MAX9779 Datasheet, PDF (11/19 Pages) Maxim Integrated Products – 2.6W Stereo Audio Power Amplifier and DirectDrive Headphone Amplifier

2.6W Stereo Audio Power Amplifier and
DirectDrive Headphone Amplifier
allows the MAX9779 headphone amplifier output to be
biased about GND, almost doubling the dynamic range
while operating from a single supply. With no DC compo-
nent, there is no need for the large DC-blocking capaci-
tors. Instead of two large capacitors (220µF typ), the
charge pump requires only two small ceramic capacitors
(1µF typ), conserving board space, reducing cost, and
improving the frequency response of the headphone
amplifier. See the Output Power vs. Charge-Pump
Capacitance and Load Resistance graph in the Typical
Operating Characteristics for details of the possible
capacitor values.
Previous attempts to eliminate the output-coupling
capacitors involved biasing the headphone return
(sleeve) to the DC bias voltage of the headphone
amplifiers. This method raised some issues:
1) The sleeve is typically grounded to the chassis. Using
this biasing approach, the sleeve must be isolated
from system ground, complicating product design.
2) During an ESD strike, the amplifier’s ESD structures
are the only path to system ground. 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 large ground-loop current and possible
damage to the amplifiers.
Low-Frequency Response
In addition to the cost and size disadvantages, the DC-
blocking capacitors limit the low-frequency response of
the amplifier and distort the audio signal:
1) The impedance of the headphone load to the DC-
blocking capacitor forms a highpass filter with the
-3dB point determined by:
f_−3dB
=
1
2πR_LC_OUT
where R_L is the impedance of the headphone and
C_OUT is the value of the DC-blocking capacitor.
The highpass filter is required by conventional sin-
gle-ended, single-supply headphone amplifiers to
block the midrail DC component of the audio signal
from the headphones. Depending on the -3dB point,
the filter can attenuate low-frequency signals within
the audio band. Larger values of C_OUT reduce the
attenuation but are physically larger, more expen-
sive capacitors. Figure 3 shows the relationship
between the size of C_OUT and the resulting low-fre-
quency attenuation. Note that the -3dB point for a
0
-5
-10
-15
-20
-25
-30
-35
10
LOW-FREQUENCY ROLLOFF
(R_L = 16Ω)
DirectDrive
330µF
220µF
100µF
33µF
100
1000
FREQUENCY (Hz)
Figure 3. Low-Frequency Attenuation of Common DC-Blocking
Capacitor Values
16Ω headphone with a 100µF blocking capacitor is
100Hz, well within the audio band.
2) The voltage coefficient of the capacitor, the change
in capacitance due to a change in the voltage
across the capacitor, distorts the audio signal. At
frequencies around the -3dB point, the reactance of
the capacitor dominates, and the voltage coefficient
appears as frequency-dependent distortion. Figure
4 shows the THD+N introduced by two different
capacitor dielectrics. Note that around the -3dB
point, THD+N increases dramatically.
The combination of low-frequency attenuation and fre-
quency-dependent distortion compromises audio
reproduction. DirectDrive improves low-frequency
reproduction in portable audio equipment that empha-
sizes low-frequency effects such as multimedia lap-
tops, and MP3, CD, and DVD players.
Charge Pump
The MAX9779 features a low-noise charge pump. The
550kHz switching frequency is well beyond the audio
range, and does not interfere with the audio signals. The
switch drivers feature a controlled switching speed that
minimizes noise generated by turn-on and turn-off tran-
sients. Limiting the switching speed of the charge pump
minimizes the di/dt noise caused by the parasitic bond
wire and trace inductance. Although not typically
required, additional high-frequency ripple attenuation
can be achieved by increasing the size of C2 (see the
Block Diagram).
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