max9789a Maxim Integrated Products, Inc., max9789a Datasheet - Page 14

• Manufacturer Part Number: max9789a
• Description: Windows Vista-compliant, Stereo Class Ab Speaker Amplifiers And Directdrive Headphone Amplifiers Integrated Products
• Manufacturer: Maxim Integrated Products, Inc.

Windows Vista-Compliant, Stereo Class AB Speaker
Amplifiers and DirectDrive Headphone Amplifiers
The output voltage is set by the following equation:
where V
To simplify resistor selection:
Since the input bias current at LDO_SET is typically
less than 500nA (max), large resistance values can be
used for R1 and R2 to minimize power consumption
without compromising accuracy. The parallel combina-
tion of R1 and R2 should be less than 1M .
Conventional single-supply headphone amplifiers have
their outputs biased about a nominal DC voltage
(V
capacitors are needed to block this DC bias from the
headphones. Without these capacitors, a significant
amount of DC current flows to the headphone, resulting
in unnecessary power dissipation and possible dam-
age to both headphone and headphone amplifier.
Maxim’s patented DirectDrive architecture uses a
charge pump to create an internal negative supply volt-
age. It allows the MAX9789A/MAX9790A headphone
amplifier output to be biased about GND. With no DC
component, there is no need for the large DC-blocking
Figure 1. Adjustable Output Using External Feedback
Resistors.
14
DD
______________________________________________________________________________________
/ 2) for maximum dynamic range. Large coupling
LDO_SET
MAX9789A
V
LDO OUT
R
LDO_OUT
LDO_SET
1
= 1.21V.
_
=
GND
R
2



=
V
LDO OUT
V
LDO SET
1 21
R2
R1
.
_
_
−



1
1



+
R
R
1µF
2
DirectDrive
1



1µF
capacitors. Instead of two large capacitors (330µF typi-
cally required to meet Vista magnitude response speci-
fications), the MAX9789A/MAX9790A charge pump
requires only two small 1µF ceramic capacitors, con-
serving board space, reducing cost, and improving the
low-frequency response of the headphone amplifier.
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:
• The sleeve is typically grounded to the chassis.
• During an ESD strike, the amplifier’s ESD structures
• 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:
• The impedance of the headphone load and the DC-
• The highpass filter is required by conventional single-
Using this biasing approach, the sleeve must be iso-
lated from system ground, complicating product
design.
are the only path to system ground. The amplifier
must be able to withstand the full ESD strike.
other equipment, the bias voltage on the sleeve may
conflict with the ground potential from other equip-
ment, 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
ended, single-supply headphone amplifier 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
but are physically larger, more expensive capacitors.
Figure 2 shows the relationship between the size of
C
Note the Vista’s magnitude response specification
calls for a -3dB point at 20Hz at the headphone jack.
The -3dB point at 20Hz for a 32 headphone requires
a 330µF blocking capacitor (Table 2).
OUT
OUT
is the value of the DC-blocking capacitor.
and the resulting low-frequency attenuation.
L
is the impedance of the headphone and
f
−
3
dB
=
2π
Low-Frequency Response
OUT
R C
L OUT
1
reduce the attenuation,