Rainbow Electronics MAX98500 - Manual (Page 12)

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MAX98500
Boosted 2.2W Class D Amplifier
with Automatic Level Control
Route audio signals on the middle layers of the PCB to
allow the 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
effective shielding.
Additional RF immunity can also be obtained from 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 capacitors
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 MAX98500. For
these capacitors to be effective, they must have a low-
impedance, low-inductance path to the ground plane.
Do not use microvias to connect to the ground plane as
these vias do not conduct well at RF frequencies.
Speaker Component Selection
Optional Ferrite Bead Filter
Additional EMI suppression can be achieved using a
filter constructed from a ferrite bead and a capacitor to
ground (Figure 3). Use a ferrite bead with low DC resis-
tance, high-frequency (> 100MHz) impedance between
100I and 600I, and rated for at least 1A. The capacitor
value varies based on the ferrite bead chosen and the
actual speaker lead length. Select a capacitor less than
1nF based on EMI performance.
Input Capacitor (C IN
)
An input capacitor, C IN , in conjunction with the input
impedance of the MAX98500 speaker inputs forms a
highpass filter that removes the DC bias from an incom-
ing analog 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:
-3dB
IN IN
1
f
2 R C
=
π
Choose C IN such that f -3dB is well below the lowest fre-
quency of interest. For best audio quality, use capacitors
whose dielectrics have low-voltage coefficients, such as
tantalum or aluminum electrolytic. Capacitors with high-
voltage coefficients, such as ceramics, could result in
increased distortion at low frequencies.
Boost Converter Component Selection
Inductor Selection
In most step-up converter designs, a reasonable induc-
tor value can be derived from the following equation.
This equation sets peak-to-peak inductor current at 1/2
the DC inductor current:
L = (2 x V BATT x D x (1-D))/(I OUT(MAX) x f SW )
where f
SW
is the switching frequency, and D is the duty
factor given by D = 1 - (V BAT /V OUT ). Using L from the
equation above results in a peak-to-peak inductor cur-
rent ripple of 0.5 x I
OUT /(1 - D), and a peak inductor
current of 1.25 x I OUT /(1 - D). Ensure the peak (satura-
tion) current rating of the inductor meets or exceeds this
requirement.
The recommended nominal inductance for the MAX98500
is 2.2FH. Nominal inductance decreases as the inductor
current increases. If the decrease from the nominal
inductance is severe, the boost converter may become
unstable or shut down at lower output power levels than
expected. Ensure the minimum inductance at the peak
inductor current is 1.0FH.
Output Capacitor (C VCCOUT )
An output capacitor, C VCCOUT , is required to keep the
output voltage ripple small and to ensure regulation loop
stability. The output capacitor must have low imped-
ance at the switching frequency. Ceramic capacitors
are highly recommended due to their small size and low
ESR. Ceramic capacitors with X5R or X7R temperature
characteristics generally perform well. The recommend-
ed nominal capacitance for the MAX98500 is 22FF (0805
case size or larger). Ensure the minimum capacitance at
5.5V is 6.8FF.
Input Capacitor (C VBAT )
An input capacitor, C VBAT , reduces the current peaks
drawn from the battery or input power source and reduc-
es switching noise in the IC. The impedance of the input
capacitor at the switching frequency should be kept very
low. Ceramic capacitors are highly recommended due
to their small size and low ESR. Ceramic capacitors with
X5R or X7R temperature characteristics generally per-
form well. One 10FF ceramic capacitor is recommended
with a system bulk capacitance of 22FF or larger.
Figure 3. Optional Class D Ferrite Bead Filter
MAX98500
SPKP
SPKN