max9752aetit Maxim Integrated Products, Inc., max9752aetit Datasheet - Page 14

• Manufacturer Part Number: max9752aetit
• Description: 2.2w, Low-emi, Stereo, Class D Power Amplifiers With Directdrive Headphone Amplifiers
• Manufacturer: Maxim Integrated Products, Inc.

Conventional single-supply headphone amplifiers have
their outputs biased about a nominal DC voltage (typi-
cally half the supply) for maximum dynamic range.
Large coupling capacitors are needed to block the DC
bias from the headphones.
Maxim’s patented DirectDrive architecture uses a charge
pump to create an internal negative supply voltage. This
allows the MAX9752/MAX9753/MAX9754 headphone
amplifier output to be biased about GND, almost doubling
the 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 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 capac-
itors 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
2) During an ESD strike, the amplifier’s ESD structures
2.2W, Low-EMI, Stereo, Class D Power Amplifiers
with DirectDrive Headphone Amplifiers
Figure 6. Low-Frequency Attenuation of Common DC-Blocking
Capacitor Values
14
this biasing approach, the sleeve must be isolated
from system ground, complicating product design.
are the only path to system ground. The amplifier
must be able to withstand the full ESD strike.
______________________________________________________________________________________
-12
-15
-3
-6
-9
0
10
LOW-FREQUENCY ROLLOFF
100
FREQUENCY (Hz)
33μF
(R
100μF
L
Headphone Amplifier
220μF
= 16Ω)
1k
330μF
DirectDrive
10k
DirectDrive
100k
3) When using the headphone jack as a line out to
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 and the DC-
2) The voltage coefficient of the capacitor, the change in
other equipment, the bias voltage on the sleeve
may conflict with the ground potential from other
equipment, resulting in large ground-loop current
and possible damage to the amplifiers.
blocking capacitor form a highpass filter with the
-3dB point determined by:
where R
C
The highpass filter is required by conventional single-
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
uation, but are physically larger, more expensive
capacitors. Figure 6 shows the relationship between
the size of C
attenuation. Note the -3dB point for a 16Ω head-
phone with a 100µF blocking capacitor is 100Hz, well
within the audio band.
capacitance due to a change in the voltage across
the capacitor, distorts the audio signal. At frequen-
cies around the -3dB point, the reactance of the
capacitor dominates, and the voltage coefficient
appears as frequency-dependent distortion. Figure 7
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
frequency-dependent distortion compromises audio
reproduction. DirectDrive improves low-frequency
reproduction in portable audio equipment that
emphasizes low-frequency effects such as multi-
media laptops, MP3, CD, and DVD players.
OUT
is the value of the DC-blocking capacitor.
L
is the impedance of the headphone and
f
−
OUT
3
dB
and the resulting low-frequency
=
2 π
Low-Frequency Response
R C
L OUT
1
OUT
reduce the atten-