______________________________________________________________________________________ 11

Boosted 2.2W Class D Amplifier

with Automatic Level Control

Shutdown

The MAX98502 features two active-low shutdown inputs

(SDSPK and SDBST). Table 2 shows the different shut-

down configurations.

Click-and-Pop Suppression

The MAX98502 speaker amplifier features Maxim’s com-

prehensive click-and-pop suppression. During startup,

the click-and-pop suppression circuitry reduces any

audible transient sources internal to the device. When

entering shutdown, the differential speaker outputs ramp

down to SPKPGND quickly and simultaneously.

Current-Limit and Thermal Protection

The IC features overcurrent and thermal protection.

to about 80% of the expected output. The IC also

enters into shutdown when the die temperature exceeds

+165NC. The device remains in shutdown until power

is reset or SDBST is toggled low and back high after

the fault condition has been removed. The IC speaker

amplifier also features a 2A (typ) short-circuit protection

scheme.

Boost Converter

Soft-Start

The MAX98502 features a two-stage, soft-start, power-

(typ), see the Soft-Start graph in the Typical Operating

Characteristics. The maximum load current is available

after the soft-start is completed.

Undervoltage Lockout (UVLO)

The undervoltage lockout (UVLO) circuit compares the

ensure that the input voltage is high enough for reliable

threshold, the soft-start begins. When the input voltage

falls below the UVLO threshold, the boost converter and

speaker amplifier turn off.

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

filter adds cost, increases the solution size of the ampli-

fier, and can decrease efficiency and THD+N perfor-

mance. The traditional PWM scheme uses large differen-

tial output swings (2 x supply voltage peak-to-peak) and

causes large ripple currents. Any parasitic resistance in

the filter components results in a loss of power and low-

ers the efficiency.

The MAX98502 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. Eliminating the output filter results

in a smaller, less costly, and more efficient solution.

Because the frequency of the MAX98502 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 dam-

aged. 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 MAX98502 is designed specifically to reject RF

signals; however, PCB layout has a large impact on the

susceptibility of the end product.

In RF applications, improvements to both layout and

component selection decrease the MAX98502’s suscep-

tibility 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

functioning as antennas and coupling RF signals into the

MAX98502. The wavelength (

l) in meters is given by:

of interest.

Table 2. Shutdown Configurations

SDBST

SDSPK

BOOST

STATUS

SPEAKER

STATUS

Low

Low

Off

Off

Low

High

Off

Off

High

Low

On

Off

High

High

On

On