DirectDrive
Conventional single-supply headphone amplifiers have
their outputs biased about a nominal DC voltage (typical-
ly half the supply) for maximum dynamic range. Large
coupling capacitors are needed to block this DC bias
from the headphone. Without these capacitors, a signifi-
cant amount of DC current flows to the headphone,
resulting in unnecessary power dissipation and possible
damage to both headphone and headphone amplifier.
Maxim’s DirectDrive architecture uses a charge pump
to create an internal negative supply voltage. This
allows the MAX9720 output to be biased about GND,
almost doubling 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 MAX9720 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 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 raised some issues:
• 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 amplifier’s ESD structures
are the only path to system ground. The amplifier
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
equipment, resulting in large ground-loop current
and possible damage to the amplifiers.
• When using a combination microphone and speak-
er headset (in a cell phone or PDA application), the
microphone typically requires a GND return. Any
DC bias on the sleeve conflicts with the microphone
requirements (Figure 2).
Low-Frequency Response
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-
blocking capacitor form a highpass filter with the
-3dB point determined by:
where R
L
is the impedance of the headphone and
C
OUT
is the value of the DC-blocking capacitor.
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
OUT
reduce the attenuation,
but are physically larger, more expensive capacitors.
Figure 3 shows the relationship between the size of
C
OUT
and 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.
