Rainbow Electronics MAX9792 - Manual (Page 19)

MAX9791/MAX9792
Windows Vista-Compliant Class D Speaker
Amplifiers with DirectDrive Headphone Amplifiers
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In addition to active emission limiting, the MAX9791/
MAX9792 feature patented spread-spectrum modulation
that flattens the wideband spectral components.
Proprietary techniques ensure that the cycle-to-cycle
variation of the switching period does not degrade audio
reproduction or efficiency (see the
Typical Operating
Characteristics ). In spread-spectrum modulation mode,
the switching frequency varies randomly by ±15kHz
around the center frequency (530kHz). The effect is to
reduce the peak energy at harmonics of the switching
frequency. Above 10MHz, the wideband spectrum looks
like noise for EMI purposes (see Figure 2).
Speaker Current Limit
When the output current of the speaker amplifier
exceeds the current limit (2A, typ) the MAX9791/
MAX9792 disable the outputs for approximately 100µs.
At the end of 100µs, the outputs are re-enabled. If the
fault condition still exists, the MAX9791/MAX9792 con-
tinue to disable and re-enable the outputs until the fault
condition is removed.
DirectDrive Headphone Amplifier
Traditional single-supply headphone amplifiers bias the
outputs at a nominal DC voltage (typically half the sup-
ply). Large coupling capacitors are needed to block
this DC bias from the headphone. Without these capac-
itors, a significant amount of DC current flows to the
headphone, resulting in unnecessary power dissipation
and possible damage to both headphone and head-
phone amplifier.
Maxim’s patented DirectDrive architecture uses a
charge pump to create an internal negative supply volt-
age. This allows the headphone outputs of the
MAX9791/MAX9792 to be biased at GND while operat-
ing from a single supply (Figure 3). Without a DC com-
ponent, there is no need for the large DC-blocking
capacitors. Instead of two large (220µF, typ) capaci-
tors, the MAX9791/MAX9792 charge pump requires two
small 1µF ceramic capacitors, conserving board space,
reducing cost, and improving the frequency response
of the headphone amplifier.
The MAX9791/MAX9792 feature a low-noise charge
pump. The nominal switching frequency of 530kHz is
well beyond the audio range, and thus does not inter-
fere with audio signals. The switch drivers feature a
controlled switching speed that minimizes noise gener-
ated by turn-on and turn-off transients. By limiting the
switching speed of the charge pump, the di/dt noise
caused by the parasitic trace inductance is minimized.
CLASS D EMI PLOT
FREQUENCY (MHz)
AMPLITUDE (dB
μ
V/m)
100
10
15
20
25
30
35
40
5
30 1000
Figure 2. EMI with 30cm of Speaker Cable
V DD
V DD /2
GND
CONVENTIONAL AMPLIFIER BIASING SCHEME
DirectDrive AMPLIFIER BIASING SCHEME
+V DD
GND
-V DD
V OUT
V OUT
Figure 3. Traditional Amplifier Output vs. MAX9791/MAX9792
DirectDrive Output