Finding the Right System Location for the Loudspeaker Driver
The one output driver that poses particular issues for integration in 28-nm processes is the loudspeaker driver. Typically, a loudspeaker driver using a 3.3 V supply will be able to deliver up to 500 mW of power. In order to have reasonable audio performance, loudspeakers should not drive less than 250 mW. Still, with only 1.8 V available to drive the gates of the loudspeaker driver, the output devices must be significantly increased to support the high current requirements leading to an often unacceptable silicon area cost.
As a result, integrating the loudspeaker driver in a 28-nm SoC may not always be technically feasible or practical, making it necessary for designers to consider system-level options. Figure 6 shows the four common options for implementing a loudspeaker driver in a mobile multimedia system. The first is to completely integrate the driver into the SoC (Figure 6a). The second is to transfer the entire audio codec functionality to a dedicated audio IC and use the I2S digital interface between the dedicated audio IC and the SoC (Figure 6b).
The third option is to integrate all of the audio functionality into the SoC with the exception of the loudspeaker driver and use a low-cost, dedicated loudspeaker driver (Figure 6c). The fourth option is to integrate the loudspeaker driver into the power management IC (PMIC) (Figure 6d). Because the PMIC already supports high voltage and currents, it is a logical location for a high power circuit. In addition, Table 1 below outlines the advantages and disadvantages of each option.
Figure 6 - Implementation options for a loudspeaker driver in a mobile multimedia system
Table 1: Advantages and disadvantages of implementing a loudspeaker driver in a mobile multimedia system
In summary, due to the high power and current consumption requirements of the loudspeaker driver, it is the most difficult output driver to integrate with the 1.8 V supply in an area efficient manner. In order to support a loudspeaker driver, SoC designers must decide whether the function can be integrated into another block in the system, such as the power management IC, or supported with an external loudspeaker driver.