Datasheet

MAX98357A/MAX98357B
PCM Input Class D Audio Power Amplifiers
28Maxim Integrated
Class D Speaker Amplifier
The filterless Class D amplifier offers much higher efficiency
than Class AB amplifiers. The high efficiency of a Class
D amplifier is due to the switching operation of the output
stage transistors. Any power loss associated with the
Class D output stage is mostly due to the I
2
R loss of the
MOSFET on-resistance and quiescent current overhead.
Ultra-Low EMI Filterless Output Stage
Traditional Class D amplifiers require the use of external
LC filters, or shielding, to meet EN55022B electromag-
netic-interference (EMI) regulation standards. Maxim’s
active emissions-limiting edge-rate control circuitry and
spread-spectrum modulation reduces EMI emissions
while maintaining up to 92% efficiency.
Maxim’s spread-spectrum modulation mode flattens
wideband spectral components while proprietary tech-
niques ensure that the cycle-to-cycle variation of the
switching period does not degrade audio reproduction or
efficiency. The ICs’ spread-spectrum modulator randomly
varies the switching frequency by Q20kHz around the
center frequency (330kHz). Above 10MHz, the wideband
spectrum looks like noise for EMI purposes (Figure 14).
Speaker Current Limit
If the output current of the speaker amplifier exceeds the
current limit (2.8A typ), the IC disables the outputs for
approximately 100Fs. At the end of the 100Fs, the out-
puts are re-enabled. If the fault condition still exists, the
IC continues to disable and reenable the outputs until the
fault condition is removed.
Gain Selection
The ICs offer five programmable gain selections through
a single gain input (GAIN_SLOT) in I
2
S/left justified
mode. Gain is referenced to the full-scale output of the
DAC, which is 2.1dBV (Table 8). In TDM mode, the gain
is automatically set at a fixed 12dB. Assuming that the
desired output swing is not limited by the supply voltage
rail, the IC’s output level can be calculated based on
the digital input signal level and selected amplifier gain
according to the following equation:
Output signal level (dBV) = input signal level (dBFS) +
2.1dB + selected amplifier gain (dB)
where 0dBFS is referenced to 0dBV.
Click-and-Pop Suppression
The IC speaker amplifier features Maxim’s comprehen-
sive click-and-pop suppression. During startup, the click-
and-pop suppression circuitry reduces audible transient
sources internal to the device by ramping the input signal
from mute to 0dB. When entering shutdown, the differen-
tial speaker outputs simultaneously drop to GND.
The comprehensive click-and-pop suppression of the
MAX98357 is unaffected by power-up or power-down
sequencing. Applying the DAI clocks before or after the
transition of SD_MODE yields the same click-and-pop
performance. The MAX98357 does not have a volume
ramp-down response when entering shutdown. For opti-
mal click-and-pop performance, ramp down the digital
data on SDIN before powering down the MAX98357.
Table 8. Gain Selection
Figure 14. EMI with 12in of Speaker Cable and No Output
Filtering
GAIN_SLOT I
2
S/LJ GAIN (dB)
Connect to GND through 100kI
Q5% resistor
15
Connect to GND 12
Unconnected 9
Connect to V
DD
6
Connect to V
DD
through 100kI
Q5% resistor
3
FREQUENCY (MHz)
EMISSIONS LEVEL (dBµV/m)
900800600 700200 300 400 500100
10
30
50
70
90
-10
0 1000