The speaker amplifier can be automatically shut down to effectively save power consumption of the mobile phone

       The AutoShutdown circuit shown in Figure 1 describes the transient characteristics of a telephone call. When the circuit detects that the call is interrupted, it puts the speaker amplifier into a low-power shutdown mode to save battery energy and extend the call time. When the call is resumed, the audio amplifier will start up immediately to avoid losing voice information.

                                Figure 1. This circuit saves battery power by automatically shutting down the speaker driver when no audio signal is present.


       The main drain on a cell phone battery is the RF power amplifier and the speaker (Class AB speaker amplifier). Ideally, an 8Ω speaker with 1W of power consumes 353mARMS of current, and will also consume milliamps of current when no audio signal is input. For this reason, most speaker driver ICs have a shutdown mode that reduces the chip's current consumption when the driver is not in use. The


       quiescent current of a circuit is very different from the current in shutdown mode. For example, U1 in Figure 1 consumes 5mA of quiescent current when no signal is input, but when U1 is placed in shutdown mode, it consumes only 10nA of current. In Figure 1, the power consumption of the entire circuit can be reduced from 4.7mA to 22μA when the input audio signal is below the preset threshold at a defined time interval.


       The window comparator (U2 or U3) compares the input signal to 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 as follows:

       The difference between the threshold values ​​(VHIGH - VLOW) is set to prevent noise signals 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 will be pulled to ground potential (as shown in Figure 2). When the input audio signal is within the window, both outputs will be turned off, and C6 and the pull-up resistor R8 will slow down the rise of the VSHDN voltage to the threshold that turns off U1.

Figure 2. The shutdown signal (VSHDN) in Figure 1 is driven low when the input exceeds a preset threshold. The large time constant created by C6 and R8 prevents the audio signal from being switched on and off unnecessarily. 



       Wake-up from the shutdown state is very fast because C6 discharges directly to ground through the open-drain output of the comparator. The fast turn-on ensures that no speech information is lost (Figure 3).

                                                   Figure 3. The speaker shown in Figure 1 turns off quickly when the input signal is removed.


This design idea appeared in the April 18, 2005 issue of EE Times.

AutoShutdown is a trademark of Maxim Integrated Products, Inc.