12 _____________________________________________________________________________________

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 MAX98502. 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 for an 8I load

and 2A for a 4I load. 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 (CIN)

An input capacitor, CIN, in conjunction with the input

impedance of the MAX98502 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

2R C

=

π

Choose CIN 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 VBATT x D x (1-D))/(IOUT(MAX) x fSW)

where fSW is the switching frequency, and D is the duty

factor given by D = 1 - (VBAT/VOUT). Using L from the

equation above results in a peak-to-peak inductor cur-

rent ripple of 0.5 x IOUT/(1 - D), and a peak inductor

current of 1.25 x IOUT/(1 - D). Ensure the peak (satura-

tion) current rating of the inductor meets or exceeds this

requirement.

The recommended nominal inductance for the MAX98502

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 (CVCCOUT)

An output capacitor, CVCCOUT, 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 MAX98502 is 22FF (0805

case size or larger). Ensure the minimum capacitance at

5.5V is 6.8FF.

Input Capacitor (CVBAT)

An input capacitor, CVBAT, 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