MAX9791AETI+T Maxim Integrated Products, MAX9791AETI+T Datasheet - Page 20

• Manufacturer Part Number: MAX9791AETI+T
• Description: IC AMP AUDIO 2.2W STER D 28TQFN
• Manufacturer: Maxim Integrated Products
• Series: DirectDrive™r
• Type: Class Dr

Specifications of MAX9791AETI+T

• Output Type: 2-Channel (Stereo) with Stereo Headphones and Subwoofer
• Max Output Power X Channels @ Load: 2.2W x 2 @ 4 Ohm; 180mW x 2 @ 32 Ohm
• Voltage - Supply: 4.5 V ~ 5.5 V
• Features: Depop, LDO, Short-Circuit and Thermal Protection, Shutdown
• Mounting Type: Surface Mount
• Package / Case: 28-TQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Windows Vista-Compliant Class D Speaker
Amplifiers with DirectDrive Headphone Amplifiers
In addition to active emission limiting, the MAX9791/
MAX9792 feature spread-spectrum modulation that flat-
tens 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 fre-
quency 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
20
______________________________________________________________________________________
40
35
30
25
20
15
10
DirectDrive Headphone Amplifier
5
30
EN55022B LIMIT
CLASS D EMI PLOT
FREQUENCY (MHz)
100
Speaker Current Limit
1000
Maxim’s DirectDrive architecture uses a charge pump
to create an internal negative supply voltage. This
allows the headphone outputs of the MAX9791/
MAX9792 to be biased at GND while operating from a
single supply (Figure 3). Without a DC component,
there is no need for the large DC-blocking capacitors.
Instead of two large (220µF, typ) capacitors, 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
GND
+V
GND
-V
V
V
DD
DD
DD
DD
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