MAX9726 Maxim Integrated Products, MAX9726 Datasheet - Page 11

• Manufacturer Part Number: MAX9726
• Description: Headphone Amplifier
• Manufacturer: Maxim Integrated Products

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The MAX9726 features a low-noise charge pump. The
610kHz switching frequency is well beyond the audio
range, and does not interfere with the audio signals.
This enables the MAX9726 to achieve an SNR of
102dB. The switch drivers feature a controlled switch-
ing speed that minimizes noise generated by turn-on
and turn-off transients. Limiting the switching speed of
the charge pump also minimizes di/dt noise caused by
the parasitic bond wire and trace inductance.
In conventional single-supply headphone amplifiers,
the output coupling capacitor is a major contributor of
audible clicks and pops. The amplifier charges the
coupling capacitor to its output bias voltage at startup.
During shutdown, the capacitor is discharged. This
charging and discharging results in a DC shift across
the capacitor, which appears as an audible transient at
the headphone speaker. Since the MAX9726 head-
phone amplifier does not require output-coupling
capacitors, no audible transients occur.
Additionally, the MAX9726 features extensive click-and-
pop suppression that eliminates any audible transient
sources internal to the device. The Power-Up/Power-
Down graph in the Typical Operating Characteristics
shows that there are minimal transients at the output
upon startup or shutdown.
In most applications, the preamplifier driving the
MAX9726 has a DC bias of typically half the supply.
The input-coupling capacitor is charged to the pream-
plifier’s bias voltage through the MAX9726’s input resis-
tor (R
across the capacitor creates an audible click-and-pop.
Delay the rise of SHDN by at least four time constants
(4 x R
avoid clicks/pops caused by the input filter.
The MAX9726 features a 8µA, low-power shutdown
mode that reduces quiescent current consumption and
extends battery life. Shutdown is controlled by a hard-
ware and software interface. Driving the SHDN input low
disables the drive amplifiers, bias circuitry, charge
pump, and sets the headphone amplifier output resis-
tance to 50kΩ. Similarly, the MAX9726 enters shutdown
when bit seven (B7) in the control register is set to 0 (see
the Command Register section). SHDN and B7 must be
high to enable the MAX9726. The I
active and the contents of the command register are not
affected when in shutdown. This allows the master to
write to the MAX9726 while in shutdown.
IN
IN
) during startup. The resulting voltage shift
x C
IN
) relative to the start of the preamplifier to
______________________________________________________________________________________
BassMax, I
Click-and-Pop Suppression
DirectDrive, Headphone Amplifier with
2
C/SMBus interface is
Charge Pump
Shutdown
2
C, Volume and Gain Control
Typical headphones do not have a flat-frequency
response. The small physical size of the diaphragm
does not allow the headphone speaker to efficiently
reproduce low frequencies. This physical limitation
results in attenuated bass response. The MAX9726
includes a bass-boost feature that compensates for the
headphone’s poor bass response by increasing the
amplifier gain at low frequencies.
The DirectDrive output of the MAX9726 has more head-
room than typical single-supply headphone amplifiers.
This additional headroom allows boosting the bass fre-
quencies without the output signal clipping.
Program the BassMax gain and cutoff frequency with
external components connected between OUT_ and
BM_ (see the Gain-Setting Components section and the
Functional Diagram/Typical Operating Circuit). Use the
I
ister to enable/disable the BassMax circuit.
BM_ is connected to the noninverting input of the out-
put amplifier when BassMax is enabled. BM_ is pulled
to SGND when BassMax is disabled. The typical appli-
cation of the BassMax circuit involves feeding a low-
pass version of the output signal back to the amplifier.
This is realized using positive feedback from OUT_ to
BM_. Figure 2 shows the connections needed to imple-
ment BassMax.
Figure 2. BassMax External Connections
2
ATTENUATOR
C-compatible interface to program the command reg-
STAGE
FROM
MAX9726
BassMax
ENABLE
R
R
BassMax (Bass Boost)
OUT_
BM_
R1
R2
TO HEADPHONE
SPEAKER
C3
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