max9702etit Maxim Integrated Products, Inc., max9702etit Datasheet - Page 19

• Manufacturer Part Number: max9702etit
• Description: Max9702 1.8w, Filterless, Stereo, Class D Audio Power Amplifier And Directdrive Stereo Headphone Amplifier
• Manufacturer: Maxim Integrated Products, Inc.

Amplifier and DirectDrive Stereo Headphone Amplifier
In conventional single-supply headphone amplifiers,
the output-coupling capacitor is a major contributor of
audible clicks and pops. Upon startup, the amplifier
charges the coupling capacitor to its bias voltage, typi-
cally half the supply. Likewise, during shutdown the
capacitor is discharged to GND. This results in DC shift
across the capacitor, which in turn, appears as an
audible transient at the speaker. Since the MAX9702
headphone amplifier does not require output-coupling
capacitors, this does not arise.
The MAX9702 offers four headphone amplifier gain set-
tings controlled through the I
amplifier gains of -2dB, +1dB, +4dB, and +7dB are set
by command register bits 3 and 4 (Table 5).
Additionally, the MAX9702 features extensive click-and-
pop suppression that eliminates any audible transient
sources internal to the device.
In most applications, the output of the preamplifier dri-
ving the MAX9702 has a DC bias of typically half the
supply. During startup, the input-coupling capacitor is
charged to the preamplifier’s DC bias voltage through
the RF of the MAX9702, resulting in a DC shift across
the capacitor and an audible click-and-pop. An internal
delay of 40ms eliminates the clicks-and-pops caused
by the input filter.
Traditional single-supply headphone amplifiers have
outputs biased at a nominal DC voltage (typically half
the supply) for maximum dynamic range. Large-cou-
pling 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 dissipation and possible dam-
age to both headphone and headphone 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
MAX9702 to be biased at GND, almost doubling
dynamic range while operating from a single supply.
With no DC component, there is no need for the large
DC-blocking capacitors. Instead of two large (220µF,
typ) tantalum capacitors, the MAX9702 charge pump
requires two small ceramic capacitors, conserving
board space, reducing cost, and improving the fre-
quency response of the headphone amplifier. See the
Output Power vs. Charge-Pump Capacitance and Load
______________________________________________________________________________________
1.8W, Filterless, Stereo, Class D Audio Power
Headphone Amplifier
2
C interface. Headphone
DirectDrive
Resistance graph in the Typical Operating
Characteristics for details of the possible capacitor
sizes. There is a low DC voltage on the driver outputs
due to amplifier offset. However, the offset of the
MAX9702 is typically 1.1mV, which, when combined
with a 32Ω load, results in less than 56µA 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:
1) The sleeve is typically grounded to the chassis.
2) During an ESD strike, the driver’s ESD structures
3) When using the headphone jack as a line out to
Figure 7. Traditional Amplifier Output vs. MAX9702 DirectDrive
Output
Using the midrail biasing approach, the sleeve must
be isolated from system ground, complicating prod-
uct design.
are the only path to system ground. Thus, the driver
must be able to withstand the full ESD strike.
other equipment, the bias voltage on the sleeve may
conflict with the ground potential from other equip-
ment, resulting in possible damage to the drivers.
CONVENTIONAL AMPLIFIER BIASING SCHEME
DirectDrive BIASING SCHEME
V
V
GND
+V
GND
-V
DD
DD
DD
DD
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