Texas Instruments TPA721 User Manual page 19

The TPA721 amplifier IC, like most other amplifiers, exhibits its best distortion
and noise performance at lower gain levels (see the TPA721 data sheet). Even
so, the TPA721 at its highest gain setting has significantly less distortion than
most low-cost speakers.
Gain versus total harmonic distortion (THD) should be considered in each
application. Both the module input signal level and the TPA721 MSOP module
gain should be adjusted to obtain the lowest overall distortion level for a
particular overall gain. A quick rule of thumb (everything else being equal): the
module input signal level should be as high as possible without clipping or
overloading the TPA721 input, and the TPA721 gain should be kept as low as
possible.
3.2.4 Shutdown
When the shutdown terminal of the TPA721 amplifier IC is taken high, the IC
ceases operation and enters a low-power state. This is accomplished by
applying a control signal to the module shutdown pin (S1) or by pressing the
shutdown switch on the module. When the control signal goes low or is
removed (or the switch is released), amplifier operation resumes.
The plug-n-play platform can generate the shutdown (mute) signal for the
module either when a plug is inserted into the platform headphone output jack
or when the plug is removed, as selected by a platform jumper (JP8).
3.2.5 Internal Depop Circuitry
Internal depop circuitry reduces or eliminates the various turn-on transients
that might appear at the output during the transition from power off or shutdown
mode to normal operation. These output transients are often heard as a pop
from the speaker and occur when the two amplifiers in the IC turn on or off at
different rates, causing a voltage spike to appear across the output. The depop
circuitry adds a delay with an RC network at the amplifier IC bypass pin to help
ensure that the amplifiers turn on and off at the same rate, thus eliminating the
pop. For best results, bypass capacitor C1 should have a value between
0.1 µF and 2.2 µF. A 1 µF unit was selected for the EVM.
The TPA721 MSOP Audio Power Amplifier Evaluation Module
Details 3-5