Rainbow Electronics MAX9770 User Manual

Page 14

The theoretical best efficiency of a linear amplifier is
78%; however, that efficiency is only exhibited at peak
output powers. Under normal operating levels (typical
music reproduction levels), efficiency falls below 30%,
whereas the MAX9770 still exhibits >80% efficiencies
under the same conditions (Figure 5).

DirectDrive

Traditional single-supply headphone drivers have their
outputs biased about a nominal DC voltage (typically
half the supply) for maximum dynamic range. Large
coupling capacitors are needed to block this DC bias
from the headphone. Without these capacitors, a signif-
icant amount of DC current flows to the headphone,
resulting in unnecessary power dissipation and possi-
ble damage to both headphone and headphone driver.

Maxim’s patented DirectDrive architecture uses a charge
pump to create an internal negative supply voltage. This
allows the headphone outputs of the MAX9770 to be
biased about GND, almost doubling dynamic range
while operating from a single supply. With no DC compo-
nent, there is no need for the large DC-blocking capaci-
tors. Instead of two large (220µF, typ) tantalum
capacitors, the MAX9770 charge pump requires two
small ceramic capacitors, conserving board space,
reducing cost, and improving the frequency response of
the headphone driver. See the Output Power vs. Charge-
Pump Capacitance and Load Resistance graph in the
Typical Operating Characteristics for details of the possi-
ble capacitor sizes. There is a low DC voltage on the dri-
ver outputs due to amplifier offset. However, the offset of
the MAX9770 is typically 5mV, which, when combined
with a 32

Ω load, results in less than 160µA of DC current

flow to the headphones.

In addition to the cost and size disadvantages of the DC-
blocking capacitors required by conventional head-
phone amplifiers, these capacitors limit the amplifier’s
low-frequency response and can distort the audio signal.

Previous attempts at eliminating the output-coupling
capacitors involved biasing the headphone return
(sleeve) to the DC bias voltage of the headphone
amplifiers. This method raises some issues:

1) When combining a microphone and headphone on

a single connector, the microphone bias scheme
typically requires a 0V reference.

2) The sleeve is typically grounded to the chassis.

Using the midrail biasing approach, the sleeve must
be isolated from system ground, complicating prod-
uct design.

3) During an ESD strike, the driver’s ESD structures are

the only path to system ground. Thus, the driver
must be able to withstand the full ESD strike.

4) 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.

Charge Pump

The MAX9770 features a low-noise charge pump. The
switching frequency of the charge pump is 1/2 the
switching frequency of the Class D amplifier, regardless
of the operating mode. When SYNC is driven externally,
the charge pump switches at 1/2 f

SYNC

. When SYNC =

, the charge pump switches with a spread-spectrum

pattern. The nominal switching frequency is well beyond
the audio range, and thus does not interfere with the
audio signals, resulting in an SNR of 101dB. The switch
drivers feature a controlled switching speed that mini-
mizes noise generated by turn-on and turn-off tran-
sients. By limiting the switching speed of the charge
pump, the di/dt noise caused by the parasitic bond wire
and trace inductance is minimized. Although not typical-
ly required, additional high-frequency noise attenuation
can be achieved by increasing the size of C2 (see
Typical Application Circuit). The charge pump is active
in both speaker and headphone modes.

Input Multiplexer/Mixer

The MAX9770 features an input multiplexer/mixer that
allows three different audio sources to be selected/
mixed. Driving a SEL_ input high selects the input chan-
nel (see Table 2), and the audio signal is output to the
active amplifier. When a stereo path is selected in
speaker mode (SEL1 or SEL2 = 1), the left and right

MAX9770

1.2W Low-EMI, Filterless, Mono Class D Amplifier
with Stereo DirectDrive Headphone Amplifiers

______________________________________________________________________________________

100

0.1

0.2

0.4

0.3

0.5

0.6

EFFICIENCY vs. OUTPUT POWER

OUTPUT POWER (W)

EFFICIENCY (%)

MAX9770

CLASS AB

= 3.3V

f = 1kHz
R

- 8

Ω

Figure 5. MAX9770 Efficiency vs. Class AB Efficiency