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The AutoShutdown â„¢ circuit shown in Figure 1 introduces the transient characteristics of a phone call. When the circuit detects a call interruption, it will put the speaker amplifier into a low-power shutdown mode to save battery power and extend the call time When restarting, the audio amplifier will start immediately to avoid losing voice information. 
Figure 1. This circuit saves battery power consumption by automatically turning off the speaker driver when there is no audio signal.
Mobile phone batteries are mainly consumed by RF power amplifiers and speakers (Class AB speaker amplifiers). Ideally, when driving an 8Ω speaker at 1W, it consumes 353mARMS of current, and when there is no audio signal input, it also consumes milliamps of current. For this reason, most speaker driver ICs are related to off mode, reducing the current consumption of the chip when the driver is not used.
The quiescent current of the circuit is very different from the current in shutdown mode. Taking U1 in Figure 1 as an example, it consumes 5mA of quiescent current when there is no signal input, and when U1 is placed in shutdown mode, it consumes only 10nA Current. In Figure 1, when the input audio signal is below the preset threshold at a defined time interval, the power consumption of the entire circuit can be reduced from 4.7mA to 22µA.
The window comparator (U2 or U3) compares the input signal with a preset reference voltage proportional to VCC. R6 and R7 bias the input audio signal to the midpoint of the comparator window. VCC / 2, R3 to R5 set the reference voltage of the window comparator. The high and low thresholds are calculated according to the following formula: 
Setting a certain threshold difference (VHIGH-VLOW) is to prevent the noise signal from turning on the speaker amplifier.
The open-drain output of the comparator is connected to the SHDN input of U1. When the peak-to-peak value of the audio input exceeds the window range set by R3 to R5, the open-drain output will automatically control the speaker driver to turn on, and the VSHDN voltage is pulled to ground ( as shown in picture 2). When the input audio signal is in the window, both outputs will be turned off. C6 and the pull-up resistor R8 will slow the voltage of VSHDN to the threshold of turning off U1. 
Figure 2. When the input exceeds the preset threshold, the shutdown signal (VSHDN) in Figure 1 is driven low. The larger time constants generated by C6 and R8 can avoid unnecessarily turning on and off the audio signal.
Waking up from the off state is very fast, because C6 discharges directly to ground through the open-drain output of the comparator. Quick start can ensure that the voice information will not be lost (Figure 3). 
Figure 3. After the input signal disappears, the speaker shown in Figure 1 will quickly turn off.
This article was published in the April 18, 2005 issue of EE TImes.
Abstract: This circuit will automatically turn off the speaker driver when it detects that there is no audio signal input to save the battery power of the mobile phone.