MAX9879 Maxim Integrated Products, MAX9879 Datasheet - Page 20

• Manufacturer Part Number: MAX9879
• Description: Stereo Class D Audio Subsystem
• Manufacturer: Maxim Integrated Products

www.datasheet4u.com
Stereo Class D Audio Subsystem
with DirectDrive Headphone Amplifier
Traditional single-supply headphone amplifiers have
outputs biased at a nominal DC voltage (typically half
the supply). Large coupling capacitors are needed to
block this DC bias from the headphone. Without these
capacitors, a significant amount of DC current flows to
the headphone, resulting in unnecessary power dissi-
pation and possible damage to both the headphone
and headphone amplifier.
Maxim’s patented DirectDrive
charge pump to create an internal negative supply volt-
age. This allows the headphone outputs of the
MAX9879 to be biased at GND while operating from a
single supply (Figure 5). Without a DC component, there
is no need for the large DC-blocking capacitors. Instead
of two large (220µF, typ) capacitors, the MAX9879
charge pump requires two small ceramic capacitors,
Figure 4. EMI with 152mm of Speaker Cable
DirectDrive is a registered trademark of Maxim Integrated
Products, Inc.
20
______________________________________________________________________________________
DirectDrive Headphone Amplifier
40
35
25
20
15
10
40
35
30
25
20
15
10
5
300
30
350
60
®
400
architecture uses a
80
450
100
500
120
550
140
FREQUENCY (MHz)
FREQUENCY (MHz)
600
160
650
180
700
conserving board space, reducing cost, and improving
the frequency response of the headphone amplifier. See
the Output Power vs. Load Resistance graph in the
Typical Operating Characteristics for details of the pos-
sible capacitor sizes. There is a low DC voltage on the
amplifier outputs due to amplifier offset. However, the
offset of the MAX9879 is typically ±1.5mV, which, when
combined with a 32Ω load, results in less than 47µA of
DC current flow to the headphones.
In addition to the cost and size disadvantages of the
DC-blocking capacitors required by conventional head-
phone amplifiers, these capacitors limit the amplifier’s
low-frequency response and can distort the audio sig-
nal. Previous attempts at eliminating the output-cou-
pling capacitors involved biasing the headphone return
(sleeve) to the DC bias voltage of the headphone
amplifiers. This method raises some issues:
200
750
220
800
240
850
260
900
280
950
1000
300
MAX9879 OUTPUT
MAX9879 OUTPUT
TEST LIMIT
TEST LIMIT