Rainbow Electronics MAX9796 - Manual (Page 17)

due to amplifier offset. However, the offset of the
MAX9796 is typically 1.4mV, which, when combined
with a 32Ω load, results in less than 44nA 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.
Using the midrail biasing approach, the sleeve must
be isolated from system ground, complicating prod-
uct design.
2) During an ESD strike, the driver’s ESD structures are
the only path to system ground. Thus, the amplifier
must be able to withstand the full ESD strike.
3) When using the headphone jack as a lineout to other
equipment, the bias voltage on the sleeve may con-
flict with the ground potential from other equipment,
resulting in possible damage to the amplifiers.
Charge Pump
The MAX9796 features a low-noise charge pump. The
switching frequency of the charge pump is half the
switching frequency of the Class D amplifier, regard-
less of the operating mode. The nominal switching fre-
quency is well beyond the audio range, and thus does
not interfere with the audio signals, resulting in an SNR
of 93dB. Although not typically required, additional
high-frequency noise attenuation can be achieved by
increasing the size of C2 (see the
Typical Application
Circuit ). The charge pump is active in both speaker
and headphone modes.
Signal Path
The audio inputs of the MAX9796 (INA, INB, and INC)
are preamplified and then mixed by the input mixer to
create three internal signals: left (L), right (R), and mono
(M). Tables 5a and 5b show how the inputs are mixed to
create L, R, and M. These signals are then independent-
ly volume adjusted by the L, R, and M volume control
and routed to the output mixer. The output mixer mixes
the internal L, R, and M signals to create a variety of
audio mixes that are output to the headphone, speaker,
and the mono receiver amplifiers. Figure 6 shows the
signal path that the audio signals take.
Signal amplification takes place in three stages. In the
first stage, the inputs (INA, INB, and INC) are preampli-
fied. The amount by which each input is amplified is
determined by the bits INA+20dB (B4 in the Input Mode
Control Register) and IN+6dB (B3 in the Global Control
Register). After preamplification, they are mixed in the
input mixer to create the internal signals L, R and M.
In the second stage of amplification, the internal L, R,
and M signals are independently volume adjusted.
Finally, each output amplifier has its own internal gain.
The speaker, headphone, and mono receiver amplifiers
have fixed gains of 12dB, 3dB and 3dB, respectively.
MAX9796
2.3W, High-Power Class D Audio Subsystem
with DirectDrive Headphone Amplifiers
______________________________________________________________________________________ 17
V DD / 2
V DD
GND
+V DD
-V DD
GND V
OUT
V OUT
CONVENTIONAL DRIVER-BIASING SCHEME
DirectDrive BIASING SCHEME
Figure 5. Traditional Amplifier Output vs. MAX9796 DirectDrive
Output