Switched-power amplifiers, also referred to as Class-D amplifiers, are in common use in audio applications due to their efficiency and compact size compared to traditional analog (linear) power amplifiers. The efficiency and/or noise performance of switched-power amplifiers can be further improved by reducing the power supply voltage provided to the switched-power output stage when the signal level being reproduced is low in amplitude. One manner in which the power supply voltage may be controlled uses a volume control value to adjust the power supply rails. Other schemes determine the signal amplitude after any volume control value has been applied and adjust the power supply rail voltages according to the expected output signal level.
In audio applications, the supply rail(s) of a class-D amplifier may be chosen from a set of discrete voltages in order to implement a digital volume control scheme. In such a scheme, a reference generator circuit typically selects one of a number of discrete voltage levels depending on the digital volume control level. The power supply rail voltage is generated in conformity with the volume control level. For example, a reference voltage generated by the reference generator may be buffered by a low dropout regulator (LDO) and applied as a power supply rail of the switched-power output stage. By changing the supply rail(s) of the amplifier in conformity with the digital volume control level, the output power is modulated.
When the power supply rail voltage changes between two discrete levels of amplifier supply rail(s), audible pops/clicks and or wideband spectral leakage can occur. The pops are typically not tolerable and must be prevented. Typically, the pops are prevented by increasing the number of steps and only stepping between adjacent levels until the desired volume change is achieved. However, the number of discrete levels required to ensure that all of the steps are inaudible may grow prohibitively large due to the increased area, cost, and complexity.
Therefore, it would be desirable to provide techniques for controlling the power supply rail voltage of a switched-power amplifier without requiring a prohibitively large number of discrete power supply output voltages.