max9758etjt Maxim Integrated Products, Inc., max9758etjt Datasheet - Page 19

• Manufacturer Part Number: max9758etjt
• Description: 2.3w Stereo Speaker Amplifiers And Directdrive Headphone Amplifiers With Automatic Level Control
• Manufacturer: Maxim Integrated Products, Inc.

Maxim’s patent-pending DirectDrive architecture uses a
charge pump to create an internal negative supply volt-
age. This allows the MAX9756/MAX9757/MAX9758 head-
phone amplifier output to be biased at 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 MAX9756/MAX9757/
MAX9758 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 graph in the Typical Operating
Characteristics for details of the possible capacitor
values.
In addition to the cost and size disadvantages, the DC-
blocking capacitors limit the low-frequency response of
the amplifier. The impedance of the headphone load to
the DC-blocking capacitor forms 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.
Figure 10. Low-Frequency Attenuation of Common DC-
Blocking Capacitor Values
Headphone Amplifiers with Automatic Level Control
OUT
is the value of the DC-blocking capacitor.
L
2.3W Stereo Speaker Amplifiers and DirectDrive
is the impedance of the headphone and
-10.5
-12.0
-13.5
-15.0
-1.5
-3.0
-4.5
-6.0
-7.5
-9.0
0
10
f
−
______________________________________________________________________________________
3
LOW-FREQUENCY ROLLOFF
dB
100
FREQUENCY (Hz)
=
33µF
(R
100µF
L
Low-Frequency Response
2 π
220µF
= 16Ω)
1k
330µF
R C
DirectDrive
L OUT
1
10k
100k
Larger values of C
physically larger, more expensive capacitors.
shows the relationship between the size of C
the resulting low-frequency attenuation. Note that the
-3dB point for a 16Ω headphone with a 100µF-blocking
capacitor is 100Hz, well within the audio band.
The MAX9756/MAX9757/MAX9758 feature a low-noise
inverting charge pump to generate the negative rail
necessary for DirectDrive headphone operation. The
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.
The headphone sense input (HPS) monitors the head-
phone jack and automatically configures the MAX9756/
MAX9757/MAX9758 based upon the voltage applied at
HPS. A voltage of less than 0.8V enables the speaker
amplifier. A voltage of greater than 2V disables the
speaker amplifiers and enables the headphone ampli-
fiers. For automatic headphone detection, connect HPS
to the control pin of a 3-wire headphone jack as shown
in Figure 11. With no headphone present, the output
impedance of the headphone amplifier pulls HPS low.
When a headphone plug is inserted into the jack, the
control pin is disconnected from the tip contact and
HPS is pulled to V
Figure 11. HPS Configuration
SHUTDOWN
CONTROL
Headphone Sense Input (HPS)
DD
MAX9756/
MAX9757/
HPS
HPL
HPR
MAX9758
OUT
14kΩ
with 35µA.
reduce the attenuation but are
35µA
14kΩ
V
DD
Charge Pump
Figure 10
OUT
and
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