A typical portable speaker consists of a lithium battery, a boost converter, an audio amplifier, and so forth. The main power supply (PVDD) of the audio amplifier is provided by the lithium battery, with the voltage stepped up by the boost converter. The signal source to be amplified by the audio amplifier is music, which usually features a wide dynamic range. An appropriate amplitude of PVDD helps prevent clipping distortion of music peaks. For most smaller-amplitude signals in music, however, a fixed PVDD voltage is a waste of energy: It can be found from the equations above that the audio amplifier generates higher losses when operating at a higher PVDD voltage, and the boost converter also has a lower efficiency in this case. As a result, the system efficiency is lower than that at a relatively low PVDD voltage.

The boost converter's efficiency is also low when it outputs a relatively high step-up voltage, and the ratio of output to input voltages is large. As shown in Figure 2-1 below, taking the efficiency data from the specifications for the TPS61288, TI's mainstream boost converter, as an example: Under the same conditions of VIN = 3.6V and IOUT = 0.1A, the efficiency at VOUT = 5.5V is about 3% higher than the efficiency at VOUT = 13V.

Similarly, audio amplifiers also have lower efficiency at a higher PVDD voltage. As shown in Figure 2-2 below, taking the efficiency data from the specifications for the TAS5825P, TI's mainstream audio amplifier, as an example: Under the same output power (for example, 10W), the efficiency at PVDD = 12V shows a significant difference, being nearly 10% higher than the efficiency at PVDD = 24V.

Therefore, if the PVDD voltage dynamically follows the music waveform dynamics to achieve envelope tracking, it will be possible to improve efficiency for both the boost converter and the audio amplifier. Figure 2-3 shows that the PVDD voltage changes with the amplitude variations of the music waveform: