength of the RF frequency of interest.
Minimizing the trace lengths prevents them from function-
ing as antennas and coupling RF signals into the IC. The
wavelength (
l) in meters is given by: l = c/f where c = 3
x 108 m/s, and f = the RF frequency of interest.
Route audio signals on middle layers of the PCB to allow
ground planes above and below to shield them from RF
interference. Ideally, the top and bottom layers of the
PCB should primarily be ground planes to create effec-
tive shielding.
Additional RF immunity can also be obtained by rely-
ing on the self-resonant frequency of capacitors as it
exhibits the frequency response similar to a notch filter.
Depending on the manufacturer, 10pF to 20pF capaci-
tors typically exhibit self resonance at RF frequencies.
These capacitors when placed at the input pins can
effectively shunt the RF noise at the inputs of the IC. For
these capacitors to be effective, they must have a low-
impedance, low-inductance path to the ground plane.
Avoid using microvias to connect to the ground plane
whenever possible as these vias do not conduct well at
RF frequencies.
Startup/Shutdown Sequencing
To ensure proper device initialization and minimal click-
and-pop, program the IC’s control registers in the correct
order.
Table 13 lists the correct startup sequence for the
device. To shutdown the IC, simply set SHDN = 0.
Table 13. Startup Sequence
SEQUENCE
DESCRIPTION
REGISTERS
1
Ensure SHDN = 0
0x12
2
Configure inputs
0x03, 0x04
3
Configure mixers
0x05, 0x06
4
Configure volume
0x07, 0x08, 0x09
5
Configure output gain
0x10
6
Enable amplifiers
0x12
7
Configure expander and DRC
0x0A–0x0F
10
Set SHDN = 1
0x12