MAX9725 Maxim Integrated Products, MAX9725 Datasheet - Page 7

• Manufacturer Part Number: MAX9725
• Description: Stereo Headphone Amplifier
• Manufacturer: Maxim Integrated Products

Pump Capacitance and Load Resistance graph in the
Typical Operating Characteristics for details of the possi-
ble capacitor sizes.
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 raises some issues:
•
•
•
Figure 1. Traditional Driver Output Waveform vs. MAX9725
Output Waveform (Ideal Case)
1V, Low-Power, DirectDrive, Stereo Headphone
The sleeve is typically grounded to the chassis.
Using this biasing approach, the sleeve must be
isolated from system ground, complicating product
design.
During an ESD strike, the driver’s ESD structures
are the only path to system ground. The driver must
be able to withstand the full ESD strike.
When using the headphone jack as a line out to
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.
V
V
OUT
OUT
CONVENTIONAL DRIVER-BIASING SCHEME
_______________________________________________________________________________________
DirectDrive BIASING SCHEME
V
V
GND
V
GND
-V
DD
DD
DD
DD
/ 2
Amplifier with Shutdown
Large DC-blocking capacitors limit the amplifier’s low-
frequency response and can distort the audio signal:
1) The impedance of the headphone load and the DC-
2) The voltage coefficient of the DC-blocking capacitor
The combination of low-frequency attenuation and fre-
quency-dependent distortion compromises audio
reproduction in portable audio equipment that empha-
sizes low-frequency effects such as multimedia lap-
tops, as well as MP3, CD, and DVD players. These
low-frequency, capacitor-related deficiencies are elimi-
nated by using DirectDrive technology.
The MAX9725 features a low-noise charge pump. The
580kHz 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
transients. The di/dt noise caused by the parasitic bond
wire and trace inductance is minimized by limiting the
turn-on/off speed of the charge pump. Additional high-
blocking capacitor forms a highpass filter with the
-3dB point set by:
where R
C
highpass filter is required by conventional single-
ended, single power-supply headphone drivers to
block the midrail DC-bias component of the audio
signal from the headphones. The drawback to the
filter is that it can attenuate low-frequency signals.
Larger values of C
in physically larger, more expensive capacitors.
Figure 2 shows the relationship between the size of
C
Note that the -3dB point for a 16Ω headphone with
a 100µF blocking capacitor is 100Hz, well within the
normal audio band, resulting in low-frequency
attenuation of the reproduced signal.
contributes distortion to the reproduced audio signal
as the capacitance value varies when the function of
the voltage across the capacitor changes. At low
frequencies, the reactance of the capacitor domi-
nates at frequencies below the -3dB point and the
voltage coefficient appears as frequency-dependent
distortion. Figure 3 shows the THD+N introduced by
two different capacitor dielectric types. Note that
below 100Hz, THD+N increases rapidly.
OUT
OUT
is the value of the DC-blocking capacitor. The
and the resulting low-frequency attenuation.
L
is the impedance of the headphone and
f
-3dB
=
OUT
2 R C
π
L OUT
reduce this effect but result
Low-Frequency Response
1
Charge Pump
7