MAX9776EVKIT+ Maxim Integrated Products, MAX9776EVKIT+ Datasheet - Page 22

• Manufacturer Part Number: MAX9776EVKIT+
• Description: EVALUATION KIT FOR MAX9776
• Manufacturer: Maxim Integrated Products
• Series: DirectDrive™r

Specifications of MAX9776EVKIT+

• Amplifier Type: Class D
• Output Type: 1-Channel (Mono) with Stereo Headphones
• Max Output Power X Channels @ Load: 1.5W x 1 @ 4 Ohm; 60mW x 2 @ 16 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Board Type: Fully Populated
• Utilized Ic / Part: MAX9776
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

2 x 1.5W, Stereo Class D Audio Subsystem
with DirectDrive Headphone Amplifier
The MAX9775/MAX9776 use Maxim’s unique modula-
tion scheme that eliminates the LC filter required by tra-
ditional Class D amplifiers, improving efficiency,
reducing component count, conserving board space
and system cost. Conventional Class D amplifiers out-
put a 50% duty-cycle square wave when no signal is
present. With no filter, the square wave appears across
the load as a DC voltage, resulting in finite load current,
increasing power consumption, especially when idling.
When no signal is present at the input of the
MAX9775/MAX9776, the outputs switch as shown in
Figure 4. Because the MAX9775/MAX9776 drive the
speaker differentially, the two outputs cancel each
other, resulting in no net idle mode voltage across the
speaker, minimizing power consumption.
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 DirectDrive architecture uses a charge pump to
create an internal negative supply voltage. This allows the
headphone outputs of the MAX9775/MAX9776 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
MAX9775/MAX9776 charge pump requires two small
ceramic capacitors, conserving board space, reducing
cost, and improving the frequency response of the head-
phone amplifier. See the Output Power vs. Load
Resistance and Charge-Pump Capacitor Size graph in
the Typical Operating Characteristics for details of the
possible capacitor sizes. There is a low DC voltage on
the amplifier outputs due to amplifier offset. However, the
offset of the MAX9775/MAX9776 is typically 1.4mV,
which, when combined with a 32Ω load, results in less
than 44nA of DC current flow to the headphones.
22
Filterless Modulation/Common-Mode Idle
______________________________________________________________________________________
DirectDrive
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 are
3) When using the headphone jack as a lineout to
Figure 4. Outputs with No Input Signal
V OUT+ - V OUT- = 0V
Using the midrail biasing approach, the sleeve must
be isolated from system ground, complicating prod-
uct design.
the only path to system ground. Thus, the amplifier
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 amplifiers.
V IN = 0V
OUT+
OUT-