Rainbow Electronics MAX9877 User Manual

Page 20

MAX9877

Low RF Susceptibility, Mono Audio
Subsystem with DirectDrive Headphone Amplifier

______________________________________________________________________________________

1) The sleeve is typically grounded to the chassis.

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

2) During an ESD strike, the amplifier’s ESD structures

are the only path to system ground. Thus, the
amplifier must be able to withstand the full energy
from an ESD strike.

3) 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 possible damage to the
amplifiers.

The MAX9877 features a low-noise charge pump. The
switching frequency of the charge pump is

of the

Class D switching frequency, regardless of the operating
mode. When the Class D amplifiers are operated in
spread-spectrum mode, the charge pump also switches
with a spread-spectrum pattern. The nominal switching
frequency is well beyond the audio range, and thus does
not interfere with audio signals. The switch drivers fea-
ture a controlled switching speed that minimizes noise
generated by turn-on and turn-off transients. By limiting
the switching speed of the charge pump, the di/dt noise

caused by the parasitic trace inductance is minimized.
Although not typically required, additional high-frequen-
cy noise attenuation can be achieved by increasing the
size of C2 (see the

Typical Application Circuit

). The

charge pump is active only in headphone modes.

Headphone Current Limit

The headphone amplifier current is limited to 140mA (typ).
The current limit clamps the output current, which appears
as clipping when the maximum current is exceeded.

Shutdown Mode

The MAX9877 features two ways of entering low-power
shutdown. The hardware shutdown function is controlled
by pulsing BIAS low for 1ms. While BIAS is low the ampli-
fiers are shut down. Following an 80ms reset period, the
MAX9877 reverts to its power-on-reset condition. Pull
BIAS low using an open-drain output that is not pulled up
with a resistor (see the

Typical Application Circuit

)

The

open-drain output leakage must not exceed 100nA and
must be able to sink at least 1mA.

The device can also be placed in shutdown mode by
writing to the SHDN bit in the Output Control Register.

Click-and-Pop Suppression

The MAX9877 features click-and-pop suppression that
eliminates audible transients from occurring at startup
and shutdown.

Use the following procedure to start up the MAX9877:

1) Configure the desired output mode and pream-

plifier gain.

2) Set the SHDN bit to 1 to start up the amplifier.

3) Wait 10ms for the startup time to pass.

4) Increase the output volume to the desired level.

To disable the device simply set SHDN to 0.

During the startup period, the MAX9877 precharges the
input capacitors to prevent clicks and pops. If the output
amplifiers have been programmed to be active they are
held in shutdown until the precharge period is complete.

When power is initially applied to the MAX9877, the
power-on-reset state of all three volume control registers
is mute. For most applications, the volume can be set to
the desired level once the device is active. If the click-
and-pop is too high, step through intermediate volume
settings with zero-crossing detection disabled. Stepping
through higher volume settings has a greater impact on
click-and-pop than lower volume settings.

For the lowest possible click-and-pop, start up the device
at minimum volume and then step through each volume
setting until the desired setting is reached. Disable zero-
crossing detection if no input signal is expected.

GND

CONVENTIONAL AMPLIFIER BIASING SCHEME

DirectDrive AMPLIFIER BIASING SCHEME

GND

-V

)

Figure 5. Traditional Amplifier Output vs. MAX9877 DirectDrive
Output