max9791 Maxim Integrated Products, Inc., max9791 Datasheet - Page 19

• Manufacturer Part Number: max9791
• Description: Windows Vista-compliant Class D Speaker Amplifiers With Directdrive Headphone Amplifiers
• Manufacturer: Maxim Integrated Products, Inc.

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).
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.
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.
Figure 2. EMI with 30cm of Speaker Cable
Amplifiers with DirectDrive Headphone Amplifiers
40
35
30
25
20
15
10
5
DirectDrive Headphone Amplifier
30
Windows Vista-Compliant Class D Speaker
______________________________________________________________________________________
CLASS D EMI PLOT
FREQUENCY (MHz)
100
Speaker Current Limit
1000
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.
Figure 3. Traditional Amplifier Output vs. MAX9791/MAX9792
DirectDrive Output
V
V
OUT
OUT
CONVENTIONAL AMPLIFIER BIASING SCHEME
DirectDrive AMPLIFIER BIASING SCHEME
V
V
GND
+V
GND
-V
DD
DD
DD
DD
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