International Rectifier IRAUDAMP4, Руководство пользователя

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RD-0617
Startup and Shutdown
One of the most important aspects of any audio amplifier is the startup and shutdown
procedures. Typically, transients occurring during these intervals can result in audible
pop- or click-noise on the output speaker. Traditionally, these transients have been kept
away from the speaker through the use of a series relay that connects the speaker to the
audio amplifier only after the startup transients have passed and disconnects the
speaker prior to shutting down the amplifier. It is interesting to note that the audible noise
of the relay opening and closing is not considered “click noise”, although in some cases,
it can be louder than the click noise of non-relay-based solutions.
The IRAUDAMP4 does not use any series relay to disconnect the speaker from the
audible transient noise, but rather a shunt-based click noise reduction circuit that yields
audible noise levels that are far less that those generated by the relays they replace.
This results in a more reliable, superior performance system.
For the startup and shutdown procedures, the activation (and deactivation) of the click-
noise reduction circuit, the Class D power stage and the audio input (mute) controls
have to be sequenced correctly to achieve the required click noise reduction. The overall
startup sequencing, shutdown sequencing and shunt circuit operation are described
below.
Click-Noise Reduction Circuit (Solid-State Shunt)
To reduce the turn-on and turn-off click noise, a low impedance shunting circuit is used
to minimize the voltage across the speaker during transients. For this purpose, the
shunting circuit must include the following characteristics:
1) An impedance significantly lower than that of the speaker being shunted. In this
case, the shunt impedance is ~100 m Ω , compared to the nominal 4 Ω speaker
impedance.
2) When deactivated, the shunting circuit must be able to block voltage in both
directions due to the bi-directional nature of the audio output.
3) The shunt circuit requires some form of OCP. If one of the Class D output
MOSFETs fails, or is conducting when the speaker mute (SP MUTE) is activated,
the shunting circuit will effectively try to short one of the two supplies (+/-B).
The implemented click-noise reduction circuit is shown in Figure 9. Before startup or
shutdown of the Class D power stage, the click-noise reduction circuit is activated
through the SP MUTE control signal. With SP MUTE signal high, the speaker output is
shorted through the back-to-back MOSFETs (U9 for Channel 1) with an equivalent on
resistance of about 100 m
Ω . The two transistors (U7 for Channel 1) are for the OCP
circuit.