In legacy envelope tracking circuits, such as the reference design in TIDA-050024 shown in Figure 2-3, an analog op amp is used to sample the music waveform at the audio amplifier output. After shaping and filtering, the PVDD voltage is adjusted by the boost converter. However, the PVDD voltage tracking lags behind the music waveform, resulting in severe clipping distortion. In addition, complex analog op-amp circuits are required, leading to high circuit complexity and BOM cost.

TI's innovative Hybrid-Pro algorithm for PVDD music envelope tracking is integrated into the audio amplifier’s DSP. The TAS5825P is the first audio amplifier solution to innovatively use digital Hybrid-Pro, also commonly referred to as Class-H. This algorithm controls the output voltage of the boost circuitry by providing easily controlled PWM feedback. In addition, with the delay between the music waveform and the Class-H waveform, it provides advance control of the PVDD voltage, which enables accurate adjustment of the PVDD voltage before music peaks arrive, perfectly preventing clipping distortion of the music waveform.

The amount of delay between the music peak output and the PVDD adjustment control is mainly affected by two factors. One is the delay length that the customer can accept, which is determined by the customer's product type. For example, in some Partybox products, a microphone input is present. The total delay of the entire link from the microphone input to the audio amplifier output is typically required to be less than 5ms. In this case, the audio amplifier delay must be as small as possible, so that listeners cannot perceive a delay between the original voice and the sound output from the speaker. The other consideration is that the time delay must be greater than the time required for PVDD to rise to its target level, so as to avoid clipping distortion. In this case, the main influencing factors include the boost converter's loop reaction time, the output capacitance, and the current power with load. Given so many factors, debugging must be done based on real-world conditions. To debug the Class-H mode, TI provides easy-to-use PurePath™ - Console 3 (PPC3) software.

The Class-H mode is implemented on the TAS5825P as shown in Figure 2-5 below. The music signal enters the audio amplifier's DSP and is split into two processing paths. In the first path, the music signal is processed by the Class-H algorithm, which calculates the signal level and outputs a PWM feedback signal with varying duty cycles related to the music amplitude. This feedback signal is then filtered by a second-order RC low-pass filter (LPF) and fed to the FB (feedback) control pin of the boost converter, adjusting the PVDD output voltage to the level just sufficient to prevent music clipping, thereby minimizing the power consumption. In the other path, the music signal waits in the DSP for an adjustable delay before being output from the amplifier until the PVDD voltage is adjusted to the required level.

The PPC3 Class-H mode debugging interface is shown in Figure 2-6 below. This GUI tool makes it easy to set the PVDD voltage range, calculate the feedback resistor parameters of the boost converter, and calculate the RC filter parameters.

As shown in Figure 2-7, the efficiency data from the TAS5825P specifications shows that a significant improvement in audio amplifier efficiency is achieved by using the Class-H mode: The blue line shows up to a 15% improvement in efficiency when the Hybrid-pro (Class-H) mode is enabled.