A key design challenge for creating an audio amplifier is generating its power supply voltage. When using a single-cell lithium battery as the power source, a boost converter raises this voltage to bias the audio amplifier. The boosted-voltage level is a trade-off between audio quality and power consumption. You would like to boost the supply voltage high enough to never distort or clip portions of the audio signal, which have a higher peak power. But you also don’t want to dissipate a lot of excess voltage during those other audio signals, which have a lower peak power. Can you achieve both?
If the power supply is capable of adjusting to the incoming audio signal, then the answer is yes. The arrival of a higher power signal – which requires a higher supply voltage to not distort – boosts the audio amplifier’s supply voltage higher. And when this signal goes away and returns to a lower level, the supply voltage reduces. Dynamically keeping the power supply voltage optimized to the changing audio signal is called envelope tracking. The Envelope Tracking Power Supply Reference Design for Audio Power Amplifiers adjusts the supply voltage from 5.5V all the way to 11.75V from the single-cell lithium battery. Figure 1 shows the implementation.
Figure 1: Block diagram of PMP9774 with envelope tracking of an audio signal
The key to envelope tracking is to measure the strength of the incoming audio signal. The OPA4377 is a rail-to-rail output, low-cost operational amplifier (op amp) that conditions both the left and right audio inputs and creates the “envelope signal.” This signal then drives a transistor that adjusts the output voltage of the TPS61088 boost converter. The boost converter contains internal power transistors and provides over 20W of output power from a single-cell lithium battery to drive the audio amplifier.
Compared to the traditional solution, which just sets the boost converter to the highest output voltage, envelope tracking requires just the additional op amp and passives to condition the audio signal. With this small amount of additional circuitry and envelope tracking, large power savings are possible in the audio amplifier.
In what other applications can envelope tracking save you power?