AutoShutdown™ for Speaker Amplifier Conserves Cell-Phone Power
The AutoShutdown circuit of Figure 1 exploits the transient nature of cell-phone conversations. When the circuit detects a lull in conversation, it places the speaker amplifier in a low-power shutdown state that conserves battery life and extends talk time. When conversation picks up again, the speaker amplifier quickly turns on to avoid loss of audio information.
Figure 1. This circuit conserves battery power by automatically shutting down the speaker driver when no audio signal is present.
Cell-phone batteries supply current to power-hungry circuits such as the RF power amplifier and, in speakerphone mode, the class AB speaker amplifier. An ideal speaker amplifier draws 353mARMS from the battery when driving 8Ω at 1W. However, with no signal present, it still draws milliamps of supply current. For that reason, most speaker-driver ICs include a shutdown mode that reduces supply current when the driver is not in use.
As a result, the quiescent operating and shutdown currents can differ dramatically. Speaker amplifier U1 of Figure 1, for instance, draws 7mA typical when no signal is applied. Forcing U1 into shutdown reduces that quiescent supply current to a paltry 10nA. In Figure 1, the total supply current drops from approximately 7mA to 22µA when the audio input dips below a preset level for a defined time interval.
A window comparator (U2 or U3) compares the input signal with a preset reference voltage proportional to VCC. R6 and R7 bias the input signal to the middle of the comparator's window, which is VCC/2. R3-R5 set the window comparator reference, in which the high and low thresholds are:
Setting the difference in thresholds (VHIGH - VLOW) sufficiently large ensures that noise cannot turn on the speaker amplifier.
The comparators' open-drain outputs connect to U1's SHDN input. When the peak-to-peak value of the audio input is outside the window set by R3-R5, the open-drain outputs turn on alternately, maintaining VSHDN at ground as shown in Figure 2. When the audio signal is within the window, both outputs turn off. C6 and pullup resistor R8 then cause VSHDN to slowly rise to U1's shutdown threshold.
Figure 2. The shutdown signal in Figure 1 (VSHDN) is driven low when the input exceeds preset levels. The large time constant created by C6 and R8 prevents excessive gating of the audio signal.
Waking up from shutdown is much quicker, because C6 discharges directly to ground through one of the open-drain comparator outputs. A quick turn-on ensures that no audio information is lost (Figure 3).
Figure 3. The speaker amplifier in Figure 1 shuts down quickly when the input signal is removed.