Rainbow Electronics MAX98500 - Manual (Page 10)

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MAX98500
Boosted 2.2W Class D Amplifier
with Automatic Level Control
Detailed Description
The MAX98500 is a high-efficiency Class D audio ampli-
fier that features an integrated boost converter to deliver a
constant output power over a large range of battery supply
voltages. The boost converter operates at 2MHz, requiring
only a small (2.2FH) external inductor and output capaci-
tor. The amplifier has differential inputs and an internal fully
differential design with three gain settings (6dB, 15.5dB,
and 20dB) that are selectable with a logic input.
The MAX98500 also features automatic level control. The
automatic level control reduces the output swing when
the battery voltage decreases to prevent the collapse of
battery voltage.
Class D Speaker Amplifier
The MAX98500 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.
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
reduces EMI emissions, while maintaining up to 92%
efficiency (speaker only). Above 10MHz, the wideband
spectrum looks like noise for EMI purposes.
Automatic Level Control
The MAX98500 features an automatic level control circuit
that limits the maximum speaker output swing. This helps:
U Avoid clipping
U Save the battery from collapsing, which could cause
a reset of the system
The limiter keeps the peak voltage below a value that is a
function of battery voltage, as shown in Figure 1.
The full output swing of 5.2V is maintained for battery
voltages down to the knee voltage, while for lower battery
voltages the maximum V
PEAK -swing is reduced by 3V/V.
The knee voltage can be changed by applying different
resistors between RKNEE and AGND. The typical track-
ing function is shifted horizontally with different R
KNEE
resistor values (Figure 2).
The preamplifier gain reduces as the automatic level
control activates. The maximum gain reduction is 8dB
with a resolution of 0.5dB steps.
The attack (gain reduction) happens immediately (20Fs/
dB), while the release is set to 1.6s/dB.
GAIN Select
The MAX98500 features three internal gain settings that
are selectable with the GAIN input. Table 1 shows the
gain settings.
Figure 1. Typical Tracking Function
Table 1. Gain Settings
Figure 2. The Relationship of R KNEE and V KNEE
PEAK AMPLIFIER OUTPUT VOLTAGE
vs. BATTERY VOLTAGE
BATTERY VOLTAGE (V)
PEAK AMPLIFIER OUTPUT VOLTAGE (V
PEAK
)
5 2 3 4 1
1
2
4
3
5
6
0
0 6
KNEE VOLTAGE
SLOPE
GAIN AMPLIFIER GAIN (dB)
AGND 6
Unconnected 15.5
V BAT 20
V
KNEE
vs. R
KNEE
R KNEE (k I)
V
KNEE
(V)
60 50 40 30 20
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
2.9
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